Feedback Factors

Introduction

Predictions of the future effects and severity of climate change are complicated by a number of feedback factors. A feedback, in this case, is an effect of global warming that amplifies or dampens the warming influences of anthropogenic (human-caused) greenhouse-gas emissions. These feedbacks arise from a number of different sources, including changes in ocean temperature, changes in cloud cover and altitude, changes in snow and ice cover, changes in vegetation range and growth, changes in soil temperature, changes in wildfire cycles, and others.

Feedback factors typically work in one of two ways: 1) By influencing how much carbon dioxide (CO₂), methane, and other greenhouse gases are released into or absorbed from the atmosphere through processes such as decay and photosynthesis, or 2) directly through albedo changes— where alterations in land, water, and clouds directly influence the amount of sunlight absorbed or reflected by the planet, and on a smaller scale, by its ecosystems. Feedbacks can be both positive, leading to further warming, or negative, moderating warming or leading to cooling.

Because there are many potential feedbacks to global warming from earth, ocean, and atmosphere, and because they work at different magnitudes, in concert or against one another, it is difficult to work these parameters into computer models for consistent long-term predictions. However, an increasingly large body of evidence suggests

that the net effects of various feedback mechanisms on human-caused global warming will ultimately be positive, further increasing warming.

Historical Background and Scientific Foundations

Antarctica's Vostok ice cores have captured evidence of climatic variations and changing atmospheric gas concentrations during glacial and interglacial cycles over the last 420,000 years. They show that past warming has typically resulted in increased levels of CO₂ and methane, which in turn contributed to more warming—a positive feedback. The same may be true today.

Scientists have identified several feedback mechanisms that may exacerbate current warming in localized and global ways. Several of these involve water in various stages. For example, warmer air can hold more water vapor, a strong greenhouse gas. Currently, water vapor may exert a warming effect 1 to 2 times greater than that exerted by greenhouse gases released by human activity. Global warming may also affect the oceans' ability to store and redistribute excess heat. Decreasing snow and ice cover at high latitudes and altitudes, meanwhile, reduces the reflectivity of Earth's surface, resulting in more localized warming and more snow and ice melt, which in turn spurs more warming. The effect of clouds, which can cool or warm Earth's surface depending on their thickness and altitude, is also important, but poorly understood.

WORDS TO KNOW

ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.

CARBON CYCLE: All parts (reservoirs) and fluxes of carbon. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments (includes fossil fuels). The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest pool of carbon near the surface of Earth, but most of that pool is not involved with rapid exchange with the atmosphere.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F (0°C) for several years.

PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photosynthesis in some plants. Plants create a very important reservoir for carbon dioxide.

THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

About half of human greenhouse-gas emissions are currently absorbed by the ocean, vegetation, and soils, but this absorption is sensitive to climate change. For example, organic materials preserved in soils—which contain much of the world's stored carbon—may provide an important positive feedback as they warm and decay more rapidly in a warming climate, cycling more CO₂ and methane into the atmosphere. The melting of permafrost, a type of frozen soil that can contain millennia worth of frozen organic materials like root fragments and leaf litter, may release massive amounts of carbon into the atmosphere that have been absent from the carbon cycle for thousands of years, providing a particularly strong warming effect.

Impacts and Issues

Many of these positive feedbacks will likely be moderated by negative feedbacks. An influx of freshwater from melting Arctic ice sheets, as well as increased precipitation, may weaken the global thermohaline ocean current (which has a warming effect) and slow Arctic and Nordic warming. Increasing concentrations of CO₂, which pants convert into sugars for food, may stimulate more photo-synthesis, and thus more plant growth. Such growth would, in turn, suck more CO₂ from the air.

Melting permafrost and warmer conditions will facilitate the advance of forests and shrubs in latitude and altitude, potentially providing another growing sink for CO₂. But plant growth may ultimately be limited by the amount of available nutrients in the soil, which in turn may be influenced by climate change. Meanwhile, the increasing incidence of large wildfires, which release massive pulses of greenhouse gases into the air, as well as warming resulting from the lower albedo of growing forests and shrublands, may dampen or obscure the benefits of these negative feedbacks.

