APES Chapter 19: Global Change

Explain the process underlying the greenhouse effect.

The physical and biogeochemical systems that regulate temperature at the surface of Earth are essential to life on our planet. The ultimate source of almost all energy on Earth is the sun. The sun emits solar radiation that strikes Earth. As the planet warms, it emits radiation back toward the atmosphere. However, the types of energy radiated from the sun and Earth are different. Because the sun is very hot, most of its radiated energy is in the form of high-energy visible radiation and ultraviolet radiation. When this radiation strikes Earth, the planet warms and radiates energy. Earth is not nearly as hot as the sun, so it emits most of its energy as infrared radiation. Differences in the types of radiation emitted by the sun and Earth, in combination with processes that occur in the atmosphere, cause the planet to warm. As radiation from the sun travels towards Earth, about one-third of the radiation is reflected back into space. Although some ultraviolet radiation is absorbed by the ozone layer in the stratosphere, the remaining ultraviolet radiation, as well as visible light, easily passes through the atmosphere. Once it has passed through the atmosphere, this solar radiation strikes clouds and the surface of Earth. Some of this radiation is reflected from the surface of the planet back into space. The remaining radiation is absorbed by clouds and the surface of Earth, which become warmer and begin to emit lower-energy infrared radiation back toward the atmosphere. Unlike ultraviolet and visible radiation, infrared radiation does not easily pass through the atmosphere. It is absorbed by gases. The warmed gases emit infrared radiation out into space and back toward the surface of Earth. The infrared radiation that is emitted toward Earth causes Earth's surface to become even warmer. This absorption of infrared radiation by atmospheric gases and reradiation of the energy back toward Earth is the greenhouse effect. In the sun-Earth heating system, the net flux of energy is zero; the inputs of energy to Earth equal the outputs from Earth. Over the long term, the system has been in a steady state. However, in the shorter term, inputs can be slightly higher or lower than outputs. Factors that influence short-term fluctuations include changes in incoming solar radiation from increased solar activity and changes in outgoing radiation from an increase in atmospheric gases that absorb infrared radiation. If incoming solar energy is greater than the sum of reflected solar energy and radiated infrared energy, then the energy accumulates faster than it is dispersed and the planet becomes warmer. If incoming solar energy is less than the sum of the two outputs, the planet becomes cooler. The two most common gases in the atmosphere, N2 and O2, compose 99% of the atmosphere. Because these two gases do not absorb infrared radiation, they are not greenhouse gases and do not contribute to the warming of Earth. The most common greenhouse gas is water vapor. Water vapor absorbs more infrared radiation from Earth than any other compound, although a molecule of water vapor does not persist nearly as long as other greenhouse gases. Other important greenhouse gases include carbon dioxide, methane, nitrous oxide, and ozone. Ozone in the stratosphere is beneficial because it filters out harmful ultraviolet radiation. In contrast, ozone in the lower troposphere acts as a greenhouse gas and can cause infrared warming of Earth. It also is an air pollutant in the lower troposphere because it can cause damage to plants and human respiratory systems. There is one other type of greenhouse gas, chlorofluorocarbons (CFCs), which does not exist naturally. Concern about the danger of greenhouse gases is based on our understanding that an increase in the concentration of these gases- as has occurred due to human activities- can cause the planet to warm even more than usual. The contribution of each gas to global warming depends in part on its greenhouse warming potential. In calculating this potential, scientists consider the amount of infrared energy that a given gas can absorb and how long a molecule of the gas can persist in the atmosphere. Because greenhouse gases can differ a great deal in these two factors, greenhouse warming potentials span a wide range of values. The effect of each greenhouse gas depends on both its warming potential and its concentration in the atmosphere. Although carbon dioxide has a relatively low warming potential, it is much more abundant than most other greenhouse gases, except for water vapor, which can have a concentration similar to carbon dioxide. While human activity appears to have little effect on the amount of water vapor in the atmosphere, it has caused substantial increases in the amount of the other greenhouse gases. Among these, carbon dioxide remains the greatest contributor to the greenhouse effect because its concentration is so much higher than any of the others. As a result, scientists and policy makers focus their efforts on ways to reduce carbon dioxide in the atmosphere. The concentrations of each gas, and how much infrared energy each gas absorbs, we can understand how changes in the concentrations of greenhouse gases can contribute to global warming. Increasing the concentration of any historically present greenhouse gas should cause more infrared radiation to be absorbed in the atmosphere, which will then radiate more energy back toward the surface of the planet and cause the planet to warm. Likewise, producing new greenhouse gases that can make their way into the atmosphere, such as CFCs, should also cause increased absorption of infrared radiation in the atmosphere and further cause the planet to warm.

