Ch 3: Climate

medieval warm period

A warm period in the Northern Hemisphere during the 10th and 11th centuries

IPCC statement

As of 2011, this group has released four comprehensive reports, and has concluded, "Most of the observed increase in global average temperature since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations."

Evidence of Climate Change

CO2 concentrations over the last 400,000+ years graph. for 650,000 years, atmospheric CO2 has never been above this line... until now. If we look even further back in time, over the last half million years, we see a similar story. (see Figure Evidence of Climate Change) The current concentration of CO2 in the earth's atmosphere is higher than at any time in the past half million years. Where is this abundance of CO2 coming from? Which reservoirs are being depleted of their CO2 while the atmosphere takes on more? The answer lies in the burning of fossil fuels and in the deforestation of significant chunks of the earth's forest biomes. Notice the spike in CO2 concentrations beginning around 1750. This time period marks the beginning of the industrial revolution, when fossil fuels overtook wood as the primary energy source on our planet. Over the subsequent two and a half centuries, oil, coal, and natural gas have been extracted from their underground reservoirs and burned to generate electricity and power modern forms of transportation. The exhaust from this process is currently adding 30 billions of tons, or gigatons (Gt), of carbon dioxide to the atmosphere each year. Combine this addition of CO2, a known greenhouse gas, to the subtraction of one of the sinks of CO2 through deforestation and the imbalance grows even further.

CO2 Emissions for the United States and China

CO2 emissions in millions of metric tons graphed against time for the United States and China. Over the past few years, China has surpassed the United States to become the nation that emits more greenhouse gasses than any other (see Figure CO2 Emissions for the United States and China). Currently, China is responsible for just over 25% of global CO2 emissions, which are approximately 30 Gt per year, with the United States in a close second place. It is important to consider population when reviewing these numbers because there are over four times as many people living in China than in the United States. When you compare these two countries on a per capita basis, the average U.S. citizen emits approximately 19 metric tons of CO2 per year while the average Chinese citizen emits approximately five metric tons. In 2009, the United States consumed more than double the amount oil than the second largest consumer, China, according to the U.S. Energy Information Administration. Topping the list in per capita CO2 emissions is the oil rich nation of Qatar. This small country located on the Persian Gulf has the largest per capita production of oil and natural gas. It also has the world's highest gross domestic product (GDP) per capita. An average citizen in this country emits nearly 60 metric tons of CO2 into the atmosphere each year.

Global Surface Temperature Comparisons

Comparison of regional and global scale surface temperature 1900-2000. Pink shading indicates the model predicted surface temperature using only natural forcings. The blue indicates model predicted surface temperature using natural and anthropogenic forcings. The black lines represent the actual observations.

the trees absorb CO2

Explain how deforestation can lead to both a warming effect and cooling effect for global temperatures.

-a measure of the amount of solar radiation falling on a surface -this difference in energy explains why the equator has a hot climate and the poles have a cold climate -differences in insolation explain the existence of seasons -Note that the same amount of sunlight is spread out over twice the area when it strikes the surface at a 30-degree angle.

Explain why insolation controls the climate

desertification

Finally, as the planet has adjusted to warmer temperatures the proliferation of drought conditions in some regions has dramatically affected human populations. The Sahel, for example, is a border region between the Sahara Desert in the north of Africa and the tropical rainforests that occupy the central part of the continent. (see Figure The Sahel in Africa) This region is experiencing ___________ as the Sahara steadily expands southward. Since the 1970s, the amount of precipitation in this region has been steadily below normal. The combination of over irrigation and recent climate change has made the region uninhabitable and forced millions to relocate.

photosynthesis, New Moore

Further dramatic changes brought on by recent warming have been observed by scientists concerned with the world's oceans. Observations of the world's coral reefs have revealed an alarming rate of coral bleaching (which is not caused by chlorine). As the oceans attempt to uptake the abundance of CO2 and absorb nearly 80% of the heat added to the earth-atmosphere system from the enhanced greenhouse effect, the waters will inevitably warm. As these waters have warmed over the past 40 years, the delicate ecological balance within some of the world's coral reefs has been upset leading to coral bleaching. Under warmer waters the rate at which the algae, which is an important part of the coral ecosystem, undergoes _______________ is too much for the coral to manage. As a result, the coral rids itself of the algae, which leads to an exposure of the white skeleton of the coral. Another consequence of warming oceans is an increase in sea level. Since 1880, sea level has risen 20 cm (8 inches). The rise in sea level is associated both with an increase in glacial melt water and in the thermal expansion of the seawater. An interesting consequence of this rise in sea level has been the disappearance of the long-disputed ___ _____ Island between Bangladesh and India. Both countries laid claim to the shallow, uninhabited island due to the speculation that oil reserves may lie beneath it, but in 2010, the sea swallowed it. Scientists at the School of Oceanographic Studies at Jadavpur University, Kolkatta, India suggest global warming played an important part.

