Global Warming

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Global Warming

Introduction

Global warming is a long-term increase in Earth’s average surface temperature. Because global warming does not cause uniform warming in all locations and because many other changes in climate are occurring, scientists often prefer to speak of “global climate change” rather than of global warming when referring to the whole complex of climate changes being caused by human activities. Thus, scientists often reserve the phrase “global warming” for higher temperatures as such, apart from other climate changes. However, in general usage it still refers to the sum of all changes occurring in Earth’s climate as a result of human activities. A major effect of global warming is redistribution of climatic zones as defined by temperature, precipitation, and associated ecosystems.

Global climate changes, including episodes of global cooling and warming, have occurred many times throughout Earth’s history as a result of natural variations in solar radiation, atmospheric chemistry, oceanic and atmospheric circulations, volcanic eruptions, and other factors. Global warming in the last few decades, however, is primarily caused by human activities that started during the Industrial Revolution, when human burning of fossil fuels began to increase drastically, releasing large amounts of carbon dioxide (CO₂). Atmospheric concentrations of CO₂, methane (CH₄), nitrous oxide (N2_O), and artificial chemicals called halocarbons have long been increasing as a result of emissions from fossil-fuel burning and other activities. Increased atmospheric concentrations of these gases are causing Earth to warm.

As of 2008, atmospheric levels of CO₂ and CH₄ were about a third higher than at any time in at least 650,000 years, as shown by air bubbles trapped in ancient Antarctic ice. Global average surface temperature had increased by about 0.23°F (0.13°C) per decade for the last 50 years, and had increased more rapidly in recent years: The two warmest years since instrumental records began in the 1800s were 1998 and 2005.

Although a few scientists continue to doubt that recent climate warming is caused by human activities, this is no longer disputed by the great majority of climatologists, meteorologists, and geophysicists. The 2007 Assessment Report of the United Nations’ Intergovernmental Panel on Climate Change (IPCC) pronounced that global warming is now “unequivocal” and that it is more than 90% likely that most of the global warming observed since the mid-twentieth century is caused by anthropogenic releases of greenhouse gases.

Historical Background and Scientific Foundations

Solar radiation is the major source of energy arriving at Earth’s surface (a small fraction, about 1/7500, comes from Earth’s interior). Much of this incoming energy is at short wavelengths (ultraviolet and visible light). These forms of light are absorbed by Earth’s surface, where their energy drives atmospheric and oceanic circulations and biological processes like photosynthesis. The surface then re-radiates most of this energy as longer-wavelength light, variously termed heat radiation, infrared light, or infrared radiation. If Earth’s atmosphere were completely transparent to infrared radiation, this re-radiated energy would simply shine out into space and the planet would have an average surface temperature of about 0°F (–18°C).

However, certain gases in the atmosphere, mostly notably CO₂, CH₄, and water vapor (H₂O), absorb some of the infrared light radiated by Earth’s surface and are warmed. This warms the atmosphere directly and warms Earth’s surface both by contact with warmer air and by re-radiation of infrared radiation from the atmosphere. The overall effect is to slow the escape of energy from Earth, warming the planet like an invisible blanket. Gases that have this effect are termed greenhouse gases because, like the glass windows of a greenhouse, they allow solar energy to enter Earth’s atmosphere but impede the loss of heat. Earth’s atmosphere naturally contains certain levels of greenhouse gases. The resulting natural greenhouse effect maintains the planet’s average temperature at a livable 59°F (15°C). This natural greenhouse effect has been crucial to the evolution and survival of life on Earth. Today’s average surface temperature of 59°F is sufficient to maintain most of Earth’s water in liquid form, but 0°F (-18°C) would be too cold for most organisms to live or for ecological processes to function.

Although atmospheric levels of CO₂ and CH₄ have varied greatly over geologic time (hundreds of millions of years), by the time of the beginning of the Industrial Revolution around 1750, their concentrations had been fairly stable for hundreds of thousands of years. The pre-industrial concentration of CO₂ was about 280 parts per million by volume (ppm), of CH₄ about 0.7 ppm, and of NO₂ about 0.285 ppm. (The special case of water vapor is discussed later.) As of 2005, the atmospheric concentration of CO₂ had increased to about 379 ppm, a 36% increase. The concentration of CH₄ has gone up to 1.8 ppm and that of N₂O to 0.319 ppm. Concentrations of chlorofluorocarbons (CFCs) and other completely human-made greenhouse gases have increased from zero in pre-industrial times to about 0.7 ppb (parts per billion by volume).

Atmospheric concentrations of greenhouse gases have increased more quickly since the middle of the twentieth century, coinciding with rapid human population growth and global industrialization. The largest single cause of greenhouse gas releases has been the burning of fossil fuels, including oil, natural gas, and coal. Fossil fuels contain carbon which, in burning, combines with oxygen to create CO₂.