Distinguish among global change, global climate change, and global warming.

Change that occurs in the chemical, biological, and physical properties of the planet is referred to as global change. Some types of global change are natural and have been occurring for millions of years ago. In modern times, however, the rates of change have often been higher than those that occurred historically. Many of these changes are the result of human activities, and they can have significant, sometimes cascading, effects. One type of global change of particular concern to scientists is global climate change, which refers to changes in the average weather that occurs in an area over a period of years or decades. Changes in climate can be categorized as either natural or anthropogenic. Anthropogenic activities such as fossil fuel combustion and deforestation also have major effects on global climates. Global warming refers to a specific aspect of climate change: the warming of the oceans, land masses, and atmosphere of Earth.

Discuss how we estimate temperatures and levels of greenhouse gases over the past 500,000 years and into the future.

Common indirect measurements include changes in the species composition of organisms that have been preserved over millions of years and chemicals analyses of air bubbles formed in ice long ago. One commonly used biological measurement is the change in species composition of a group of small protists, called foraminefera. Foraminifera are tiny, marine organisms with hard shells that resist decay after death. In some regions, the tiny shells have been building up in sediments. The youngest sediment layers are near the top of the ocean floor whereas the oldest sediment layers are much deeper. Fortunately, different species of foraminifera prefer different water temperatures. As a result, when scientists identify the predominant species of foraminifera in a layer of sediment, they can infer the likely temperature of the ocean at the time the layer of sediment was deposited. By examining thousands of sediment layer samples, we can gain insights into temperature changes over millions of years. Scientists can determine changes in greenhouse gas concentrations and temperatures over long periods of time by examining ancient ice. The youngest ice is near the surface and the oldest ice is much deeper. Combining data from different biological and physical measurements, researchers have created a picture of how the atmosphere and temperature of Earth have changed over hundreds of thousands of years. During the past 10,000 years, CO2 is not the only greenhouse gas whose concentration has increased. Methane and nitrous oxide show a pattern of increase that is similar. For all three gases, there was little change in concentration for most of the previous 10,000 years. After 1800, however, all three rose dramatically. This increase in greenhouse gases occurred because this time period marks the start of the Industrial Revolution when humans began burning large amounts of fossil fuel and producing a variety of greenhouse gases. Most of these rapid shifts occurred during the onset of an ice age or during the transition from an ice age to a period of warm temperatures after an ice age to a period of warm temperatures after an ice age. Because these changes occurred before humans could have had an appreciable effect on global systems, scientists suspect the changes were caused by small, regular shifts in the orbit of Earth. The path of the orbit, the amount of tilt on Earth's axis, and the position relative to the sun all change regularly over hundreds of thousands of years. These changes alter the amount of sunlight that hits high northern latitudes in the winter, the amount of snow that can accumulate, and the way the albedo effect keeps energy from being absorbed and converted to heat. These changes could give rise to fairly regular shifts in temperature over a long period of time. The more important insight is the close correspondence between historic temperatures and CO2 concentrations. Scientists believe that the relationship between fluctuating levels of CO2 and the temperature is complex and that both factors play a role. The increase of CO2 in the atmosphere causes a greater capacity for warming through the greenhouse effect. However, when Earth experiences higher temperatures, the oceans warm and cannot contain as much CO2 gas and, as a result, they release CO2 into the atmosphere. What ultimately matters is the net movement of CO2 between the atmosphere and the oceans and how these different feedback loops work together to affect global temperatures. We can also examine temperatures over somewhat shorter time periods. How can we tell if the recent changes are anthropogenic? One explanation for warming temperatures during the past century is an increase in solar radiation. Another possibility is that warming is cased by increased CO2 in addition to warming caused by natural fluctuations in solar radiation. One way to approach the problem is to look for more detailed patterns in temperature changes. On the other hand, if increased solar radiation were the cause of global warming, periods of elevated solar radiation would be associated with higher temperatures more commonly when the sun is shining more. These times and locations on Earth receive the greatest amount of sunlight, so an increased intensity of solar radiation would cause these times and places on Earth to warm more than other times and places. Just as indirect indicators, can help us get a picture of what the temperature has been in the distant past, computer models can help us predict future climate conditions. We can determine how well a model approximates real-world processes by applying it to a time in the past for which we have accurate data on conditions such as air and ocean temperatures, CO2 concentration, extent of vegetation, and sea ice coverage at the poles. Modern models reproduce recent temperature fluctuations well over large spatial scales. Although climate models cannot forecast future climates with total accuracy, as the models improve scientists have been able to place more confidence in their predictions of temperature change, although they have had more difficulty predicting changes in precipitation.