Northern Ice Sheet

Glacial coverage (light blue) of the northern hemisphere during the ice ages.

Winter and Summer Precipitation Anomalies

Global temperature and precipitation projections for 2080-2099 using the A1B scenario. Top panels are temperature (left), precipitation (middle) and sea level pressure (right) for December-January-February. Bottom panels show the same variables for June-July-August.

proxy data -ice cores -water molecules -air bubbles -ice -oxygen isotope ratios -measure amount of CO2 -atmospheric dust -fossils -abundance of plant and animal life

How do we know about the Quaternary Climate?

Milankovitch Cycles

Illustration of the three variables in Earth's orbit, with periods of variation marked.

brighter

In a picture of the Earth's surface with a cloud cover removed, the poles and deserts are much ________ than the oceans and forests.

Global CO2 Emissions From Coal Combustion - The world's CO2 emissions from coal combustion in billions of metric tons are plotted against time for OECD countries and non-OECD countries.

In this figure, the world's CO2 emissions from coal combustion in billions of metric tons are plotted against time. Notice that countries of the Organization for Economic Co-operation and Development (OECD), which comprises a large group of developed and industrialized nations, have not increased their CO2 emissions from coal combustion since 1990, and future projections also reveal a flat line in CO2 emissions. Compare this to the non-OECD countries, many of which are emerging economies like China and India, and you see that CO2 emissions are set to triple in the next 25 years. There is much debate over information like this now that recent climate change has been linked so closely to anthropogenic emission of CO2. This debate revolves around the fact that developed nations used coal, oil, and natural gas during a time when the impacts of CO2 and climate change were not well researched. This meant that during the time these countries, including the United States, industrialized there were no regulations on the emissions of CO2. Now that CO2 emissions have been shown to cause global warming, pressure is being applied to these emerging economies to regulate and control their CO2 emissions. This is subject of much of the debate at the international climate summits at Kyoto, Bali, and Copenhagen. What is important to remember when discussing developed countries vs. emerging economies is that the per capita emissions of CO2 in emerging economies are approximately one third of those for developed countries.

Climate Simulation Scenarios

Left - multiple climate model projections (or scenarios) of greenhouse gas emissions (including CO2, CH4 and N2O) emissions in Gt-CO2-equivalent through 2100. Right - multiple climate model projections of globally averaged surface air temperature through 2100. The graphs in Figure Climate Simulation Scenarios show the range of model projections from different climate simulation scenarios based upon various greenhouse gas emission scenarios (left graph). Focus on the top and bottom curves in the right panel, which show the most dramatic warming and the most conservative warming. The worst-case scenario, found in the top line, shows the "business as usual" projections. If nothing is done to mitigate the emission of greenhouse gases into the atmosphere, these climate models are predicting a 4°C to 6°C increase in global average temperature by 2100. The best-case scenario, from a climate change perspective, would be for a cessation of CO2 emissions or for the current emission rates to not increase. In this case, there would still be a warming of 0.5° to 2°C by 2100 as indicated by the bottom curves.

Observed Temperatures vs. Projected Temperatures

Observed global average surface temperatures (black line) overlaid on the temperature projections of the first IPCC report (FAR), second report (SAR) and third report (TAR).

melting

One amazing depiction of polar warming can be found in the drunken forests of Siberia. Larch and spruce trees there are often seen tilted over on their sides and growing at strange angles. Why? Because the once continually frozen soil, or permafrost, in which they are rooted has been _______ in recent years. As the soil thaws it becomes more malleable and the trees begin to slant as the soil beneath them sinks.

Northern Hemisphere Surface Air

Panel (a) - Northern Hemisphere surface air temperature data from the modern instrument era from various sources. Panel (b) - Northern Hemisphere surface air temperature reconstruction dating back 1300 years from various sources. Panel (c) - Percent of overlap between the various sources of Panel (b).

Milankovitch cycles

Periodic variations in the Earth's orbit that influence its climate. These cycles are named after Milutin Milankovitch, a mathematician who quantified the theory.