Deforestation is the second-largest emitter of greenhouse gases. As they grow, trees, like all plants, take in CO₂, incorporate the carbon in their structure, and release O₂ into the atmosphere. Deforestation therefore destroys a carbon sink that tends to lower the atmospheric concentration of CO₂. Deforestation also releases CO₂ and CH₄ through the burning and decay of trees and soil carbon in deforested areas.

Agriculture is a primary cause of deforestation. It also releases large quantities of CH₄ through decomposition of organic materials in livestock digestive systems, livestock waste-treatment facilities, and flooded rice fields. Agricultural fertilizers and combustion of fossil fuels and solid wastes are the primary sources of increased N₂O emissions.

Industrial processes emit a variety of powerful synthetic greenhouse gases, including chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons

WORDS TO KNOW

ANTHROPOGENIC: Made by humans or resulting from human activities.

CARBON SINK: A location like a forest where there is net storage of carbon as sequestration exceeds release.

INFRARED RADIATION: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all member countries with the goal of mitigating climate change.

PHOTOSYNTHESIS: The process by which plants fix carbon dioxide from the atmosphere using the energy of sunlight.

RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation, resulting in warming or cooling of Earth’s surface.

RESIDENCE TIME: For a greenhouse gas, the average amounof time a given amount of the gas stays in the atmosphere before being absorbed or chemically altered.

(PFCs), and sulfur hexafluoride (SF₆). Many of these chemicals also attack the ozone layer, and their manufacture and use has been limited by the Montreal Protocol treaty of 1987.

Greenhouse gases vary greatly in their ability to absorb infrared radiation. On a per-molecule basis, methane is about 21-40 times more absorptive than carbon dioxide, nitrous oxide is 200-270 times more absorptive, and CFCs are 3,000-15,000 times more absorptive. CO₂, however, is present in the atmosphere at much higher concentrations than the more-powerful greenhouse gases and has experienced the greatest increases due to human activity. CO₂ is responsible for about 60% of the human contribution to increased atmospheric heat retention, termed radiative forcing.

Water vapor is responsible for more of the greenhouse effect than any other single gas, but is not a primary driver of global warming. This apparent contradiction is explained by the fact that the residence time of water vapor in the atmosphere is short. Increased water vapor concentrations, left to themselves, would disappear in a matter of weeks as rain or snow fall from clouds. The other greenhouse gases, however, linger for decades: These gases force the atmosphere to warm, increasing its ability

to hold water vapor. This increased level of water vapor acts to warm the world further. Therefore, water vapor acts as a feedback factor in global warming.

Impacts and Issues

Most scientists today are convinced that an increase in greenhouse gases is resulting in an intensification of Earth’s greenhouse effect, with resulting global warming and climate change. The exact climatic response to increased concentrations of greenhouse gases and its potential effects on humans are difficult to predict, but can be foretold in outline. Moreover, computer models of climate are constantly improving, allowing more realistic predictions to be made about future climate change. If global climate change proceeds as recent scientific studies forecast, it will likely have substantial negative ecological consequences.

Earth’s surface temperature is variable from place to place and over time. Furthermore, the systems that interact to maintain the planet’s temperature and climate are complex; cause-and-effect relationships between the oceans, atmosphere, land, living things, and nonliving chemistry are numerous. In spite of these scientific challenges, there is overwhelming evidence that Earth has warmed significantly during the past 150 years or so. Climate records show that from 1906 to 2005 there was a 1.3°F (0.74°C) increase in Earth’s average surface temperature, with twice as much warming as this global average occurring in the Arctic and over the West Antarctic Peninsula. The oceans have also warmed, though not as much.

Recent observations show dramatic melting of the Arctic sea ice, accelerated melting of the Greenland ice cap, and accelerated seaward flow of glaciers on the western peninsula of Antarctica. Global sea-level has risen since the mid-1800s; for 1961–2003, at about .07 in (1.8 mm) per year. Alterations have been seen in large-scale weather phenomena like the southeast Indian monsoon season, Atlantic hurricane season, El Niño/ Southern Oscillation, and North African drought cycle. Wildlife and plants are shifting their ranges to higher latitudes (closer to the poles, to track cool weather as climate warms) and to higher altitudes in mountainous regions. Many other types of observations, some of which are reviewed elsewhere in this book, independently demonstrate the reality of global warming and the complex changes it has produced in regional climates.

