GS ENVS 302 CH 12 Climate Change

Several processes were involved in the climate changes of the past thousand years:

(1) changes in Earth's orbital pattern, causing cooling; (2) volcanic eruptions; (3) changes in solar-energy output; (4) changes in sea-surface temperatures in the tropics; (5) changes in aerosol concentrations; and (6) increases in greenhouse gases in the atmosphere. There probably were also interactions between the ocean, the atmosphere, and the ice sheets that are yet to be understood.

Mitigation Options There is a widely perceived need to slow global warming. How can this be done? Various schemes to control climate have been proposed; they fall into two main categories:

(1) controlling CO2 content in the atmosphere and (2) managing solar radiation received by Earth. A third category proposes some schemes that could achieve rapid results; they are (3) fast-action strategies.

THE LAST GLACIAL MAXIMUM Growing continental glaciers reached their last maximum positions between 33,000 and 26,500 years ago due to decreases in

(1) summer insolation in the Northern Hemisphere, (2) tropical Pacific sea-surface temperatures, and (3) atmospheric CO2. The glaciers were still near their maximum positions 20,000 years ago.

The past thousand years find us in the realm of human history with ever-improving observational records. Historic records studied to learn about past climates include

(1) tax records of grain and grape crops; (2) advances and retreats of mountain glaciers; (3) paintings of winter scenes showing frozen lakes, rivers, and ports; and (4) numbers of weeks per year of sea ice around Iceland.

In the early stage of a famine, food is still available, but there is not enough. Healthy people can lose up to

10% of their body weight and still remain mentally alert and physically vigorous. • In the advanced stage, body weight decreases by around 20%, and the body reacts to preserve life itself. Body cells lower their activity levels, reducing the energy needed to keep vital functions going. People sink into apathy. • In the near-death stage, when 30% or more of body weight has been lost, people become indifferent to their surroundings and to the sufferings of others, and death approaches.

SIGNS OF CHANGE Have you seen them? Have you seen the signs of climate change? The IPCC report evaluated 75 studies that reported more than

29,000 observational data series of significant changes in physical and biological systems; more than 89% of them are consistent with the change expected from warming (table 12.9). ex. Mosquitoes carrying diseases are migrating farther north and south

LATE CENOZOIC ICE AGE (cont) Why have these changes occurred? There is no single answer. Several variables have interacted in a complex fashion to bring about the climatic cooling of the last 55 million years. The main factors appear to be related to plate-tectonic changes—that is, the changing positions of continents and oceans. (1) The climatic change is associated with the ongoing breakup of Pangaea into separate continents (see figures 2.24 and 2.25). (2) As continents drifted, seaways opened and closed, thus altering the circulation patterns within the oceans and the distribution of heat about the globe. (3) Continental masses have moved into polar latitudes, with Antarctica centering on and rotating about the South Pole, while North America and Eurasia have moved to encircle the North Pole region.

(4) As snow and ice began accumulating on polar landmasses, they reflected more sunlight (increased albedo), and thus heat, back to space. (5) Circulation of the ocean water around the equator was restricted at about 23 million years ago with the closure of the eastern Mediterranean Sea and ended at 3 million years ago when volcanism completed building Central America as a continuous north-south barrier that blocked east-west ocean-water flow. (6) The area of shallow oceans has been reduced, so less water surface is available to absorb sunlight. (7) The uplifts of the Tibetan Plateau/Himalaya Mountains in Asia and the Colorado Plateau in the western United States have deflected west-to-east atmospheric circulation in the midlatitudes with resultant airflows to the north and return flows to the south.

The Greenhouse Effect was never a "manmade" phenomenon

- our technological lifestyles have merely created a dangerous imbalance affecting the Earth's natural Greenhouse Effect.

Today, carbon dioxide makes up only

0.04% of the Earth's atmosphere, but it helps create the greenhouse effect that keeps Earth's average temperature at 16°C (61°F); this is 34°C higher than it would be if CO2 and other greenhouse gases were absent. • If CO2 were not present in the atmosphere, the average temperature at Earth's surface would be about -18°C (0°F), and much of life would be different from what we know.

Sunspots were nearly absent from the Sun between 1645 and 1715 during the Maunder Minimum. During those decades, the Sun was about

0.25% weaker, causing Earth's temperature to fall 0.2° to 0.5°C (see figure 12.27).

LATE PALEOZOIC ICE AGE Part 1 One of the major Ice Ages in Earth history began around 360 million years ago and lasted until 260 million years ago. For a glacial interval to last so long, broad-scale and long-lasting conditions are required. The major factors appear to be changes in the shapes, sizes, and orientations of the continents and oceans.

1. An initial, absolute requirement for an Ice Age is having one or more large continental masses near the poles. • A polar landmass is necessary to collect the snowfall that allows the buildup of immense, 3 km (2 mi) thick ice sheets that bury continents. In Late Paleozoic time, the continents were largely united as the single landmass Pangaea. The southern portion of the Pangaea supercontinent is Gondwanaland; it moved across the south polar region and was progressively covered by major ice sheets. South America-Africa probably first supported the great ice sheet, then Antarctica, and finally Australia (figure 12.5).

What are the main variables that come into play when volcanism affects climate? Principal factors include:

1. Size and rate of eruptions 2. Heights of eruption columns 3. Types of gases and the atmospheric level where they are placed. (For example, sulfur dioxide in the stratosphere reflects sunlight and cools the climate below, whereas carbon dioxide in the atmosphere creates a greenhouse effect.) 4. Latitude. Low-latitude eruptions spread atmospheric debris across more of the world and have greater global effects than high-latitude eruptions.

Solar Energy (sunlight) is short-wavelength radiation that without doubt penetrates the atmosphere and warms the Earth. The atmosphere reflects back approximately

1/4 of the incoming sunlight, emitting long-wavelength radiation (infrared waves or heat energy) back into space. However, due to the current increases in atmospheric carbon dioxide levels, these longer waves are now reflected back to Earth. Carbon dioxide in the atmosphere is trapping the heat that would have normally been released back into outer space. As a result, the Earth is warmer than it has been in the last one million years.

A global assessment of satellite data finds that changes in solar-energy output are responsible for less than

10% of recent warming. Thus, the major causes of global warming are changes in the greenhouse gas and aerosol contents of the atmosphere.

Returning to the earliest Earth (table 12.1), when no life was present and the atmosphere was full of CO2, the surface temperature of our planet would have been about

290°C (550°F). Why was Earth so hot? This global warming was due in part to the greenhouse effect. Because of its abundance, CO2 (an important greenhouse gas), kept Earth's climate hot. Over geologic time, as CO2 dissolved in water and CaCO3 sediments formed, the amount of atmospheric CO2 declined. • Early photosynthesizing life on Earth pulled CO2 out of the atmosphere, causing a decline. Innumerable species of animals made skeletal material out of CaCO3, reducing the amount of atmospheric CO2 even further. Biologic use of CO2 has lessened the greenhouse effect to yield the present congenial temperatures on Earth.

The Milankovitch theory explains that glaciers advance and retreat due to variations in solar radiation received at high latitudes during summer and that these variations are due to the following factors: 1. Eccentricity of Earth's orbit around the Sun. The more elliptical the orbit, the less solar radiation is received and the less snow melts. The shape of the orbit varies every 100,000 years, from nearly circular to a slightly eccentric ellipse (figure 12.11a). 2. Tilt of the Earth's axis. The spin axis of the Earth tilts away from the orbital plane in a 41,000-year-long cycle, where tilt varies from 21.5° to 24.5° (figure 12.11b). Greater tilt angles cause more increased seasonal extremes, including more snow melt. At present, the tilt is about 23.5°.

3. Precession of the equinoxes where the direction of the tilt changes even though the angle stays the same. The effect is a wobble roughly analogous to what you see in the spin of a toy top. The wobble has a double cycle with periodicities of 23,000 and 19,000 years (figure 12.11c). Currently, the wobble places Earth closest to the Sun during the Northern Hemisphere winter, giving it milder winters and summers than the Southern Hemisphere.

Until human beings dramatically reduce emissions by at least

80 percent globally, the concentration of CO2 in the atmosphere will continue to rise like water overflowing the hot tub. Ongoing atmospheric heating from rising CO2 levels means ongoing acidification of the oceans, which is already bleaching the Earth's coral reefs.

It is increasingly evident that unusually high rains in one area and drought in another are not isolated events; rather, they are both parts of a globally connected weather system.

A change that occurs in one area can trigger other changes around the world, somewhat like a falling line of dominoes—knock over one and it starts the process whereby they all fall down. Every several years, the tropical atmosphere goes through changes that link up around the world. The tropical Indian Ocean warms, and its weather pattern may blow into the western Pacific Ocean, setting off an El Niño. After the El Niño wind shifts cross the Pacific Ocean and South America, the tropical Atlantic Ocean may begin to warm. This global circuit takes about four years to move around the Earth.

LA NIÑA But El Niño has a sister called La Niña ("the girl"), and she has a different personality. La Niña occurs when cooler water moves into the equatorial Pacific Ocean (figure 12.22). During a La Niña, trade winds are stronger and other wind systems change their paths, bringing different weather patterns across North America.

A typical La Niña winter brings cold air with high rainfall to the northwestern United States and western Canada but causes below-average rainfall in most of North America. The winter of 1999-2000 was typical, with heavy rainfalls in the northwest and belowaverage rainfalls elsewhere, accompanied by numerous wildfires in the west.

Our current Ice Age is not over. We live during one of the colder intervals in Earth's history, despite the current glacial retreat.

About 10% of the continents today remain buried beneath about 32 million km3 of ice, primarily on Antarctica and Greenland. If this ice were to melt, with or without human help, sea level would rise about 65 m (210 ft), and many of the world's major cities would be submerged.

The Greenhouse Effect is actually a natural increase in temperature caused by the gases in the atmosphere trapping heat energy from sunlight.

According to Ohio State University researchers Robert Essenhigh and E.G. Bailey, Professor of Energy Conservation, Department of Mechanical Engineering, global warming is a natural geologic process that appears to be reversing itself today[ii]. Both Essenhigh and Bailey attribute the current reported rise in global temperatures to a natural cycle of warming and cooling. However, excess carbon dioxide and water vapor from manmade influences - from pollution, manufacturing, urbanization, and deforestation - that add to global warming.