Radioactive Forcings and Simulated Temperatures

Plot (a) - Radiative forcing due to volcanic eruptions over the last 1,300 years. Plot (b) - Radiative forcing due to fluctuations in solar irradiance over the last 1,300 years. Plot (c) - Radiative forcing due to all other forcing over the last 1,300 years. Plot (d) - Northern Hemisphere temperature reconstruction with overlap (shading) over the last 1,300 years. At first glance the Figure Radiative Forcings & Simulated Temperatures looks quite complicated, but let's break this graph down to understand how changes in the sun's output and volcanic eruptions have contributed to recent climate change. In the top panel (a), changes in the amount of energy, measured in W/m2, are graphed against time to show how volcanic eruptions have impacted the amount of energy the earth receives from the sun. Notice that around the year 1815, when Mt. Tambora erupted, there is a large downward spike in the plot. Now, examine the bottom panel, which shows the NH temperatures, just as Figure Northern Hemisphere Surface Air displayed, and see how the temperatures in the years following 1815 took a sharp downward turn. This is a direct consequence of the changes in albedo caused by large volcanic eruptions. Next, look at the time period between 1000 and 1300 A.D., the so-called Medieval Warm Period. In panel (b), changes in solar output are graphed against time; notice that during the Medieval Warm Period, the amount of insolation was high compared to the average. The opposite occurred during the Little Ice Age which peaked around 400 years ago.

Global influence maps (world mapper) of population, fuel imports, absolute wealth, carbon emission, poverty, forest loss

Rather than point the finger at individual countries, let's examine the bigger problem. The maps in Figure Global Influence Maps distort the size of each country based on a certain variable, like CO2 emissions, with respect to the rest of the world. In the upper left panel, the map is based on population, which is why China and India appear so large. The upper right map distorts the size of the country based upon fuel imports. Notice that the United States, much of Europe, and Japan are expanded the most, while Africa, the Middle East, and much of South America are barely visible. Compare these two maps with absolute wealth and carbon emissions and the story is quite clear. The industrialized and wealthy nations are responsible for the largest quantities of carbon emissions and fuel imports. These societies are built on the foundation of energy production through the consumption of fossil fuels. The bottom two panels tell another aspect of this story. Focus first on the graph in the lower right, which shows forest loss by country. The world's forest biomes are a large part of the CO2 cycle and with deforestation, a large sink for atmospheric CO2 is taken away. Notice that deforestation is most prevalent in Africa, South America, and Indonesia while the United States is barely visible on this map. In the United States, reforestation is practiced, but in the rainforests of the world, which are those areas in South America, Africa, and Indonesia that are ballooned on this map, deforestation is commonplace. The last graph in Figure Global Influence Maps distorts each country's size according to poverty. Much of Asia and Africa are distorted the most, and it is in these regions that we need to pay close attention over the upcoming years. Many of the nations found within these countries are what economists and politicians call "emerging economies." Although much of the current abundance of CO2 in the atmosphere is from developed countries such as the United States, CO2 emissions from these countries are not increasing with time according to a 2008 report from the Energy Information Administration.

Deforestation in the Amazon (2010)

Satellite image shows the extent of deforestation in the Amazon as of 2010.

Volcanic Outgassing

The Mt. Bromo volcano in Indonesia emitting gas into the atmosphere.

•aka thermal radiation •Light radiation in wavelengths just out of range of visibility to the human eye •Cannot travel very far before absorbed •700 nm - 1 mm

The Nature of Light Radiation in the Electromagnetic Spectrum - *infrared spectrum*

•Light radiation in the range of wavelengths visible to the human eye •Travels a long way through atmosphere before absorbed •400 nm -700 nm

The Nature of Light Radiation in the Electromagnetic Spectrum - *visible spectrum*

Ice Age Temperature

The blue and green lines depict two different Antarctic ice cores (taken from ice about 350 miles apart) and the variations in oxygen isotopes are converted into temperature changes. The red line depicts global ice volume. The y-axis shows temperature change; today's climate is at zero - the dashed line.

hydrologic cycle

The continuous movement of water, on above and below the surface of the earth. The cycle is dominated by the global equilibrium in evaporation and condensation.