Explain how global climate change has affected organisms.

Effects range from temperature-induced changes in the timing of plant flowering and animal behavior to the ability of plants and animals to disperse to more hospitable habitats. Rapid temperature changes have the potential to cause harm if organisms do not have the option of moving to more hospitable climates and do not have sufficient time to evolve adaptations. The ability to migrate is one reason that temperature shifts have not been catastrophic over the past few million years. Today, however, fragmentation of certain habitats has made movement much more difficult. In fact, this may be the primary factor that allows a warming climate to cause the extinction of species.

Explain how temperatures have increased since records began in 1800s.

Global temperatures have increased 0.8 degrees C from 1880 through 2013. The 10 warmest years on record since 1880 have all occurred between 1998 and 2013. While an increase in average global temperature of 0.8 degrees C may not sound very substantial, it's not evenly distributed around the globe. Some regions, including parts of Antarctica, have experienced cooler temperatures. Some regions, including areas of the oceans, have experienced no change in temperature. Finally, some regions, such as those in the extreme northern latitudes, have experienced increases of 1 degree C to 4 degrees C. The substantial increases in temperatures in the northern latitudes have caused, among other problems, nearly 45% of the northern ice cap to melt, which has threatened polar bears and their ecosystem. However, it is possible that such changes in temperature are simply a natural phenomenon.

Explain the global climate change goals of the Kyoto Protocol.

In 1997, representatives of the world's nations convened in Kyoto, Japan, to discuss how best to control the emissions contributing to global warming. At this meeting, they drew up the Kyoto Protocol. Due to special circumstances and political pressures, countries agreed to different levels of emission restrictions. Developing nations, including China and India, did not have emission limits imposed by the protocol because they argued that different restrictions on developed and developing countries are justified because developing countries are unfairly exposed to the consequences of global warming that in large part come from the developed nations. The main argument for the Kyoto Protocol is grounded in the precautionary principle, which states that in the face of scientific evidence that contains some uncertainty we should behave cautiously. The first option includes trying to increase fuel efficiency or switching from coal and oil to energy sources that emit less or no CO2. The second option includes carbon sequestration. Methods of carbon sequestration might include storing carbon in agricultural soils or retiring agricultural land and allowing it to become pasture or forest. Researchers are also working on cost-effective ways of capturing CO2 from the air. In developed countries, reductions in CO2 emissions would require major changes to manufacturing, agriculture, or infrastructure at significant expense and economic impact. In 2001, the Kyoto Protocol was modified to convince more developed nations to ratify it. More recently, the US government has taken stronger steps to regulate CO2 emissions. As of 2014, 192 countries have ratified the Kyoto Protocol, including most developed and developing countries, although more than 100 developing countries are exempt from any limits on CO2 emissions including China and India.