METHANE (CH4) About 16% of modern greenhouse warming has come from increasing methane in the atmosphere. Notice in table 12.6 that the global warming potential (GWP), or heat-trapping ability, of methane is 21 times greater than that of carbon dioxide.

Air trapped in ice tells us that methane concentration in the year 1750 was about 700 parts per billion (ppb), but it has risen to more than 1,815 ppb in 2014. The increase in atmospheric methane was slow in the 19th century and rapid in the 20th century.

Fisheries Food harvested from the sea by humans provides about 15% of the protein eaten by 60% of the human population. The IPCC Assessment Report 5 projects that climate change by the mid-21st century and beyond will cause global redistribution of marine species and reductions in biodiversity that will challenge the sustained productivity of fisheries (figure 12.41).

Algae are shifting poleward 10 km (6 mi) per decade and plankton are shifting 400 km (250 mi) per decade. As the base of the food pyramid shifts, other marine organisms will change with them. The fish yields in temperate latitudes could be 30-70% higher than in 2005 but tropical yields could fall 40-60%. These potential yields do not consider the effects of overfishing and ocean acidification.

El Chichón, 1982 Located in the state of Chiapas in southern Mexico is the relatively small volcano called El Chichón (which translates loosely as "bump"). Four big Plinian eruptions from El Chichón on 29 March through 4 April 1982 blew out about 0.6 km3 of material, leaving a 1 km diameter crater and killing 2,000 people (see figure 7.28).

Although the eruptions were not as big as the Mount St. Helens event in 1980, more than 100 times the volume of SO2 gases was pumped into the stratosphere, along with volcanic ash. The cloud of stratospheric gases took 23 days to circle the globe (figure 12.24). The SO2 gas combined with O2 and water vapor, converting to sulfuric acid (H2SO4) aerosol. Sunsets were spectacular for months, beginning with a purple glow high over the horizon, changing gradually to surreal yellows and oranges as the Sun set, and ending with a red afterglow when the sky normally would have been dark. World temperature from this event was lowered 0.2°C.

Precipitation It is very likely that precipitation will increase in high latitudes and decrease in most subtropical lands (figure 12.35). As precipitation and temperature patterns change around the world, there will be regions of winners and regions of losers.

And a winner in one decade may become a loser in following decades as temperatures continue to rise and precipitation decreases, increases, or shifts location. In regions where temperatures rise and precipitation falls, drought and famine can become problems.

Arctic sea ice can be called the canary in the global warming coal mine. Caged canaries have long been carried into mines because they die quickly when exposed to gases such as carbon monoxide (CO) and methane (CH4) thus warning miners to leave the mine rapidly.

And now, the Arctic sea-ice cover is dying quickly as the polar climate warms due to buildup of gases such as CO2 and CH4 in the atmosphere. Some computer models suggest that Arctic summer sea ice could disappear as early as 2030. Global warming is occurring before our eyes.

The Last 3 Million Years The ice sheet on Antarctica is older and more stable than ice in the Arctic. The cold ocean water circulating around

Antarctica (see figure 9.28) helps isolate the continent from major changes. The ice sheets on North America and Eurasia have a greater effect on global climate change because they expand and shrink in a more dynamic fashion. Their initial growth as continental glaciers occurred between 3.0 and 2.7 million years ago and coincided with Central America forming a continuous link between North and South America, it blocked westward-flowing ocean water and began diverting the warm water of the Caribbean Sea and Gulf of Mexico and forcing it to flow northward along the western Atlantic Ocean. The warm water delivered to Canada, Greenland, and Europe caused greater evaporation and formation of water vapor, which resulted in greater snowfall that accumulated to build glaciers.

It is not only area covered by sea ice that is important but also sea-ice volume and age, and both are decreasing.

Arctic sea-ice volume in 2012 was 76% smaller than in 1979. Thinner ice melts away faster, and younger ice melts more easily than older ice. In 1987, 57% of Arctic sea ice was 5 or more years old, and 14% was 9 or more years old. By 2007, only 7% of Arctic sea ice was 5 or more years old, and none was 9 or more years old. • In the recent past, Arctic sea ice covered an area about 1.5 times larger than the United States. This huge sheet of ice with high albedo reflected sunlight back to space, acting as a huge air conditioner for planet Earth. Now the Arctic Ocean has growing areas of ice-free, dark seawater absorbing solar energy and warming rapidly.

Circulation Major climatic shifts will occur if the present deep-ocean circulation system is altered (see figure 9.29). At present, in the North Atlantic Ocean, the surface water carries warm tropical water to the north, where it releases heat to the atmosphere.

As it travels north, the seawater cools and becomes denser, sinks, and then flows south at intermediate depths almost to Antarctica (figure 12.42). This deep-water circulation is driven by density contrasts in seawater. As the global ocean warms, the density contrasts become less, raising the possibility that the deep-water circulation system could slow or stop. Changing the heat-carrying ocean currents would cause significant climate change, affecting life in the sea and on land.

Why did the Late Paleozoic Ice Age end? Possibly because Gondwanaland began to break up and disperse.

As the continents moved apart, ocean circulation patterns around the world changed. Warm waters stayed near the equator, and cold waters encircled the poles, thus drastically reducing the moisture supplied to polar landmasses. Additionally, when continents move away from the poles, no platform exists for the accumulation of snow and the building of glaciers.

Why the sudden jumps or drops in temperature? One suggested cause relies on changes in the North Atlantic Ocean.

As the massive ice sheets on the continents were melting, enormous lakes of pure, cold water formed, held back by ice dams. The shape of the land surface (see last section of chapter 13) and the sediment record tell of enormous floods produced by the failure of the ice dams.

Late Paleocene "Torrid Age" (cont) Warm, oxygen-deficient, salty waters apparently sank, flowing through the ocean deeps and warming up the oceans from surface to bottom. Warm, salty water masses moving along the ocean bottoms likely affected deep-ocean life. Organisms used to living in cold, oxygen-rich bottom waters experienced the shock of their environment becoming warm and oxygen-poor.

At about 55 million years ago, the massive change in deep-sea water temperature reached a peak, causing up to 50% of unicellular deep-sea animal species to become extinct—a natural disaster.

The changes over time of the eccentricity, tilt, and wobble cycles have been calculated for the past and into the future (figure 12.12).

At present, the tilt contributes to cooling while the eccentricity and wobble (precession) work to warm the climate.

Look at figure 12.27 again, at arm's length, and notice the thousand-year pattern—a warm interval for the first few hundred years, followed by a cooling until around 1700 when temperatures began rising.

At the start of the 20th century, temperatures began exceeding the 1,000-year high; this warming trend continues today.

How much did global temperature rise in the 20th century?

Average global surface temperature rose 0.6°C (+/-0.2°C), or 1.1°F. Could a person feel the climate warming of the 20th century? No, because the climate warming is small compared to the day-to-day temperature fluctuations of weather.

How is methane released to the atmosphere? It is released during decomposition of vegetation in oxygen-poor environments such as swamps, rice paddies, and cattle digestive systems.

Bacteria remove carbon (C) from dead vegetation, and if oxygen is absent, the carbon combines with hydrogen (H) to make methane (CH4). About 40% of methane release occurs by natural decomposition, mostly in wetlands and secondarily via termites.

NITROUS OXIDE (N2O) Nitrous oxide is another contributor to the greenhouse effect (see table 12.6). About 70% of N2O is produced naturally, mostly by bacteria removing nitrogen from organic matter, especially within soils.

Human activities cause the remaining 30% of nitrous oxide release via our agricultural practices, including use of chemical fertilizers. The second important way humans release N2O is by combustion burning of fuels in car and truck engines. N2O is increasing in the atmosphere regularly and rapidly.

Equilibrium Between Tectonics, Rock Weathering, and Climate (cont) A geologic example of this process involves the tectonic collision of India pushing into Asia and uplifting the Himalaya Mountains, beginning about 40 million years ago (see figure 4.18).

Before the collision, the atmospheric concentration of CO2 was about 1,400 parts per million, but by 24 million years ago, CO2 was down to about 200 parts per million. Uplifting the huge masses of fractured, fresh rock to form the Himalayas increased the rate of rock weathering, thus drawing down CO2 levels in the atmosphere.

Arctic Sea Ice Throughout recorded history the Arctic Ocean has been mostly covered with sea ice. European explorers sailing to the Arctic Ocean looking for a direct passage to Asia were blocked by sea ice from the 15th through the 20th centuries, even during the warmest summer months.

But the 500+ year barrier is lifed in 21st-century summers as enough sea ice melts to open long-sought passages (figure 12.38). Now dozens of the world's largest ships travel across the Arctic Ocean each summer using either the Northwest Passage along Canada or the Northeast Passage along Russia.

When Did Humans Begin Adding to Greenhouse Warming? During recent decades, the greenhouse warming caused by humans has become well known. For example, we release large quantities of CO2 to the atmosphere by burning oil, natural gas, coal, and wood.

But when did human activities begin producing the greenhouse gases that warmed the climate? According to William Ruddiman of the University of Virginia, humans began warming the climate about 8,000 years ago by cutting and burning forests to clear land for agriculture; the widespread forest destruction added CO2 to the atmosphere. • Then about 5,000 years ago, rice began to be extensively grown with techniques that formed artificial wetlands, which gave off methane to the atmosphere. • Ruddiman calculates that these land uses caused the climate to warm about 0.8°C. If this estimate is correct, then the amount of human warming of climate has about tripled: from about 0.8°C by our ancestors over thousands of years to 0.4°C by us in tens of years.