IN CONTEXT: RADIATIVE FORCING

“The Earth's global mean climate is determined by incoming energy from the Sun and by the properties of the Earth and its atmosphere, namely the reflection, absorption and emission of energy within the atmosphere and at the surface. Although changes in received solar energy (e.g., caused by variations in the Earth's orbit around the Sun) inevitably affect the Earth's energy budget, the properties of the atmosphere and surface are also important and these may be affected by climate feedbacks. The importance of climate feedbacks is evident in the nature of past climate changes as recorded in ice cores up to 650,000 years old.”

“Changes have occurred in several aspects of the atmosphere and surface that alter the global energy budget of the Earth and can therefore cause the climate to change. Among these are increases in greenhouse gas concentrations that act primarily to increase the atmospheric absorption of outgoing radiation, and increases in aerosols (microscopic airborne particles or droplets) that act to reflect and absorb incoming solar radiation and change cloud radiative properties. Such changes cause a radiative forcing of the climate system …”

“Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. Positive forcing tends to warm the surface while negative forcing tends to cool it.”

SOURCE: Solomon, S., et al., eds. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press. 2007.

Based on evidence of past warming from Vostok ice core data, the combined effects of feedback factors may shift predicted average warming from 2.7–8.1°F (1.5– 4.5°C) under a doubling of atmospheric CO₂ to a range of 2.9–11°F (1.6–6°C). However, few computer models take a full range of feedbacks into consideration and several of the positive feedbacks listed earlier are still poorly understood in a global context, leading to widely variable predictions of their effects over time. Human land-use practices further complicate the picture, and may enhance or limit ecosystems' abilities to respond to or moderate climate change. More than 30% of Earth's land surface is used by humans for farming and grazing, and increases or decreases in the deforestation associated with these practices may play a significant role in the future effects of climate change.

See Also Agriculture: Contribution to Climate Change; Albedo; Anthropogenic Change; Arctic Melting: Greenland Ice Cap; Arctic Melting: Polar Ice Cap; Carbon Cycle; Carbon Dioxide (CO₂); Climate Change; Clouds and Reflectance; Forests and Deforestation; Great Conveyor Belt; Ice Core Research; Permafrost; Wildfires.

BIBLIOGRAPHY

Periodicals

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Cox, Peter, et al. “Acceleration of Global Warming Due to Carbon-Cycle Feedbacks in a Coupled Climate Model.” Nature 408 (November 9, 2000): 184–187.

Cramer, Wolfgang, et al. “Global Response of Terrestrial Ecosystem Structure and Function to CO₂ and Climate Change: Results from Six Dynamic Global Vegetation Models.” Global Change Biology 7 (April 2001): 357–373.

Field, Christopher B., et al. “Feedbacks of Terrestrial Ecosystems to Climate Changes.” Annual Review of Environment and Resources 32 (2007): 1–29.

Karl, Thomas R., and Kevin E. Trenberth. “Modern Global Climate Change.” Science 302 (December 5, 2003): 1719–1723.

Oechel, Walter C., and George L. Vourlitis. “The Effects of Climate Change on Land-Atmosphere Feedbacks in Arctic Tundra Regions.” TREE 9 (1994): 324–329.

Overpeck, J. T., et al. “Arctic Ecosystems on Trajectory to New, Seasonally Ice-Free State.” EOS 86 (2005): 309–316.

Powlson, David. “Will Soil Amplify Climate Change?” Nature 433 (January 20, 2005): 204–205.

Running, Steven W. “Is Global Warming Causing More, Larger Wildfires?” Science 313 (August, 18, 2006): 927–928.

Sturm, Matthew, and Tom Douglas “Changing Snow and Shrub Conditions Affect Albedo with Global Implications.” Journal of Geophysical Research 110 (September 2005): G01004.

Torn, Margaret S., and John Harte “Missing Feedbacks, Asymmetric Uncertainties, and the Underestimation of Future Warming.” Geophysical Research Letters 33 (May 2006): L10703.

Walker, Gabrielle. “The World Melting from the Top Down.” Nature 446 (April 12, 2007): 718–721.

Web Sites

“Feedback Loops in Global Climate Change Point to a Very Hot 21^st Century.” ScienceDaily, May 22, 2006. < http://www.sciencedaily.com/releases/2006/05/060522151248.htm> (accessed November 17, 2007).

Sarah Gilman