Explain how CO2 concentrations have changed over the past 6 decades and how emissions compare among the nations of the world.

In the first half of the twentieth century, most scientists believed that if any excess CO2 were being produced, it would be absorbed by the oceans and vegetation. In addition, because the concentration of atmospheric CO2 was low compared to gases such as oxygen and nitrogen, it was difficult to measure accurately. Charles David Keeling was the first to overcome the technical difficulties in measuring CO2. When Keeling set out to measure the precise level of CO2 in the atmosphere, most atmospheric scientists believed that two measurements several years apart would be sufficient to answer the question of whether human activities were causing increased concentrations of CO2 in the atmosphere. Keeling did not agree. After just 1 year of work, Keeling found that CO2 levels varied seasonally and that the concentration of CO2 increased from year to year. His results prompted him to take measurements for several more years, and he and his students have continued this work into the twenty-first century. The results confirm Keeling's early findings; although CO2 concentrations vary between seasons, there is a clear trend of rising CO2 concentrations across the years. This increase over time is correlated to increased human emissions of carbon from the combustion of fossil fuels and net destruction of vegetation. Each spring, as deciduous trees, grasslands, and farmlands in the Northern Hemisphere turn green, they increase their absorption rates of CO2 to carry out photosynthesis. At the same time, bodies of water begin to warm and the algae and plants also begin to photosynthesize. In doing so, these producers take up some of the CO2 in the atmosphere. Conversely, in the fall, as leaves drop, crops are harvested, and bodies of water cool, the uptake of atmospheric CO2 by algae and plants declines and the amount of CO2 in the atmosphere increases. Per capita consumption of fossil fuels and materials is greatest in developed countries. It is not surprising, then, that the production of carbon dioxide has also been greatest in the developed world. Development has been especially rapid in China and India, which together contain one-third of the world's population. From 2000 to 2009, China more than doubled its emissions of carbon dioxide as the country built many new coal-powered electrical plants that increased its ability to burn coal. China emits more than 7200 million metric tons of CO2, representing 24% of all global CO2 emissions. The US is in second place, representing 18% of all global CO2 emissions, yet the the US contains only 5% of the world's population. If we consider the amount of per capita CO2 emissions, we obtain a very different picture of which countries produce the most CO2. The US and Australia are the leading per capita emitters of CO2, followed by Saudi Arabia and Canada. Despite the fact that China and India rank among the top producers of CO2, their per capita production ranks them sixteenth and twentieth, respectively, which reflects the fact that these two countries both have very large populations.

Identify the natural and anthropogenic sources of greenhouse gases.