Acidification The increasing volume of CO2 in the atmosphere is more than just a climate-changing greenhouse gas; it is a reservoir in contact with the oceans that cover 71% of Earth's surface. CO2 readily dissolves in water, creating an acid:

CO2 + H2O yields H2CO3 (carbonic acid) Ocean acidification has been called the evil twin of global warming. Regular, direct measurements of CO2 in the atmosphere document its greater than 25% rise since 1958 (see figure 12.32). In the past 30 years, ocean pH has changed from 8.11 to 8.06, which is a rise in acidity of 12%. Projections of rising CO2 emissions indicate that by 2100 ocean acidity could increase 170%. As acidity increases, so does the dissolving of coral reefs, shells of clams and oysters, planktonic larvae, and more.

CHLOROFLUOROCARBONS (CFCs) Chlorofluorocarbons do not occur naturally. They are examples of gases produced solely by humans. CFCs are used as coolants in refrigerators and air conditioners, as foam insulation in buildings, and as solvents, among other purposes.

Chlorofluorocarbons are not only greenhouse gases (table 12.6), but they also aid in the destruction of the ozone in the stratosphere, which helps shield life from damaging ultraviolet (UV) rays. CFCs may remain in the atmosphere for a century, causing so many problems that international treaties have been signed restricting their use.

Glacial Advance and Retreat: Timescale in Thousands of Years As an Ice Age begins, ocean surface water evaporates, and some precipitates on the continents as snow. Snow accumulates, and burial pressure converts it into ice (figure 12.8).

Continental glaciers reach thicknesses of about 3 km. (2 mi), deeply burying the land.

What if eruptions from different volcanoes kept occurring at closely spaced intervals for many years? The Greenland ice cores record many years with high acid content during the Little Ice Age.

Conversely, the record of global warming from 1912 to 1952 has been partly attributed to the absence of major SO2-rich volcanic eruptions.

Solar Energy Variation Solar radiation is not a constant. One type of variation is marked by the sunspot cycle.

Dark areas called sunspots occur on the surface of the Sun; they are regions of intense magnetic activity. The abundance of sunspots varies in an 11-year cycle (figure 12.30) wherein solar radiation varies about 0.15%. • Earth's average temperature warms and cools less than 0.1°C during a cycle. The cycle is too brief for Earth to come to temperature equilibrium with the changes in solar radiation. If the cycles were longer, the temperature could vary about 0.3°C.

DROUGHT AND FAMINE The 21st century is very likely to experience more frequent and longer duration heat waves. When extended heat waves are coupled with decreased rainfall, drought can set in.

Drought does not equal desert. Drought describes times of abnormal dryness in a region when the usual rains do not appear and all life must adjust to the unexpected shortage of water. The lack, or reduction, of moisture can cause agricultural collapse or shortfalls, bringing famine, disease-causing deaths, and mass migrations to wetter areas.

The surface of the Earth is divided into temperature zones of frigid, temperate, and torrid by latitude (figure 12.4).

During a frigid period, an Ice Age, the colder climates of the high latitudes expand in area while the area of warmer climate in the low latitudes shrinks but does not disappear. Conversely, in an era of warmth, a Torrid Age, the globe is marked by expansion of the subtropical climatic zones, while the cold-climate belts shrink back toward the poles.

In the South Pacific Ocean, the shifting of weather patterns is known as the Southern Oscillation; it occurs when the usual low-pressure atmosphere is replaced by high-pressure air, as measured at Darwin on the north coast of Australia (figure 12.21).

For example, in late 1996, strong weather systems in the Indian Ocean migrated into the western Pacific Ocean before the strong El Niño of 1997-98. The combined system is called the El Niño/Southern Oscillation (ENSO).

Toba, Indonesia, About 74,000 Years Ago Tambora erupted an impressive 150 km3 of material, but if we go back 74,000 years, the eruption of Toba on Sumatra expelled about 2,800 km3 (670 mi3) of material. The Toba event is the youngest known resurgent caldera eruption. Scientists estimate that the Toba ash and H2SO4 aerosols formed a dense cloud in the stratosphere lasting for up to six years.

Global cooling may have been 3° to 5°C (5° to 9°F) for several years. A volcanic winter of this magnitude may have triggered additional climate responses that prolonged the cold climate and increased the severe drought, ecological disasters, and famine. Some researchers even speculate that the Toba eruption effects drove down the global population of humans to less than 10,000 people.

Aerosols also affect the volume of clouds, their distribution, and their albedo as measured by cloud albedo in table 12.5. Clouds are a problem for computer modelers trying to forecast 21st-century climate.

Greenhouse gases will continue to warm the atmosphere. Will the warmer air cause clouds to thicken and spread, thus shading Earth's surface with a cooling effect? Or will the warmer air hold more water vapor, with thinner and smaller clouds causing a warming effect? To date, studies of cloud effects suggest that warming will dominate.

How stable is the Greenland ice sheet? This is difficult to assess, and there are several concerns: (1) The rate of melting has increased (figure 12.40). (2) The major glaciers entering the ocean are moving faster. (3) There are more than a thousand large meltwater lakes on top of the ice sheet, and some of them are pouring huge volumes of water down through holes in the ice where it reaches the bedrock below and then flows seaward. Glaciers sitting on top of running water move faster than glaciers sitting on bedrock. (4) Common falls of large ice masses into the ocean reduce support for the toes of glaciers and raise the possibility of large-scale catastrophic collapses. So far, the computer scenarios have only modeled ice melting, not ice masses collapsing.

Greenland is losing ice faster than expected. Is this just a normal short-term cycle? Or have we passed a tipping point (see In Greater Depth) and entered a new regime? We don't know.

Haboobs Overwhelming dust storms (see figure 12.36) are becoming more common. Their name is haboob, which means "strong winds" in Arabic. Haboobs occur in arid regions around the world, including the southwestern United States. For example, Phoenix, Arizona, was overrun by several haboobs in summer 2011, causing reduced visibility, dangerous driving conditions, downed power lines, delayed airplane flights, and some structural damage.

Haboobs range up to 100 km (60 mi) wide and several kilometers high, move 35-100 km/hr (20-60 mph), may arrive with little warning, and last up to three hours. Haboobs form when thunderstorm downbursts or collapse cause air to rush down to the ground and flow outward, pushing a gust front and carrying dust, silt, and sand (figure 12.37).

Equilibrium Between Tectonics, Rock Weathering, and Climate Are plate tectonics and climate related? At first glance this may seem unlikely, but an intriguing hypothesis holds that major climatic changes of the last several hundred million years were caused in good part by variations in rates of CO2 input to the atmosphere and oceans by plate-tectonic processes.

Here is how it works: The amount of CO2 in the atmosphere is a significant control on climate. A major source of CO2 to the atmosphere is volcanism. The greater the volume of magma poured onto the land and ocean floors, the greater the volume of CO2 released to the atmosphere. • But rates of seafloor spreading are not constant; they vary markedly. When spreading is more rapid than now, huge volumes of CO2 are released. • means the rate of subduction must increase, with accompanying greater releases of magma and CO2 through subduction-zone volcanoes. • increase the negative feedback that begins removing CO2 via mountain destruction due to increased chemical weathering of rocks. Notice that CO2 was consumed in the weathering process, thus lessening CO2 in the atmosphere and reducing the greenhouse effect. Multiply this reaction by millions of years, and major climate changes occur.

Late Paleocene "Torrid Age" (cont) • Continental ice sheets apparently did not exist anywhere in the world. Evergreen (coniferous) and warm deciduous forests covered much of the land. Hot deserts and arctic tundra covered smaller percentages of the ground. The world climate lacked extremes.

How did Earth's climate become so dominated by warmth? Several factors apparently combined to turn up the heat: (1) The equatorial zones were largely covered by oceans, allowing more absorption of solar radiation. (2) As oceans warmed, areas covered by snow and ice decreased, thus exposing more land. Snow and ice reflect the Sun's rays; land absorbs heat. (3) Enormous outpourings of lavas from the opening North Atlantic Ocean are likely to have released large volumes of gases to the atmosphere, which may have increased global warming via the greenhouse effect. (4) The oceans changed their style of density differentiation. At present, cold Antarctic and Arctic waters are the densest of all waters; they sink and flow along the ocean deeps. By Paleocene time, the polar water became so much warmer that the heaviest waters may have been tropical waters that had become saltier due to evaporation.

VOLCANISM AND CLIMATE Benjamin Franklin recognized in 1784 that volcanism can affect the weather. He suggested that the haze and cold weather in Europe during 1783-84 were due to the massive outpourings of lava and gas at Laki, Iceland (see chapter 7).

How else can volcanism affect the climate? Large, explosive Plinian eruptions can blast fine ash and gas high enough to be above the normal zone of weather. Free from the cleansing effects of rainfall in the stratosphere, the volcanic products can float about for years and interfere with incoming sunlight.

The Mayan Civilization and Climate Change Weather brings us local events that can cause death and destruction, whereas climate change brings regional or global events that can lead to the fall of civilizations. For example, the Mayan civilization of Mesoamerica made great accomplishments in agriculture, irrigation, social organization, mathematics, and astronomy during a thousand-year period.

However, a centuries-long pattern of decreased rainfall caused problems that intensified during multi-year droughts, especially occurring between the years 800 to 1100 CE. The droughts set off a chain of events that led the Mayans to permanently abandon many urban areas in the southern and central lowlands (figure 12.26), to stop construction of monuments, and to experience the breakdown of social and political order leading to wars. Population in some cities dropped by 90%, and society became increasingly decentralized, forcing many people to return to a life of rural subsistence. The advanced Mayan civilization declined significantly during a string of events triggered by long-term climate change.

How do we decompose plants to release CO2?

In two main ways: burning wood and burning fossil fuels. About 15% of the increased atmospheric CO2 is due to humans burning wood to clear land for agriculture, heat homes, and make charcoal for furnaces. Fossil fuels are coal, oil, and natural gas. They form during transformation of dead plant material in swamps, river deltas, and other organic-rich environments after burial beneath sediments. More than 80% of the energy that powers our global societies is generated by burning fossil fuels, and these energy-producing processes have added about 80% of the excess CO2 now in the atmosphere.

The non-condensing greenhouse gases account for the remaining 25% of the greenhouse effect; they do not condense and precipitate from the atmosphere.