nothing

The graphs in Figure Climate Simulation Scenarios show the range of model projections from different climate simulation scenarios based upon various greenhouse gas emission scenarios (left graph). Focus on the top and bottom curves in the right panel, which show the most dramatic warming and the most conservative warming. The worst-case scenario, found in the top line, shows the "business as usual" projections. If _______ is done to mitigate the emission of greenhouse gases into the atmosphere, these climate models are predicting a 4°C to 6°C increase in global average temperature by 2100. The best-case scenario, from a climate change perspective, would be for a cessation of CO2 emissions or for the current emission rates to not increase. In this case, there would still be a warming of 0.5° to 2°C by 2100 as indicated by the bottom curves. In addition to predicting warming of the atmosphere, climate models also suggest that sea level will continue to rise. Since 1880, sea level has risen 20 cm (approximately 8 inches) as seen in Figure Sea Levels since 1880. This rise has been primarily the result of the water thermally expanding as it warms along with the atmosphere. Polar ice cap melt from land-based ice sheets and glaciers has also added to increase in sea level. The current projection is that sea level will rise at the rate of at least 2 mm per year over the next century, with an overall increase ranging from 15 to 60 inches cm.

last glacial maximum

The time at which ice sheets were at their greatest extent during the latest glacial period

Changes in Greenhouse Gases from Ice Core and Modern Data

The top panel in Figure Changes in Greenhouse Gases from Ice Core and Modern Data shows the past 10,000 years of atmospheric CO2concentrations. Before 1750, the amount of CO2 in the atmosphere was relatively steady at 280 ppm. Since 1750 there has been a dramatic increase in CO2 concentrations.

interglacial period

The warm periods of the Quaternary in which glaciers and ice sheets retreat. Occur between the longer glacial periods.

uhh tilt??

There are three different ways in which the Earth's orbit changes through time. What combination of orbital parameters would be most likely to start an ice age? (Hint: Ice ages require cool northern summers.)

Vostok Petit Data

These graphs depict how changes in temperature—inferred from changes in isotope ratios (blue line)--correspond to changes in atmospheric carbon dioxide (green line) and dust (red line) over the last 400,000 years as recorded in an ice core extracted from Antarctica. Carbon dioxide varies directly with temperature - the warmer the climate the higher the carbon dioxide level. Atmospheric dust is highest during the coolest periods (such as 25,000 and 150,000 years ago).

Earth's Annual Carbon Cycle chart

This diagram shows a simplified representation of the contemporary global carbon cycle. Changes are measured in gigatons or carbon per year (GtC/y). Numbers in parentheses refer to stored carbon pools. Red indicates carbon from human emissions. Humans contribute a net increase of 4 GtC/y to atmospheric carbon. Summary of diagram Description is based on the cited public-domain source (US DOE, 2008): Values in parentheses are estimates of the main carbon reservoirs in gigatons (Gt) as reported in Houghton (2007). The natural flux between the terrestrial biosphere and the atmosphere is about 120 Gt of carbon per year, and that between the oceans and atmosphere is about 90 Gt per year (Denman et al., 2007). The atmosphere is a carbon pool of 800 Gt. In the terrestrial biosphere, photosynthesis removes about 120 Gt of carbon from the atmosphere. Some of the carbon from photosynthesis flows to: •plant biomass, which is a carbon pool of 550 Gt; •soil carbon. Decomposition of biological material and respiration from plants and soil microbes return 120 Gt/y of carbon to the atmosphere: •60 Gt/y due to plant respiration, •60 Gt/y due to microbial respiration and decomposition The soil is a carbon pool of 2300 Gt. There is a fossil pool of 10,000 Gt in the Earth's crust. In the oceans, the marine biosphere does not take up CO2 directly from the atmosphere. Each year the oceans absorb and release about 90 Gt of carbon largely via diffusion across the air-ocean interface. The surface ocean is a carbon pool of 1000 Gt. The physical processes controlling the sinking of carbon dioxide (CO2) into colder, deeper waters (where CO2 is more soluble) and the mixing of ocean water at intermediate depths are known collectively as the "solubility pump." Phytoplankton photosynthesis converts CO2 into organic carbon that is largely returned to ocean water as CO2 via microbial respiration and decomposition. The "biological pump" refers to the small fraction of organic carbon that forms into degradation-resistant clumps and sinks to the ocean floor. Together the solubility and biological pumps control the amount of carbon transported to ocean depths and the exchange of CO2between ocean and atmosphere. The deep ocean is a carbon pool of 37,000 Gt. Reactive sediments at the bottom of the ocean are a carbon pool of 6000 Gt. Human activities (primarily fossil fuel use) emit about 9 Gt of carbon each year. About 4 Gt of this human-contributed carbon remain in the atmosphere; 3 Gt are taken up by natural terrestrial processes, and another 2 Gt are removed by the ocean (Canadell et al. 2007).