Natural sources of greenhouse gases include volcanic eruptions, decomposition, digestion, denitrification, evaporation, and evapotranspiration. Over the scale of geologic time, volcanic eruptions can add a significant amount of carbon dioxide to the atmosphere. Other gases and the large quantities of ash released during volcanic eruptions can also have important, short-term climatic effects. A volcanic eruption emits a large quantity of ash into the atmosphere. The ash reflects incoming solar radiation back out into space, which has a cooling effect on Earth. When decomposition occurs under high-oxygen conditions, the dead organic matter is ultimately converted into carbon dioxide. Methane is created when there is not enough oxygen available to produce carbon dioxide. This is a common occurrence at the bottom of wetlands where plants and animals decompose and oxygen is in low supply. A similar situation occurs when certain animals digest plant matter. Animals that consume significant quantities of wood and grass require gut bacteria to digest the plant material. Because the digestion occurs in the animal's gut, the bacteria do not have access to oxygen and methane is produced as a by-product. Nitrous oxide is a natural component of the nitrogen cycle that is produced through the process of denitrification. Denitrification occurs in the low-oxygen environments of wet soils and at the bottoms of wetlands, lakes, and oceans. Nitrate is converted to nitrous oxide gas, which then enters the atmosphere as a powerful greenhouse gas. Water vapor is the most abundant greenhouse gas in the atmosphere and the greatest natural contributor to global warming. Because the amount of evaporation into water vapor varies with climate, the amount of water vapor in the atmosphere can vary regionally. There are many anthropogenic sources of greenhouse gases. The most significant of these are the burning of fossil fuels, agricultural practices, deforestation, landfills, and industrial production of new greenhouse chemicals. Organisms were sometimes buried without first decomposing into carbon dioxide. When humans burn these fossil fuels, we produce CO2 that goes into the atmosphere. Because of the long time required to convert carbon into fossil fuels, the rate of putting carbon into the atmosphere by burning fossil fuels is much greater than the rate at which producers take CO2 out of the air and both the producers and consumers contribute to the pool of buried fossil carbon. Because fossil fuels differ in how they store energy, each type of fossil fuel produces different amounts of carbon dioxide. For a given amount of energy, burning coal produces the most CO2. The production of fossil fuels, such as the mining of coal, and the combustion of fossil fuels can also release methane and, in some cases, nitrous oxide. Particulate matter may also play an important role in global warming. Particulates that fall on ice and snow in the higher latitudes absorb more energy of the sun by lowering on the albedo. As the snow and ice begin to melt, the particulates become more concentrated on the surface. The increased concentration raises the amount of solar radiation absorbed, which increases melting. Agricultural practices can produce a variety of greenhouse gases. Agricultural fields that are overirrigated, or those that are deliberately flooded for cultivating crops such as rice, create low oxygen environments similar to wetlands and therefore can produce methane and nitrous oxide. Synthetic fertilizers, manures, and crops that naturally fix atmospheric nitrogen can create an excess of nitrates in the soil that are converted to nitrous oxide by the process of denitrification. Raising livestock can also produce large quantities of methane. Each day, living trees remove CO2 from the atmosphere during photosynthesis, and decomposing trees add CO2 to the atmosphere. This part of the carbon cycle does not change the net atmospheric carbon because the inputs and outputs are approximately equal. However, when forests are destroyed by burning or decomposition and not replaced, as can happen during deforestation, the destruction of vegetation will contribute to a net increase in atmospheric CO2. This is because the mass of carbon that made up the trees is added to the atmosphere by combustion or decomposition. Landfills receive a great deal of household waste that slowly decomposes under layers of soil. When the landfills are not aerated properly, they create a low-oxygen environment in which decomposition causes the production of methane as a by-product. The creation of new industrial chemicals often has unintended effects on the atmosphere. CFCs were used in the past until scientists discovered that they were damaging the protective ozone layer. The nations of the world joined together to sign the Montreal Protocol on Substances That Deplete the Ozone Layer, which phased out the production and use of CFCs by 1996. The three major contributors of methane in the atmosphere are the digestive processes of livestock, landfills, and the production of natural gas and petroleum products. The major contributor of nitrous oxide is agricultural soil because they receive nitrogen from synthetic fertilizers, combustion, and industrial production of fertilizers and other products. Approximately 94% of all CO2 emissions come from industrial processes and the burning of fossil fuels.

Identify the future changes predicted to occur with global climate change.