Instead, the non-condensing GHGs are well mixed in the atmosphere and individual molecules may remain up in the atmosphere for centuries; they provide a stable temperature structure that sustains the water vapor and clouds (table 12.6). Without the radiative forcing provided by the non-condensing GHGs, especially CO2, Earth's global climate could cool into an icy state.

Permafrost Permafrost is perennially frozen ground occurring where mean annual temperature is at or below 0°C (32°F). In the Northern Hemisphere, permafrost underlies about 24% of exposed land from depths of several meters to 150 m (500 ft) or more, primarily in Siberia, northern Canada, Alaska, and Greenland.

It is estimated that permafrost contains about 0.022% of the water on Earth. During summer, the ice may melt at and just below the surface, creating an active soil zone where plant growth occurs, such as stunted shrubs, mosses, and lichens. Winter ice entombs the plant material. Over thousands of years, a tremendous reservoir of organic remains has been preserved within permafrost ice.

Where has all the CO2 in the early Earth's atmosphere gone?

It is stored in physical form in several ways, but about 80% of that CO2 is now chemically tied up in limestone (table 12.2). Limestone is rock composed of CaCO3. Most limestone is made from the hard parts of oceanic life, such as shells and reefs. TABLE 12.2 Carbon on the Earth

LA NIÑA (cont) There are hazards associated with the La Niña cooling in the Pacific Ocean. La Niña allows the growth of hurricanes in the Atlantic Ocean, spelling trouble for the eastern United States and Gulf of Mexico coastal areas.

La Niña leads to decreased rainfall in the American Southwest, helping dry out the El Niño-fed vegetation and leading to wildfires.

Climate Variations: Timescale in Hundreds of Years The air temperature over the Greenland ice cap proceeds through significant warm stages followed by colder intervals, as recorded by oxygen-isotope ratios in ice layers (figure 12.16).

Look at the temperature conditions in figure 12.16 since 20,000 years ago: (1) Conditions began to warm; (2) the warming was interrupted by the Older Dryas cold stage; (3) the cold interval was suddenly replaced by the elevated temperatures of the Bølling period; (4) the higher temperatures deteriorated through the Allerød interval; (5) temperatures plunged back into the depths of the Ice Age during the Younger Dryas stage from 12,900 to 11,700 years ago; (6) last came the current interglacial period.

Climatic conditions during the Little Ice Age were far from constant, as smaller-scale warmings and coolings occurred (figure 12.27). One colder interval between 1645 and 1715 CE is known as the

Maunder Minimum During this time, minimal sunspot activity was noted by astronomers, and the Sun may have been 0.25% weaker.

After 20,000 years ago, summer insolation increased in the Northern Hemisphere and started a rapid melting of glaciers with a resulting abrupt rise in sea level (figure 12.15).

Melting of ice in West Antarctica increased significantly about 14,700 years ago, adding significantly to sea-level rise.

Another significant source of methane release is through "mud volcanoes," cone-shaped piles of mud and rock built by methane rising from depth through the ground surface and into the atmosphere.

More than 900 mud volcanoes have been located in 26 countries. Azerbaijan has the most and the largest mud volcanoes, with one standing 700 m (2,300 ft) tall. About 60% of methane is given off by human activities, listed in order of importance: • burning fossil fuels, • growing rice, and maintaining livestock, with • lesser amounts emitted from ○ landfills, ○ burning wood, and ○ rotting animal waste and human sewage.

How much of the 20th century warming was due to natural processes and how much was due to human activities?

Natural processes appear to have caused a net increase in temperature of about 0.2°C. Changes in Earth's orbital patterns caused a cooling of -0.02°C that was offset by a hotter Sun, netting a warming of +0.2°C. • Human activities probably were responsible for the remaining 0.4°C increase in global temperature.

Did it warm continuously through the 20th century?

No, most of the warming occurred during two time intervals: • 1910 to 1944 and • since 1977. It appears that the early warming was largely due to a hotter Sun and a lack of global volcanism. The present warming is likely due to increases in greenhouse gases in the atmosphere.

The 21st Century The largest coordinated effort is the Intergovernmental Panel on Climate Change (IPCC), which delivered its Assessment Report 4 in 2007. The effort involved more than 450 lead authors, 800 contributing authors, and 2,500 expert reviewers representing more than 130 countries. Their consensus marks a milestone in scientific effort and was judged to be so significant that they shared the 2007

Nobel Peace Prize. Some of their main, unanimous conclusions are listed in table 12.7. For the past three decades, the GISS surface temperature record shows an increase of about 0.2°C (0.36°F) per decade. Since 1880, a distinct warming trend has occurred, albeit with leveling-off trends. In total, average global surface temperatures have increased about 0.9°C (1.6°F) since 1880.

The sediment- and ice-core records seem to show that glacial advances and retreats are synchronous in both the

Northern and Southern Hemispheres. How are ice masses around the opposing poles affected simultaneously by astronomical tilts and wobbles? Probably by heat transfer within the world ocean and atmosphere. Any increased heat received in one hemisphere is shared with the other.

ICE MELTING Glacial ice holds 2.15% of the water on Earth. If all glacial ice melted, global sea level would rise about 67 m (220 ft) and flood vast areas of land.

Nothing this drastic will happen in the foreseeable future, but there is concern, about certain ice masses. Mountain glaciers, summer snow cover, and Arctic sea ice are declining in volume. This trend is very likely to continue.

Mount Pinatubo, 1991 After a slumber of 635 years, Mount Pinatubo awoke to disrupt life on the Philippine island of Luzon in the spring of 1991. The 1,745 m (5,724 ft) high summit was blasted to bits and replaced by a 2 km (1.25 mi) wide caldera as up to 5 km3 (1.2 mi3) of dense magma blew out in the form of pyroclastic debris. Despite ample advance warnings, more than 300 people died in pyroclastic flows. Adding to the tremendous destruction of property and life loss was a major storm that poured torrential rains on the loose pyroclastic debris, setting in motion numerous large-volume lahars.

Of climatic importance were the 30 million tons of SO2 gas blasted into the stratosphere (figure 12.25)—triple the volume of SO2 released by El Chichón. The H2SO4 aerosols reflected 2-4% of incoming short-wavelength solar radiation back to space, causing a 20-30% decline in solar radiation directly reaching the ground. Mean global temperatures at the ground surface dropped 0.5°C. The greatest cooling occurred in the midlatitudes of the Northern Hemisphere, including the United States, where temperatures declined by 1°C. The volcanically induced cooling from SO2 in the stratosphere more than offset the greenhouse warming that was expected in 1991-92 due to the CO2 added to the troposphere by humans burning wood, oil, coal, and natural gas.

During about 250,000 years of excess methane in the atmosphere, the Earth experienced its warmest climate of the last 66 million years.

Over time, the methane oxidized to CO2, which was withdrawn and used by life, and then finally global temperatures began to decrease. Earth changed from a Late Paleozoic icehouse to a Late Paleocene hothouse. But climatic change is the way of the world.

OZONE (O3) Ozone is a greenhouse gas in the troposphere, but in the stratosphere it provides negative feedback (table 12.5). It is a gaseous molecule composed of three atoms of oxygen rather than the usual two-atom molecule (O2).

Ozone is also a principal component of the smog that chokes urban atmospheres. Our automobiles and industries emit gases, some of which react with sunlight to produce the ozone that makes our eyes water and our lungs ache. The ozone story is well described by the saying that pollutants are merely resources that are in the wrong place. • Ozone in the stratosphere shields us from killing UV rays, but • ozone in the air we breathe weakens us and shortens our lives.

RADIATIVE FORCING Solar energy is always flowing through the atmosphere, and much is absorbed by the Earth. The warm Earth radiates energy out into cold space. Energy flow in and out of the atmosphere is measured in watts per square meter (W/m2) at the boundary between the troposphere (lower atmosphere) and the stratosphere.

Radiative forcing is a measure of changes in these energy flows. Since the year 1750, many of the changes in the energy balance reflect human-caused changes in greenhouse gas and aerosol concentrations, albedo, cloud cover, and more. A positive forcing warms Earth's climate, and a negative forcing cools it. The biggest change since 1750 has been positive forcing due to increasing CO2 content (table 12.5). The total of this radiative forcing since 1750 is estimated to have increased the effect of incoming solar energy by about 1%.

Temperature All the computer-model scenarios of 21st-century climate are provisional and subject to change. All computer models forecast increasing volumes of greenhouse gases and increasing Earth-surface temperatures. Figure 12.34 shows a geographic pattern of surface warming expected by the end of the 21st century in an IPCC rapid growth of the global economy (A1) model.

Regional changes include the greatest warming occurring over northern lands in high latitudes; this means there will be less area covered by snow and ice; decreased sea ice; increased thawing of frozen ground; disappearance of more mountain glaciers and late summer water runoff; and increased evaporation from farmland. It is very likely that heat waves and droughts will increase in the middle and low latitudes. Wildfire seasons will be longer and fires more frequent. Changes in global precipitation amounts and locations are another concern.

LATE PALEOCENE TORRID AGE The world was warming during Paleocene time (66 to 55 million years ago). There was more heat in the Paleocene oceans and atmosphere than at any time since.

Sea-surface temperatures in the Southern Ocean around Antarctica were 10° to 15°C (18° to 27°F) warmer than today based on measurements of oxygen isotopes (figure 12.6).

At about 7,000 years ago, average global temperatures were warmer, and rainfall totals had risen. At this time, known as the "climatic optimum," even North Africa had enough rainfall to support civilizations.

Since then, there has been a 7,000-year-long lowering of global average temperature totaling about 2°C. However, the cooling trend has had several smaller cycles of glacial expansion and contraction superimposed on it (figure 12.17). It seems that climatic cycles can be found at any timescale we choose to use.

What is a worst-case scenario for volcanic effects on climate? Probably the hypothesis that blames flood-basalt volcanism for the great die-off of species, including non-avian dinosaurs, that occurred 66 million years ago.