The aforementioned observations agree with predictions by computerized mathematical models of global climate processes. These three-dimensional general circulation models (GCMs) simulate the complex movements of energy and mass involved in the global circulation of the atmosphere, oceans, and living things. Scientists use GCMs to predict the effects of changes in specific variables, like the atmospheric concentration of CO₂, on the rest of the global climate system. Because of the complexity of the computational problem, GCMs that attempt to predict global climate change have some what variable results. However, modelers now agree that increased concentrations of atmospheric greenhouse gases have resulted, and will continue to result, in global warming. For example, one GCM that doubles the present CO₂ concentration to about 700 ppm predicts a 2-6°F (1.1-3.3°C) rise in global temperature and suggests that the warming would be 2 to 3 times more intense at high latitudes—nearer the poles—than in the tropics. These predictions are consistent with changes seen so far, or, if anything, tend to underestimate the pace of warming.

Other predicted consequences of warming include large-scale shifts in atmospheric and oceanographic circulation patterns, partial melting of the polar ice caps, global sea-level rise, reorganization of Earth’s climatic zones, and establishment of new large-scale weather patterns. Changes in the distribution of heat, precipitation, and weather phenomena like storms and floods will affect the productivity and distribution of natural and managed vegetation. Animals and microorganisms will experience dramatic changes in their habitats and face higher rates of species extinction. According to the Intergovernmental Panel on Climate Change (IPCC), a rise of about 4°F (2.2°C) above the pre-industrial average will likely commit 15-37% of all plant and animal species to extinction. Most biologists agree that global warming is a serious threat to biodiversity and to the health of ecosystems worldwide.

The predicted climatic and biological changes associated with anthropogenic global warming are likely to have negative outcomes for Earth’s human population, although it is likely that some regions will be more severely affected than others. As of 2000, about 400 mil-

IN CONTEXT: AUTHORITATIVE ARGUMENTS AND EVIDENCE ABOUT GLOBAL WARMING

“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.”

…“Eleven of the last twelve years (1995–2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).”

SOURCE: Solomon, S., et al, eds. “IPCC, 2007: Summary for Policymakers.” In: 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.

lion people lived within 65.6 ft (20 m) of sea level and within 12.4 mi (20 km) of a coast. Even small increases in global sea level, accompanied by increasingly intense coastal storms and floods, can therefore threaten the lives and property of a large number of people. Changes in regional temperature, precipitation, and extreme weather events would affect the managed agriculture, fishing, and forestry that provide food and shelter for Earth’s human population.

Most scientists and many international policymakers now consider global warming to be a credible threat to

Earth’s natural environment and human population. However, attempts to prevent anthropogenic global warming, especially measures that require socioeconomic sacrifice or unfamiliar forms of behavior, have been controversial. The 1997 Kyoto Protocol—an extension of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by most nations—acknowledges that human activities can alter global climate and commits signatory nations to reducing their greenhouse-gas emissions. As of January 2008, 177 nations had ratified the Kyoto Protocol. The United States, the world’s second largest producer of greenhouse gases (after China) and one of the biggest emitters on a person-by-person basis, was the only industrialized country in the world to have not ratified the treaty. (Australia, another holdout, signed in late 2007.) The U.S. government argued that the science of global warming remains inconclusive and that the economic consequences of action would be too great. Negotiations for a follow-up treaty to the Kyoto Protocol began in November 2007.

Although there are ongoing research efforts to study methods of carbon capture and storage to reduce greenhouse-gas emissions, most proposals to inject large quantities of CO₂ into underground reservoirs or oceans have, so far, met with economic, technological, and environmental barriers.

Increased extreme weather is a predicted consequence of global climate change. In August 2007, scientists at the World Meteorological Organization, an agency of the United Nations, announced that during the first half of 2007, Earth showed significant increases above long-term global averages in both high temperatures and frequency of extreme weather events, including heavy rainfalls, cyclones, and wind storms. Global average land temperatures for January and April of 2007 were the warmest recorded for those two months since records began in the 1800s.

BIBLIOGRAPHY

Books

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, PA: Rodale Press, 2006.

Lal, Rattan, et al. Climate Change and Global Food Security. New York: CRC, 2005.

McCaffrey, Paul. Global Climate Change. Minneapolis, MN: H. W. Wilson, 2006.

Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

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.

Weart, Spencer. The Discovery of Global Warming. Cambridge, MA: Harvard University Press, 2004.

Web Sites

Intergovernmental Panel on Climate Change (United Nations). “Intergovernmental Panel on Climate Change.” 2008. http://www.ipcc.ch (accessed March 24, 2008).

United Nations. “United Nations Framework Convention on Climate Change.” 2008. http://unfccc.int/2860.php (accessed March 24, 2008).

U.S. Environmental Protection Agency. “Climate Change.” March 14, 2008. http://epa.gov/climatechange/index.html (accessed March 24, 2008).

Larry Gilman

Environmental Science: In Context