Ice Cores

Three different sections of an ice core. The seasonal layers are most clear in the middle section (note the dark and light bands). The deepest section (bottom core) is taken from almost two miles down and is colored brown by rocky debris from the ground under the ice.

High Temperature vs. Low Temperature, Champaign, IL - The average high temperature for Champaign-Urbana Illinois in black (1899-2009). The 2005 actual high temperature are graphed in red.

To better understand the differences between weather and climate, take a look at Figure High Temperature vs. Low Temperature, Champaign, IL which shows in red the actual high temperatures for each day in 2005 in Champaign, Illinois, compared to the average high temperature in black over a period beginning in 1899 and ending in 2009. It is completely normal for the temperature to vary ±20°F around this average. In 2005 there were only a handful of days where the actual high was the same as the average high. This graph shows the highly variable and chaotic behavior of weather. But, when data from a long span of time is averaged, the climatological mean emerges.

Atmospheric Transmission

Top graph - normalized spectral intensity (radiant energy) emitted by the earth and sun as a function of wavelength. Middle graph - total atmospheric absorption as a function of wavelength. Bottom graph - individual gas absorption as a function of wavelength To understand why these gases are so efficient at keeping the planet warm, let's examine Figure Atmospheric Transmission. The top panel of this figure shows the normalized intensity of the radiation emitted by both the sun and earth as a function of wavelength. The middle panel shows the total atmospheric absorption spectrum and the bottom panel shows the individual gas absorption spectrum (excluding Nitrogen and Argon). Notice from the top panel that the sun's peak energy emission falls within the visible portion of the spectrum and suffers very little atmospheric absorption (middle panel). The peak emission wavelength for the earth is in the thermal infrared (IR), and it is effectively absorbed by water vapor (H20), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N02). The primary purpose of this figure is to show that the gases in the earth's atmosphere are transparent to the sun's peak energy emission (visible light) but not the earth's peak emission (thermal IR). It is through the absorption of the earth's outgoing thermal infrared radiation that the global average temperature warms approximately 60°F over what it would be without greenhouse gases.

Model Simulations

Top panel - Climate model simulations of the global mean surface temperature compared to the observed global mean surface temperature in black. Each yellow line is one of 58 climate model simulations of which the red line is the ensemble mean. Bottom panel - 19 climate model simulations in blue with the ensemble mean in dark blue. These simulations were run without anthropogenic influences. The thick black line is the observed global mean surface temperature. For a description of each scenario, please click here.

Oxygen Schematic

Water becomes lighter as it travels toward the poles. The heavy (18O) water drops out of the atmosphere (as rain or snow) before reaching the ice sheet. This means that the snow that forms the glacial ice is lighter than the ocean water (has more 16O than 18O, compared to ocean water).

Annual Global Temperature Anomalies

We have ruled out the first two mechanisms (i.e., changes in albedo and insolation) as reasons for the recent increase in global temperatures. But when we look at panel (c) in Figure Radiative Forcings & Simulated Temperatures, we notice that the "all other forcing" curves point to a rapid increase in the amount of energy retained by the earth-atmosphere system over the last 200 years. What is responsible for the increasing tail on this graph? Have humans altered the composition of the Earth's atmosphere to make it more efficient at absorbing the infrared radiation that would have otherwise been lost to space? Is there proof of a human enhancement to the natural greenhouse effect? Can we explain the recent warming on an anthropogenic adjustment to the greenhouse gases like carbon dioxide (CO2)? Is an "enhanced greenhouse effect" to blame for the fact that the top ten warmest years since the modern era of instrument measurements have occurred since 1995, as seen in Figure Annual Global Temperature Anomalies.

1. fire ants 2. drunken forests 3. mosquitoes 4. coral bleaching 5. melting 6. cherry blossoms

What are six observed effects of climate change?

-the average global temperatures will have a 0.5 degree C to 2 degree C increase (best case) or a 4 degree C to 6 degree C increase (worst case) -the sea level will rise at a rate of 2 mm/year -there will be precipitation consisting of droughts and flash floods

What are the climate predictions for 2100?

1. eccentricity 2. obliquity 3. axial precession

What are the three factors in the Milankovitch Cycles?

1. eccentricity 2. axial tilt or obliquity 3. axial precession

What are the three factors in the Milankovitch Cycles?