Some scientists suggest that the extinctions resulted from the climatic effects of the voluminous flood basalts erupted to form the Deccan Plateau in India. This hypothesis is based on the fact that about 2.6 million km3 (625,000 mi3) of basaltic lavas poured forth in as short a time as 700,000 years. What might the climatic effects have been? Proponents of this hypothesis estimate that atmospheric CO2 increased significantly, with consequent global warming that raised the average world temperature as much as 10°C (18°F). The elevated surface temperatures, more acidic ocean waters (carbonic, sulfuric, and nitric acids), and possibly a depleted ozone layer all combined to deal punishing blows to life on land and in the uppermost layer of the ocean.

A peak of sunspot activity occurred in 2000 during Solar Cycle 23. Regions with concentrated ultraviolet light radiating from the solar atmosphere were obvious (figure 12.31a).

Temperatures within these ultraviolet pulsations were about 1,000 times hotter than the Sun itself. Sunspot activity began declining in early 2002 and in 2009 was exceptionally low (figure 12.31b). Solar Cycle 24 was a weak one, reaching a peak of 82 in 2014.

GLOBAL CLIMATE MODELS What changes will occur in the global climate in the 21st century? Added complications arise because a change in one variable may cause other variables to change in unknown ways. Questions are addressed by constructing global climate models (GCMs) involving complex computer simulations.

The IPCC Assessment Report 4 analysis set up different economic and political scenarios for computers to consider in modeling climate (table 12.8). IPCC Assessment Report 5 (AR5) was released in three parts from late 2013 to 2014. Documentation of ongoing changes shows the atmosphere and ocean warming, snow and ice diminishing, and sea level rising. The latest report especially emphasizes the major climate risks for humans, the greatest reasons for concern.

Looking at figure 12.27 reveals a warm period from about 1000 to 1300 CE referred to as the Medieval Maximum. During this time, northern Europeans emigrated to Iceland, where the almost ice-free coast helped fishers build thriving industries.

The Little Ice Age affected Europe from about 1350 to the mid-1800s CE. It was originally defined by Francois Matthes in 1939 as an "epoch of renewed but moderate glaciation." Late in the Little Ice Age, part of northeastern Canada had accumulated permanent snowfields and the beginning of an ice sheet. Cold winters in Europe led to shorter growing seasons, with reduced crop yields leading to local famine. Mountain glaciers advanced throughout Europe. The fishing industry in Iceland was slowed by many weeks of sea ice each year.

Greenland Greenland is buried beneath 3 million km3 of ice. If all that ice melted, global sea level would rise 7 m (23 ft).

The Northern Hemisphere continental glaciers have retreated from Canada, Scandinavia, and Siberia, but the Greenland continental glacier remains; it is a relic left over from the last major glacial advance.

PACIFIC DECADAL OSCILLATION In recent years, another weather-influencing cycle, the Pacific Decadal Oscillation (PDO), has been recognized by studying sea-surface temperatures in the north Pacific Ocean.

The PDO cycles last 20 to 30 years (figure 12.23) and occur as midlatitude conditions of the Pacific Ocean that have secondary effects on the tropics. Compare this to El Niño, which lasts 6 to 18 months as low-latitude (tropical) conditions of the Pacific Ocean with secondary effects on the midlatitudes. The PDO has a warm phase accompanied by increased storminess and rainfall. Warm phases occurred from 1925 to 1946 and from 1977 to 1998. Cool phases with decreased numbers of storms were in effect from 1890 to 1924 and from 1947 to 1976. The cause of the PDO is not known. But even without a theoretical understanding, PDO data can help climate forecasts for North America because PDOs tend to persist for multiple years.

Satellite monitoring of the Arctic Ocean began in 1979. In some years, sea ice grew in area, and in other years it shrank, but an unmistakable trend developed.

The area covered by sea ice shrinks significantly every decade, by about 12%. September 2012 sea-ice area was 49% less than September 1979. The 10 Septembers from 2005 through 2014 had the 10 smallest sea-ice areas on record.

The Past Thousand Years During the past thousand years, the combined effects of Earth's orbital patterns of eccentricity, tilt, and wobble caused a cooling trend, but climate records show numerous variations, testifying to other processes also at work.

The climate variations seen in figure 12.27 are actively being studied to learn more about (1) the extent of the temperature fluctuations, (2) whether they were regional or occurred simultaneously around the world, and (3) the causes of the changes. Information is being sought by analyzing glacial ice layers for • oxygen isotopes, • air bubbles, and • volcanic debris; and • annual growth rings of corals and trees.

What was Earth like around 20,000 years ago when glacial ice masses were at peak extent?

The continental ice sheets contained about 70 million km3 (17 million mi3) of ice, and the ice masses had spread out to cover about 27% of today's land, including virtually all of Canada and part of the northeastern United States (figure 12.13). Each ice sheet had its own cell of atmospheric high pressure that displaced midlatitude storm systems to the south.

U.S. Dust Bowl, 1930s (cont) The drought began in 1930, a particularly bad time. Only months before, in October 1929, the U.S. stock market had crashed, and the nation's economy began sinking into the Great Depression. By 1931, farmers were becoming desperate. For example, a group of 500 armed farmers went to the town of England in Arkansas to seek food from the Red Cross. They were denied aid, so they went to the town's stores and took the food they needed.

The dust storms became even worse in 1934 and 1936. Some of the blame for the Dust Bowl was heaped onto the farmers for plowing deeply through drought-tolerant native grasses and exposing bare soil to the winds. The plowed lands were sowed with seeds of plants that could not handle drought and thus died, exposing more soil. The farming practices were not the best, but they did not cause the drought; they just accentuated its effects. Evidence showing that droughts are common is found in the archaeological record. For example, droughts in the past probably led to the downfall of the Anasazi civilization of the southwestern United States and the migration of its people to areas with more dependable water supplies.

AEROSOLS Aerosols are microscopic or very small particles of solids and liquids suspended in air. Our understanding of how aerosols affect Earth's climate is much less certain than our knowledge of the effects of greenhouse gases.

The effects of aerosols are complex, and sometimes they seem contradictory. For example, an urban atmosphere of brownish air can block some incoming sunlight, causing cooling, but particles of black soot in that same air absorb heat, causing warming. The scattering of light by aerosols is visible as haze; an example is the spraying of whitish salts into the air by breaking ocean waves. Aerosols mostly reflect sunlight, thus cooling the Earth's atmosphere, as measured by the direct effect in table 12.5.

LATE PALEOZOIC ICE AGE Part 2 2. Another important consideration is ocean-water circulation. No matter how much the Sun's brightness varies, equatorial waters receive more solar energy than polar waters. Without continents present to block ocean water flow, the warm equatorial waters simply circulate latitudinally (east-west) due to the spin of the Earth.

The geologic record shows that Ice Ages are favored when oceanic circulation is more longitudinal (north-south) than latitudinal (east-west). When the continents are aligned in a north-south direction, they block latitudinal circulation of ocean water, thus sending warmer equatorial waters toward the poles, where evaporation can form the clouds that yield the snowfall that builds up on polar landmasses as glaciers. • The continents had a north-south alignment during the Late Paleozoic time, as they do today (see figure 9.28) in the current Ice Age.

Earth 20,000 years ago The displacement of storm systems increased midlatitude rain, turned the desert basins of the southwestern United States into a series of lakes, and produced much heavier rainfall over the Mediterranean region.

The high pressure over Asia kept away its monsoonal rains, thus increasing the aridity of the Indian subcontinent. The amount of seawater required to build the glaciers caused sea level to drop 130 m (425 ft) below the level of today (figure 12.14). But extensive ice masses carry some of the seeds of their own destruction: shrinking ocean surface area plus colder ocean water mean less water evaporation with less snowfall. Cutting down on evaporation reduces the supply of snow necessary to maintain glaciers, and thus they shrink.

Antarctica Antarctica is the fifth largest continent; it supports a continental ice sheet holding 29 million km3 of ice. If all that ice melted, global sea level would rise about 60 m (200 ft).

The ice sheet on the continental interior seems relatively stable, but ice losses are occurring along the coastlines where glaciers flow faster and ice shelves exist. Ice shelves are wide bodies of ice, hundreds of meters thick, flowing off the continent and now partially floating on seawater. An example is the West Antarctic ice sheet, a Texas-size mass of ice containing enough water to raise global sea level 5 m (16 ft). West Antarctica warmed 2.4°C (4.3°F) between 1958 and 2010, making it one of the fastest-warming areas on Earth.

Today, near-surface air temperatures in the high latitudes are warming about twice as fast as the global average (see figure 12.33).

The increasing warmth is being referred to as Arctic amplification of global warming. Warmer summers are thawing more permafrost, thus exposing greater amounts of carbon-rich organic material. As soils defrost, microscopic organisms decompose the ancient organics and release CO2 and methane (CH4) to the atmosphere. Some surface areas are drying out, allowing wildfires to release huge volumes of CO2 to the atmosphere after hundreds of years of storage in ice.

EL NIÑO (cont) The warm surface waters then flow "downhill" toward South and Central America. Some surface currents are reversed as some winds from the west blow surface water to the east.

The reversal places a huge mass of warm water against the Americas (figure 12.20), where it evaporates more readily and produces more clouds. The warm, moist air flowing eastward off the ocean into the Americas commonly leads to heavier rains than the coastal deserts can handle.

How is CO2 removed from the atmosphere?

The scorecard for the latter part of the 20th century shows about 20% is removed by plants during photosynthesis, about 30% dissolves in ocean water, and the remaining 50% stays in the atmosphere and traps radiated heat.

SEA-LEVEL RISE How much will global sea level rise in the 21st century? The IPCC Assessment Report 5 model that continues present trends through the 21st century yields a sea-level rise up to 0.82 m (2.7 ft).

The sea-level rise mostly comes from (1) melting of glaciers, sending water to the seas, and (2) thermal expansion of seawater as warming causes an increase in water volume. Isostatic adjustments also occur where (1) seawater floods the edges of continents causing land to sink and the shoreline to move farther inland, and where (2) glaciers melt and remove their weight, causing the land to rise.

LATE CENOZOIC ICE AGE Beginning from the temperature peak at 55 million years ago, Earth began the long-term cooling trend that has carried us into our current Ice Age (see figure 12.6).