Predicted future changes have some amount of uncertainty because they are based on computer models of complicated systems. As temperatures increase, long periods of hot weather are likely to become frequent. Heat waves caused an increased an energy demand for cooling homes and offices. They increase the risk of death for those without air conditioning and cause heat and drought damage to crops. With global temperatures rising, minimum temperatures are expected to increase over most land areas. Such conditions would have two major positive effects for humans: fewer deaths due to cold temperatures and a decrease in the risk of crop damage from freezing temperatures. It may make new areas available for agriculture, and warmer temperatures would decrease the energy needed to heat buildings. However, a decrease in freezing temperatures that normally would cause the death of some pest species might allow them to expand their range. Because warmer temperatures should drive increased evaporation from the surface of Earth as part of the hydrologic cycle, global warming is projected to alter precipitation patterns. Regions receiving increased precipitation would benefit from an increased recharge to aquifers and higher crop yields, but they could also experience more flooding, landslides, and soil erosion. In contrast, other regions of the world are predicted to receive less precipitation, making it more difficult to grow crops and requiring greater efforts to supply water. Although it is impossible to link any single weather event to climate change because of the multiple factors that are always involved, ocean warming may be increasing the intensity of Atlantic storms. Global ocean currents may shift as a result of more fresh water being released from melting ice. If the currents change, the distribution of heat on the planet could be disrupted. Scientists are particularly concerned about the thermohaline circulation. Global warming and climate change could also affect many aspects of our lives. Some people may have to relocate from such vulnerable areas as coastal communities and some ocean islands. On the other hand, certain areas that have not been suitable for human habitation might become more hospitable if they become warmer, although other factors, such as water availability, might still limit their habitability. Climate changes has the potential to affect human health. Continued warming of the planet could affect the geographic range of temperature-limited disease vectors. It will also have economic consequences.

Explain the role of feedbacks on the impacts of climate change.

The global greenhouse system is made up of several interconnected subsystems with many potential positive and negative feedbacks. Positive feedbacks amplify changes. Because of this, positive feedback often leads to an unstable situation in which small fluctuations in inputs lead to large observed effects. On the other hand, negative feedbacks dampen changes. There are many ways that a rise in temperatures could create a positive feedback. Higher temperatures are expected to increase the biological activity of decomposers in these soils. Because this decomposition leads to the release of additional CO2 from the soil into the atmosphere, the temperature change will be amplified even more. As atmospheric concentrations of CO2 from anthropogenic sources increase, the Arctic regions become substantially warmer and the frozen tundra begins to thaw. As it thaws, the tundra develops areas of standing water with little oxygen available under the water as the thick organic layers of the tundra begin to decompose. As a result, the organic material experiences anaerobic decomposition that produces methane, which is stronger than CO2, which should lead to even more global warming. One of the most important negative feedbacks occurs as plants respond to increases in atmospheric carbon. Because carbon dioxide is required for photosynthesis, an increase in CO2 can stimulate plant growth. A second negative feedback exists in the oceans. As CO2 concentrations increase in the atmosphere, more CO2 is absorbed by the oceans. Most of the feedbacks are limited by features of the systems in which they take place. The magnitude and direction of many feedbacks are complex.

Discuss how global climate change has affected the environment.

Warming temperatures are expected to have a wide range of impacts on the environment, including the melting of polar ice caps, glaciers, permafrost, rising sea levels, an increased frequency of heat waves, fewer and less-intense cold spells, altered precipitation patterns and storm intensity, and shifting ocean currents. Over the next 70 years, the Arctic is predicted to warm by an additional 4 degrees C to 7 degrees C compared to the mean temperatures experienced from 1980 to 1990. If this prediction is accurate, large openings in sea ice will continue to expand and the ecosystem of the Arctic region will be negatively affected. At the same time, though, there may also be benefits to humans. Global warming also has caused the melting of many glaciers around the world. The loss of glaciers is not simply a loss of an aesthetic natural wonder. In many parts of the world, the melting of glaciers starting each spring provides a critical source of water for many communities. As warmer temperatures cause ice caps and glaciers to melt, it is not surprising that areas of permafrost are also melting. The melting of the permafrost causes overlying lakes to become smaller as the lake water drains deeper down into the ground. Melting can also cause substantial problems with human-built structures that are anchored into the permafrost. This also means that the massive amounts of organic matter contained in the tundra will begin to decompose. Because this decomposition would be occurring in wet soils under low-oxygen conditions, it would release substantial amounts of methane. This chain of events could produce a positive feedback. The rise in global temperature affects sea levels in two ways. First, the water from melting glaciers and ice sheets on land adds to the total volume of ocean water. Second, as the water of the oceans becomes warmer, it expands.