1. glacial period 2. interglacial period 3. last glacial maximum

What are the three periods in the quaternary climate?

■Thirty years is the shortest time period over which weather can be averaged to extract climate information. ■Cannot attribute single weather events on climate change ■Climate models forecast changes in the flux of energy between earth, its atmosphere and space.

What are three climate projections?

1. temperature of the core 2. properties of the water that make up the ice 3. trapped dust 4. tiny entombed bubbles of ancient atmosphere

What four different types of information does the ice record?

1. water vapor 2. carbon dioxide 3. methane 4. ozone

What four gases make up Greenhouse gases (1%)?

Mt. Pinatubo Erupting in the Philippines in 1991

What is this a photo of?

The "Keeling Curve" of CO2 concentrations measured in Mauna Loa, Hawaii, since the 1950s.

What is this a picture of?

1. insolation 2. albedo 3. composition of the Earth's Atmosphere

What three things control the climate?

1. oxygen 2. nitrogen

What two gases make up non-greenhouse gases (1%)?

1. medieval warm period 2. little ice age

What were the two events observed in temperature records?

little ice age

a cool period in the Northern Hemisphere, primarily in Europe from the 16th to the 19th century.

albedo

a measure of how reflective the earth is. the higher the albedo, the more reflective the material

ice sheet

glaciers big enough to cover a continent; currently found in Antarctica and Greenland; during glacial periods, covered other land masses, including North America

2007 Sea Ice Extent in the Arctic

it is going down

quaternary period

the most recent geological period, spanning the time from 2.6 million years ago to today

greenhouse effect

the process by which the atmosphere acts to trap heat, warming the climate

climate

the typical, average, atmospheric conditions. The average of the weather in degrees Celsius

Sea Levels Since 1880 - The different colors represent different data sets used to make this graph.

they are rising

the typical, or average, atmospheric conditions; average of the weather; scientists use the Celsius scale when discussing climate

weather vs *climate*?

obliquity

•Axial tilt •The angle between a planet's axis of rotation and the line perpendicular to the plan in which it orbits •The Earth's current axial tilt is 23.5 degrees.

eccentricity

•Orbital shape •A measure of how much an ellipse departs from circularity. The Earth's orbital shape cycles between being nearly circular and being mildly elliptical.

axial precession

•The direction of the Earth's axis of rotation, which changes in the direction of the Earth's axis of rotation relative to the stars.

Snowball Earth Hypothesis

■Condition in which the entire earth is covered in ice ■thought to have occurred 650 million years ago ■May have been caused by an initial change which created a positive feedback: -a runaway process which amplifies the effect of an initial change

the short term state of the atmosphere (wind, air pressure, precipitation, humidity, temperature)

*weather* vs climate?

polar vs equatorial insolation

A cartoon of how latitude is important in determining the amount of insolation. The same amount of sunlight (yellow bars) is spread out over twice the planet's surface area when the rays strike the Earth at an angle (compare the length of the dark lines at the equator and at the poles).

Earth Atmosphere Cartoon

A cartoon of the greenhouse effect. (Top) Visible light radiation emitted by the sun (yellow arrows) strikes the Earth and reflects as infrared radiation (orange arrow); (middle) an atmosphere reflects some of the infrared radiation back toward the planet; (bottom) a thickened atmosphere reflects greater amounts of infrared radiation. This effect is shown in Figure Earth Atmosphere Cartoon. The visible light radiation enters the atmosphere, and quickly exits as infrared radiation if there is no atmosphere (top Earth). With our atmosphere (the middle Earth), visible light enters unhindered but the infrared light is partially reflected back to the surface, increasing the amount of energy and thus the temperature at the Earth's surface. If the atmosphere is made thicker (bottom Earth) the infrared radiation is trapped for longer, further warming the planet's surface.

Five Myr Climate Change

A comparison of the age of sediment (x-axis) and the change in temperature over time (left y-axis) as derived from oxygen isotope ratios (right y-axis). The dashed line shows today's climate. Note that the climate is cooling over the last few million years, but it is highly variable. In the last one million years the climate alternates between warm and cool conditions on a 100,000-year time scale ("100 kyr cycle"), before this it alternated on a 41,000 year cycle. Both these period lengths are the same as Milankovitch cycles. These cores suggest that today's temperature is higher than almost all of that of the Quaternary (the last 2.6 Million years).

glacial period

A long period of time in which ice sheets and glaciers are advanced, covering entire continents and land masses.