The sequence of events included: • After 55 million years ago, intervals of high temperature still occurred, but the torrid climate had begun a long-term cooling trend. • At 40 million years ago, Antarctica was surrounded by cold water. • At 34 million years ago, glaciers were widespread in Antarctica. • At 14 million years ago, a continental ice sheet existed on Antarctica, and mountain glaciers were in the Northern Hemisphere. • At 5 million years ago, the Antarctic ice sheet had expanded. • At 2.7 million years ago, continental ice sheets existed in the Northern Hemisphere.

Tambora, 1815 In the early 1800s, Mount Tambora, on the island of Sumbawa in Indonesia, stood 4,000 m (13,000 ft) tall. After the explosive eruptions of 10-11 April 1815, Tambora was only 2,650 m (8,700 ft) high and had a caldera 7 km (4+ mi) wide and l.1 km (0.7 mi) deep. About 150 km3 (36 mi3) of rock and magma were blasted out during the eruption, producing 175 km3 of ash and other pyroclastic debris. The eruption has been called the greatest in historic times, killing about 10,000 people outright by pyroclastic flows and another 117,000 indirectly through famine and disease.

The volcanic ash and aerosols, especially sulfur dioxide (SO2), blown into the stratosphere blocked enough sunshine to make 1816 known as "the year without a summer." Agricultural production was down throughout the world as global temperatures lowered another 0.3°C during an already cold series of years. On the other side of the world in the northeastern United States, snow or frost occurred in every month of the year. An estimated 8% of the residents of Vermont were forced to leave the state due to agricultural failures. In India, the Tambora cooled climate induced famine and weakened the population, possibly triggering the cholera epidemic that soon followed. The disease then slowly migrated around the world, killing people who lived under the harshest, least sanitary conditions.

EL NIÑO (cont) Every two to seven years, the typical ocean-atmosphere pattern breaks down for about 6 to 18 months. The trade winds weaken, the atmospheric low pressure over Indonesia moves out over the central Pacific Ocean, and winds blow into the Pacific basin from the west (figure 12.19).

The warm surface waters then flow "downhill" toward South and Central America. Some surface currents are reversed as some winds from the west blow surface water to the east.

WATER VAPOR Water vapor is Earth's most abundant greenhouse gas. Water vapor is a vast, natural control on temperature and climate. As the atmosphere warms, more water vapor can be absorbed into the warmer air in a positive feedback cycle that leads to ever greater warmth.

The warmer the air, the greater the percentage of water vapor it can hold, and this includes the warmer air resulting from the CO2 and CH4 put into the atmosphere by human activities. • Water is a condensing greenhouse gas (GHG); it responds quickly to changes in air temperature and pressure by condensing and precipitating, or evaporating. • Although water vapor and clouds are highly variable, they provide about 75% of Earth's greenhouse effect.

Extreme ocean conditions make weather prediction easier. However, there are also times when the tropical Pacific Ocean is neither excessively warm nor markedly cool but neutral.

The weaker signal from the ocean makes weather more difficult to predict. NASA oceanographer William Patzert has suggested the term La Nada for this neutral condition.

Research conducted in 2010 documented that mega-floods flowed around 13,000 years ago through northwest Canada in the Mackenzie River into the Arctic Ocean.

These huge pulses of cold freshwater in the Arctic would ultimately have flowed into the North Atlantic Ocean. Megafloods also flowed down the Saint Lawrence River into the North Atlantic. These cold surface-water layers could alter the ocean-circulation pattern shown in figure 9.29 by stopping Arctic seawater from sinking and by blocking the northward inflow of Gulf Stream warm water.

How the Greenhouse Effects Works Due to the thinning of the ozone by both natural and unnatural forces, an increase in high intensity solar radiation now penetrates into the lower layers of the atmosphere. This penetration contributes to the melting of continental ice sheets, an event taking place at the same time the Earth is cycling back into the Ice Age. Modern technology has created a "manmade" Greenhouse Effect that has contributed to an increase in the volume of liquid/water at both the North and South Poles, within the oceans, and on the continents.

This addition of warm water is adding to unstable planetary imbalances, which is the cause of melting ice at the poles today. In essence, human technology is pushing the Earth into a corner. The two principal constituents in the atmosphere that absorb infrared are water vapor and carbon dioxide. For several hundred years, the concentration of carbon dioxide levels has been gradually increasing both from natural events, such as volcanic eruptions and wildfires, and from the increase in manmade technology and pollution.

Recent analyses of carbon isotopes in Late Paleocene sedimentary rocks suggest that a major release of methane occurred about 55 million years ago during a 10,000-year-long interval.

This is a very short time for the atmosphere to receive such a large volume of a powerful greenhouse gas (methane has a 21-times-stronger capacity to trap heat than carbon dioxide).

But when Arctic surface water is fresh and cold, its low density prevents sinking, and its cold temperature results in colder air temperatures.

This is apparently what happened during the Younger Dryas (figure 12.16). Glacial meltwater floods from 12,900 to 11,700 years ago put cold freshwater on top of the North Atlantic Ocean, shutting down the circulation system of figure 9.29. It took centuries for solar energy to return the ocean surface to its warmer, saltier condition and the present circulation system. Remember that sea level was 130 m (425 ft) lower at the peak of continental glaciation and that the removed water was stored on land as glacial ice. When the glaciers retreated, sea level rose by the inflow of cold freshwater freed by the melting of glacial ice. The return of this massive volume of meltwater affected the oceanic distribution of heat; this is a climate-modifying process. The last major melting of the ice sheets is recorded by the sea-level rise curve (see figure 12.15).

A Cold Decade: 1810-1819 In late 2009, analyses of ice cores from Greenland and Antarctica identified unique sulfur isotopes and ash in the year 1809 ice layers; they came from the same volcanic eruption.

To spread volcanic debris over the whole Earth, the eruption must have been huge and at a near-tropical latitude. Which volcano erupted? We don't know yet, but the search is on. The 1809 eruption from the mystery volcano would have made the years 1810 and 1811 significantly colder than average. Then the eruption of Tambora in 1815 made the years 1815-1817 much colder. The decade from 1810 to 1819 must have been a cold one, with reduced agricultural production and increased human suffering.

EL NIÑO The high heat capacity of water gives it the ability to absorb and store tremendous volumes of heat upon warming and to release copious quantities of heat upon cooling. An example of ocean—atmosphere coupling is the phenomenon commonly marked in South America by the arrival of warm ocean water to Peru and Ecuador near Christmastime. This phenomenon is known as El Niño (Spanish for "the child").

Typical conditions in the central Pacific Ocean find high atmospheric pressure over the eastern Pacific, resulting in trade winds that blow toward the equator from the north and south. The trade winds push Pacific Ocean surface waters to the west within the equatorial zone, where they absorb solar energy (figure 12.18a). The winds push so hard that sea level is not level; it is about 1.5 ft higher on the western side of the ocean. The warm water piled up on the west side forms a pool of heated water that evaporates readily, helping produce heavy rainfalls for the tropical jungles of Indonesia and Southeast Asia and providing the environment for the Great Barrier Reef of Australia. Meanwhile, on the eastern side of the Pacific Ocean, the warm surface water blown west is replaced by cold waters rising from depth (figure 12.18b) and from the polar regions. The colder waters along the coast evaporate less readily, and thus deserts are common along the coasts of Ecuador, Peru, Baja California, and California because of the shortage of cloud-producing water vapor.

Climate History of Earth: Timescale in Millions of Years Many sedimentary rocks contain information about the climate at the time they formed.

Warm climates are indicated by (1) fossil reefs and most limestones; (2) aluminum ores, which form only in tropical soils; and (3) beds of salts that crystallize when water bodies evaporate under high-temperature, arid climates. Cold climates may be marked by the powerful erosion of glaciers that sculpt the landscape (figure 12.2), leaving polished and grooved surfaces beneath them (figure 12.3) and dumping massive piles of debris.

Oxygen Isotopes And Temperature Ancient temperatures can be determined from the ratio of stable isotopes of oxygen in the calcium carbonate (CaCO3) shells (fossils) of single-celled sea life. An atom of oxygen may have either 16, 17, or 18 protons and neutrons in its nucleus.

Water evaporated from oceans removes more of the lighter common oxygen (16O) and less of the heavier 18O. This 16O-enriched water is locked up on land as ice and snow, leaving the ocean with 18O-enriched water. Shells constructed from seawater incorporate the 18O/16O ratio of the seawater during their lifetime within their CaCO3 shell walls. Thus, measurement of the 18O/16O ratio in shells acts as a paleothermometer, which is used to estimate the temperatures of ancient seas. • Heavier seawater (18O-enriched) corresponds to glacial buildups and • lighter seawater (18O-depleted) corresponds to warmer climates.

Global heat supply has a profound effect on water, which exists on Earth's surface at the transition between its three phases of solid (ice), liquid, and vapor (gas).

Water has such a tremendous capacity to either absorb or release heat that it acts as a powerful control on global climate.

The presence of atmospheric gases, such as CO2, water vapor, and methane, creates a greenhouse effect whereby incoming, short wavelength solar radiation

passes through the atmosphere, but heat radiated by the Earth is in longer wavelengths that are unable to pass back through the atmosphere. Thus, Earth's surface temperature rises with the increasing abundance of greenhouse gases.

A 0.82 m (2.7 ft) sea-level rise will affect tens of millions of people around the world. But it could be worse. The IPCC Assessment Report 4 also warned of potentially large sea level rises in a sobering analysis:

a global average temperature increase to 1-4°C (relative to 1990-2000), causing a contribution to sea-level rise of 4-6 m or more." A sea-level rise of 4-6 m (13-20 ft) would cause major problems worldwide for coastal cities. It seems likely that a 1°C temperature rise will be reached about mid-21st century, and a 3°C increase may be here by the end of the century. If these temperatures are reached, the sea-level rise will not be immediate because of lag times for full response. Nonetheless, it will be too late to reverse; the world will be committed to flooding huge areas of low-lying land upon which hundreds of millions of people now live (figure 12.39).

The Greenhouse Effect plays one of the most important roles in polar shifts. There is no question that human activities have

boosted the amount of carbon dioxide in the atmosphere by more than one-quarter the normal level, and this contribution has interfered with the natural Greenhouse Effect.

However, humans are now changing the CO2 concentration in the atmosphere by

burning tremendous volumes of plants, both living (trees and shrubs) and dead (the fossil fuels of coal, oil, and natural gas). Combining the C in plants with O2 via fire returns large amounts of CO2 to the atmosphere (figure 12.1). • About 6 gigatons (1 gigaton equals 10^9 metric tons) are returned to the atmosphere each year by burning fossil fuels; about 5,000 gigatons remain to be burned.

The record of glacially deposited sediments tells of numerous glacial advances and retreats. Starting in the 1970s, our knowledge of the advance-retreat history has been leaping ahead, thanks to

cores of sediments taken from the ocean floor and cores of ice removed from the Greenland and Antarctic continental glaciers. Each core holds the cumulative record of the annual deposits of sediment or snow (ice) that may be read like the pages of a history book using techniques such as the ratios of oxygen isotopes.

All these climate-modification plans bring with them the danger of unintended consequences. It is possible that even the best intentions and the best engineering plans might

create bigger problems than they solve. Unpredictable responses could include changes in ocean currents, drying up the tropics, acidifying the oceans, and disturbing plant and animal ecology.

Lag Times The human race is provoking Earth's climate system by pouring immense volumes of greenhouse gases into the atmosphere. Changes in climate are occurring slowly, but the full effects of gases emitted today will not be felt for

decades or centuries. The atmosphere is warming, but slowly because much of the greenhouse-trapped heat is being absorbed by the oceans with their tremendous capacity for energy storage. IPCC AR4 estimates that the greenhouse gases we have already emitted will continue to warm the oceans throughout the 21st century, by about 0.6°C (1.1°F). The climate system contains significant lag times.

Our knowledge about warm climates of the past is deduced from evidence in

fossil reefs, tropical soils, evaporite mineral bodies, and widespread fossils of tropical and subtropical organisms. Our understanding of ancient cold climates is based on features such as glacially deposited debris, ice-polished and grooved rock surfaces, and wide distribution of the fossils of cold-water organisms.

An important factor in continental glacier formation is the amount of solar radiation received at high latitudes on Earth each summer. During a warm summer, all the snowfall from the previous winter can melt. But

if the winter snowfall of one year persists until the next winter snowfall begins, then glaciers can start to form. And when glaciers grow for thousands of years, continents can become buried by ice.

Earth's climate depends on the balance between

incoming and outgoing heat. At any given time, the atmosphere-ocean-continent system may be gaining or losing in its overall heat budget.

Possible causes of the Little Ice Age include

increased volcanic eruptions, changed ocean circulation, and reduced solar energy. One hypothesis now being tested suggests that low-latitude volcanic eruptions in the late 1200s put aerosols in the atmosphere that reduced the amount of solar energy reaching the surface; this caused climate cooling that allowed glaciers and sea ice to grow. • The sea ice cooled North Atlantic seawater, thus reducing northward transport of heat in seawater flowing to North America and Europe; this cooled the climate. Direct effects of volcanism last only years, but the effects on ocean circulation of heat last decades.

The immense volumes of ice deform

internally under their own weight and slowly flow out over the countryside like mega-bulldozers, scarring and reshaping the land (figure 12.9).

Notice the range of pre-1900 temperatures in figure 12.27; they rise and fall 1.2°C. In figure 12.28, the pre-1900 temperature range is only 0.4°C. The difference between 1.2°C and 0.4°C is significant because

it is a natural range of temperatures; it sets a baseline for assessing the human-caused temperature changes of the 20th and 21st centuries. At the beginning of the 20th century, both curves show sharp increases in temperatures that rise above the 1,000-year high. Much of this marked increase in temperature is climate change due to human activities.

Some pollutants, such as sulfur dioxide and nitrogen oxide, stay in the atmosphere for only a few hours or a few days at the longest. This means that sulfur dioxide emitted into the atmosphere quickly disappears. But

levels of CO2 and other greenhouse gasses aren't like this; they remain in the atmosphere; some for hundreds of years.

What was responsible for the final increase in warmth? The warming of ocean bottom waters about 8°C (14°F) probably caused

melting of icy methane hydrates on the seafloor, thus releasing methane gas to the atmosphere. What are methane hydrates? Bacteria living on the deep ocean floor release methane (CH4) as part of their life process, but the overlying water is so cold and the pressure from the weight of the overlying water is so great that the methane is locked up inside linked, near-freezing water molecules to form an ice like deposit (figure 12.7).

Industrialized nations have extracted and burned so much fossil fuel over the past 150 years that the Earth's environment is now dangerously unstable. Since the start of the Industrial Revolution, the atmospheric concentration of carbon dioxide has increased

nearly 30 percent, nitrous oxide concentrations have increased 15 percent, and methane has more than doubled.

OCEAN CHANGES Much of the increasing global warmth is being held in seawater. Oceans store

nine times as much of the Sun's heat as do the atmosphere and land combined. This heat influences weather, but many other changes are occurring within the oceans.

Methane hydrate holds more energy than all of Earth's

oil, coal, and natural gas combined. It becomes unstable if temperature rises a few degrees above freezing or if pressure is less than that of 500 m (1,640 ft) of overlying ocean. Today, about 15 trillion tons exist on the seafloor; if it were to melt, the world would see a sharp greenhouse increase in temperature.

The current concentration levels of carbon dioxide in the atmosphere are approximately .03 percent or 300 parts per million (ppm), which is actually a small amount. This carbon dioxide naturally combines with traces of water vapor to

regulate the temperature on the planet. But don't let these numbers fool you; just because the amount of carbon dioxide in the atmosphere appears small, the influence that human activities have had on this balance is proportionately large. Ultimately, humans are upsetting the Earth's temperature balance by creating too much warmth through unnatural temperature increases.

Atmospheric carbon dioxide is a key "greenhouse gas", but it is not the sole cause of global warming or recent Earth changes. Actually, the opposite is most likely true;

rising global temperatures naturally increase the levels of carbon dioxide, not the other way around[i]. Today, the shifting of the Earth's axis is causing a change in both global temperatures and carbon dioxide levels.

The distribution of fossil organisms tells much about ancient climates. For example, when fossil shells of organisms that live only in polar seas also are found in abundance in

rocks formed in midlatitudes, it suggests that the world climate must have been colder at that time. The rocks and fossils tell of extreme variations and changes in world temperature and precipitation throughout geologic time. Not only do warm and cold intervals come and go, but they do not necessarily correlate with wet and dry periods, nor is there a pattern to the arrivals and departures of various climates.

Famine is the slowest-moving of all disasters. Earthquakes, volcanic eruptions, tornadoes, and the like all hit suddenly and with great force and then quickly are gone. But famine is

slow. First, the expected rains do not arrive, and then vegetation begins to wither, food supplies shrink, and finally famine sets in. • Unlike other natural disasters, drought tends to drive people apart rather than bring them closer together. The shortages of food and water lead to conflicts as people, communities, and governments battle each other for the means to survive.

The finest volcanic ash (0.001 mm) can stay suspended for years. Most gases blown into the stratosphere disappear into space, but

sulfur dioxide (SO2) picks up oxygen and water to form an aerosol of sulfuric acid (H2SO4) that may stay aloft for years (table 12.4). The combined ash and sulfuric acid produce haze, reducing the amount of sunshine that reaches the troposphere and the ground surface; thus, climatic cooling results. TABLE 12.4 Volcanic Eruptions into Stratosphere Estimated Loading of SO4 in Stratosphere (10,000,000 metric tons)

It is important to note that humans do not cause the global Greenhouse Effect - global warming;

the Earth is the cause of global warming. Humans simply get in the way by adding to it.

The human contribution is small compared to the natural fluxes between

the atmosphere and ocean, and between the atmosphere and continents, each of which exchange in excess of 100 gigatons annually. • Although human changes in CO2 and other gases are relatively small, they can be enough to trigger climate shifts that cause major problems.

Verification of the importance of orbit and rotation cycles came in the 1980s, when computer analyses of data from sediment and ice cores were shown

to match the theoretical astronomical framework erected by the Serbian astronomer Milutin Milankovitch in the 1920s and 1930s. Milankovitch defined astronomical changes in Earth's orbit, tilt, and wobble and how they affect the amount of solar radiation received by Earth.

A positive feedback system is operating in which warming air leads to

warmer seawater, and warming land thaws more permafrost, releasing more greenhouse gases to the atmosphere which warms the air; and the cycle continues.

Greenhouse Gases and Aerosols The major greenhouse gases are

water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), and human-made chlorofluorocarbons (CFCs). These gases absorb energy radiated from the Earth and "trap" it in the lower atmosphere (see figures 9.4 and 9.11). The warming caused by increasing concentrations of gases such as CO2 and CH4 is well understood (table 12.6). Less well known are the size of the feedbacks from increasing water vapor and aerosols. 12.6 Relative Percentage Responsible for Greenhouse Warming & Ability to Trap Heat (compared to CO2 = 1)

Plate tectonics keeps supplying CO2 (source) to the atmosphere, and

weathering of uplifted rocks keeps removing CO2 (sink), thus tending toward a quasi-equilibrium that helps control climate warming and cooling over millions of years.

The high-level atmospheric winds are also affected by reversal of flow direction during an El Niño. The change brings negative and positive results for the United States. For example, in the 1997-98 El Niño,

winds flowing eastward caused heavy rains and floods in California and brought higher rainfall with some tornadoes to the southeastern United States; on the other side, El Niño winds helped break apart Atlantic and Caribbean storms, resulting in fewer hurricanes. Meanwhile, the midwestern and northern states had a warmer winter with reduced damages to crops, increased economic productivity, and a below-average number of deaths. The 1997-98 El Niño brought more economic gains than losses as well as fewer fatalities to the United States as a whole (table 12.3).

The emerging story for the past 1 million years is of worldwide glacial advances that last about 100,000 years followed by retreats that take place more rapidly

withdrawing over periods from decades to a few thousand years (figure 12.10). What causes the cycles of slow buildup and advance of glaciers followed by rapid shrinkage and retreat? The answer lies in the cyclic peculiarities of Earth's spin and its orbit around the Sun, each of which affects the amount of solar energy received by Earth.

CARBON DIOXIDE (CO2) About 60% of greenhouse warming caused by humans comes from releasing CO2 into the atmosphere (table 12.6). How does carbon cycle through Earth's surface environments? The element carbon is a major building block of life on Earth.

• CO2 is removed from the atmosphere by plants during photosynthesis to build their tissue. Upon death, much of the organic tissue is oxidized, and CO2 both returns to the atmosphere and dissolves in water. • Looking for a place to monitor atmospheric CO2 without much influence from plants or industry, Charles D. Keeling installed instruments high up on Mauna Loa in Hawaii in 1958 when CO2 was 315 ppm. By the end of 2014 it had increased to 400 ppm (figure 12.32). The increase since 1800 is greater than 40%, and 70% of that increase has occurred since 1958. CO2 concentration at 400 ppm is the highest value in the past 800,000 years, according to analyses of air bubbles in Antarctic ice.

The 20th Century When viewed in terms of the past 1,100 years, the 20th-century warming trend was unprecedented in both amount and rate (figure 12.29). What processes of the 20th century caused this dramatic increase in global temperature?

• Changes in Earth's orbit and plate-tectonic processes are both too slow to have affected climate significantly in one century. • Volcanic eruptions and La Niñas cool global climate, and El Niños warm it, but only for a year or two each time; they do not dominate the century. The processes that can change significantly and last for a long time are changes in • solar-energy output and • increased contents of greenhouse gases and aerosols in the atmosphere. Figure 12.28 Trends in average annual temperatures in the Northern Hemisphere during the last 1,000 years. The whole pattern has been called a "hockey stick."

Shorter-Term Climate Changes: Timescale in Multiple Years Several processes change climate on timescales of 1 to 20 to 30 years. Let's look at examples.

• EL NIÑO • LA NIÑA • PACIFIC DECADAL OSCILLATION • VOLCANISM AND CLIMATE

Climate change occurs at nearly every historic and geologic timescale examined. Many processes affect climate, each with its own operating principles and timetable. Climate changes as:

• Ocean basins open and continents drift • Earth's orbit around the Sun changes • Volcanism pumps ash and gas up into the stratosphere • The Sun burns hotter or colder • Global sea level rises or falls • Humans burn huge volumes of wood, oil, gas, and coal

Why has Earth's atmosphere changed? The changes have been caused in large part by life processes.

• Plants remove CO2 from the atmosphere via photosynthesis and respire O2 as a by-product that has built up O2 in the atmosphere. But the total amount of CO2 locked up in plants, dead or alive, is small compared to the amounts originally in the atmosphere.

MANAGING INCOMING SOLAR RADIATION Slowing down global warming by reducing solar radiation into Earth's climate is achievable within months. But beware—there is uncertainty about unintended effects caused by changes we initiate:

• Sulfates in the stratosphere. We could imitate volcanoes, such as El Chichón in 1982 and Mount Pinatubo in 1991. • Cloud brightening. Sea salts could be sprayed into clouds to brighten them and increase their albedo. The effects would be real, but local.

The El Niño of 1982-83 was especially strong. On the eastern side of the Pacific Ocean, the cold-water fisheries off Peru and Ecuador collapsed as ocean water warmed as much as 8°C (14°F) above average. The warm water led to greater than average evaporation, which fed torrential rainfalls. The rains caused overwhelming floods and mass movements from steep hillsides that killed 600 people in Peru and Ecuador and severely punished the economies of those nations.

• The warm coastal waters also promoted heavy rainstorms in the western United States. For example, California suffered $300 million in damages, 10,000 people were evacuated due to flooding and landsliding, and 12 people were killed. Meanwhile, out in the ocean, the tropical rain belt shifted to the central Pacific Ocean, helping form hurricanes that hit Hawaii and Tahiti. • On the western side of the Pacific Ocean, Australia and Indonesia were covered by high-pressure air and below-average rainfalls. Australia suffered its worst drought of the century, and out-of-control bushfires whipped by high winds killed 75 people and many domestic and wild animals and caused $2.5 billion in damages (see chapter 14).

Late Paleocene "Torrid Age" (cont) -about 55 million years ago -the warming was in part likely due to the equatorial zones largely covered by oceans, which absorbs solar radiation better than land & thus lowered Earth's albedo

• There was less difference in temperature between tropical and polar waters; an absence of cold, dense, sinking water at the poles; and less difference in temperature between surface and deep-ocean waters. This means that the pull of gravity was less effective and ocean circulation probably was more sluggish. • Temperature differences in the atmosphere also decreased, resulting in more peaceful weather worldwide. There was an absence of strong seasons, weather was more constant, and rainfall was more evenly distributed throughout the year. Most of the world was wetter and warmer. • The conterminous United States was covered by either tropical or subtropical climates. Along the coastal zones, subtropical conditions existed above the Arctic Circle, as evidenced by fossil crocodiles and palm trees found there.

Early Earth Climate — An Intense Greenhouse Billions of years ago, Earth's climate was dramatically different from today. The climatic regime of the early Earth, the third planet from the Sun, can be appreciated by looking at the atmospheric compositions of the inner planets (table 12.1). TABLE 12.1 Atmospheres of the Inner Planets

• Venus is the second planet from the Sun and thus receives intense solar radiation. With surface temperatures of about 477°C (890°F). • Mars is the fourth planet from the Sun, and its greater distance causes it to receive much less solar energy. The thin Martian atmosphere is also relatively rich in carbon dioxide (CO2), helping hold the heat it does receive and maximizing its surface temperature, although its temperatures of about -53°C (-63°F) are still cold. • Earth's atmosphere has undergone a radical change from being CO2-rich to CO2-poor. Early Earth CO2 - 98% | Earth Today : .04%

U.S. Dust Bowl, 1930s Might the worst conditions of the recent past serve as a prologue to the future? One of the greatest weather disasters in U.S. history occurred during the 1930s, when several years of drought turned grain-growing areas in the center of the nation into the "Dust Bowl." Failed crops and malnutrition caused abandonment of thousands of farms and the broad-scale migration of displaced people, mostly to California and other western states. This human drama was captured in many articles and books, including The Grapes of Wrath by John Steinbeck.

• What happened to cause the drought? Recurrent large scale meanders in the upper-air flow created ridges of high pressure with clockwise flows resulting in descending air (see figure 10.43). The upper-level high-pressure air was already dry, but as it sank, it became warmer, thus reaching the ground hot, dry, and thirsty. As the winds blew across the ground surface, they sucked up moisture, killing plants and exposing bare soil to erosion. Wind-blown clouds of dust built into towering masses of turbulent air and dust called rollers (figure 12.36). When they rolled across an area, the Sun was darkened, and dust invaded every possible opening on a human body and came through every crack in a home. Figure 12.36 A dust storm (a haboob) rolls into Stratford, Texas, in 1935.

LEARNING OUTCOMES Climate change has been occurring throughout Earth's history. Today there is the added concern that human beings are accelerating climate change. After studying this chapter, you should:

• know the conditions needed for Earth to experience an Ice Age. • be able to explain why continental ice sheets advance and retreat during an Ice Age. • know the conditions needed for a greenhouse to build and warm Earth into a Torrid Age. • understand the multiyear climate changes of El Niño and La Niña and their global effects. • be familiar with how volcanism can change the climate. • recognize the striking increase in global warming during the past century. • be familiar with the greenhouse gases in the atmosphere and know how they are being increased. • understand the concepts of tipping points and lag times for 21st-century climate change.

20TH-CENTURY GREENHOUSE GAS INCREASES Why did we humans release such great volumes of greenhouse gases in the 20th century? The gases were a by-product of many well-intentioned activities, such as

• providing energy for industries, homes, and personal automobiles; • growing rice; and raising livestock for human consumption. • Another significant factor in the increase of greenhouse gases was the 20th-century growth of the human population; it doubled twice, from 1.5 billion in 1900 to 3 billion in 1960, and then to 6 billion in 1999. Even conservative estimates for 21st-century population growth forecast another doubling to 12 billion people.

FAST-ACTION STRATEGIES Attention on global warming has focused on CO2, which makes up about 75% of greenhouse-gas emissions. Slowing CO2 emissions requires basing our global energy systems on new technologies instead of fossil fuels. These changes are both politically and technologically difficult; they will take a long time to accomplish. But the world can gain some time by reducing emissions of four pollutants that are potent warmers of climate but have short residence times in the atmosphere:

∙ Black carbon. Black carbon is a component of soot; it absorbs solar radiation. Black carbon is produced by incomplete combustion of diesel fuels and biofuels such as wood. ∙ Ozone in the troposphere. Ozone is a component of smog; it absorbs solar radiation. ∙ Methane. Methane is a potent greenhouse gas (see table 12.6). Reduction of human-caused emissions from landfills, farming, and coal mining is necessary. ∙ Hydrofluorocarbons (HFCs). Hydrofluorocarbons are widely used refrigerants. Leakage of HFCs to the atmosphere is significant, and they are greenhouse gases about 1,400 times more potent than CO2.

CONTROLLING CO2 CONTENT OF ATMOSPHERE The CO2 concentration in the atmosphere can be managed by reducing emissions and/or by removal. Following are some proposed methods:

∙ Changes in energy-usage technologies. There is a tremendous need for carbon-free energy technologies. ∙ Cap-and-trade. A cap-and-trade plan to limit emissions of CO2 has been set in action as the European Union emissions-trading system. It is an economy wide, market-based system that places CO2 emission allowances on companies. ∙ Air scrubbing. Machines and methods could be developed to remove (scrub) CO2 from the atmosphere. This is technologically possible, but very expensive. ∙ Ocean fertilization. The ocean could be fertilized to stimulate massive blooms of photosynthetic algae, which would draw down atmospheric CO2 as part of their life processes. ∙ Rock weathering. Mountain building, with its uplift and fracturing of rocks, triggers accelerated weathering of the rocks. This chemical decomposition process works by drawing down atmospheric CO2 to make carbonic acid, which does the rock weathering.