Global Warming

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Global Warming

Atmospheric Concentrations of Greenhouse Gases

Predictions and Evidence of Global Warming

Global warming refers to a long-term increase in the Earth’s average surface temperature that results in large-scale changes in global climate, including redistribution of climatic zones defined by temperature, precipitation, and associated ecosystems. Global climate changes and episodes of global warming, have occurred throughout geologic history as a result of natural variations in incoming solar radiation, atmospheric chemistry, and oceanic and atmospheric circulation. Anthropogenic, or human-caused, global warming and climate change are an ongoing outcome of human activities during the last 150 years, that is, since the Industrial Revolution, when human burning of fossil fuels multiplied many times. Scientific data show that atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and human-made chemicals called halocarbons are increasing as a result of emissions associated with human activities, and models predict that this environmental change may lead to global warming. Studies of Antarctic ice cores published in 2005 showed that atmospheric carbon dioxide is now 27% higher than any other point in the last 650,000 years.

Earth’s Greenhouse Effect

Solar radiation is the major source of energy to Earth’s surface. Much of that incoming short-wavelength energy is absorbed by the surface where it drives atmospheric and oceanic circulation, and fuels biological processes like photosynthesis. The land and sea surfaces then reradiate extra longer-wavelength heat, or infrared, energy. If Earth’s atmosphere were transparent to the emitted infrared radiation, the planet would cool relatively efficiently and would have an average surface temperature of about 0°F (–18°C). However, the Earth’s naturally occurring “greenhouse effect” maintains the planet’s average temperature at a more livable 59°F (15°C) by trapping some of the escaping heat within the atmosphere. Small concentrations of so-called “greenhouse gases,” also known as radiatively active gases, absorb some of the infrared energy and thereby delay its passage to space. Water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃) are the most concentrated and effective greenhouse gases. The greenhouse effect has been extremely important to the evolution and survival of life on Earth. A surface temperature of 59° F is sufficient to maintain the Earth’s reservoirs of life-sustaining liquid water, and to impel climatic processes, whereas 0°F is too cold for most organisms to live or for ecological processes to function well.

Atmospheric Concentrations of Greenhouse Gases

Prior to the modern influence of human activities on atmospheric chemistry, the naturally occurring greenhouse gases had fairly stable atmospheric concentrations: carbon dioxide about 280 ppm (or parts per million by volume), methane 0.7 ppm, and nitrous oxide 0.285 ppm. (Human activities do not appear to affect the concentration of water vapor, which varies naturally over time.) Today, the atmospheric concentration of CO₂ has increased to about 364 ppm, while that of CH₄ is 1.7 ppm, and N₂O is 0.304 ppm. The concentrations of chlorofluorocarbons (CFCs), and other completely man-made greenhouse gases, have increased from essentially zero to about 0.7 ppb (parts per billion by volume).

Atmospheric concentrations of the greenhouse gases have increased particularly quickly since the middle of the twentieth century, coinciding with rapid human population growth and intensive global industrialization. The combined effects of fossil fuel use and deforestation have increased the atmospheric concentration of CO₂. Fossil fuels, like oil, natural gas, and coal contain carbon in their chemical structure that, when liberated by combustion, combines with oxygen to create CO₂. Trees, like all plants, take in CO₂, incorporate carbon in their structure, and emit O₂ back into the atmosphere; deforestation destroys carbon “sinks” that lower the atmospheric concentration of CO₂. Fossil-fuel mining, decomposition of organic materials in human and livestock waste treatment facilities, and flooding in rice agriculture have led to increased emissions of CH₄. Agricultural fertilizers, and combustion of fossil fuels and solid wastes account for increased N₂O emissions. Industrial processes emit a variety of powerful synthetic greenhouse gases like CFCs, hydrofluorcarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF₆).

The greenhouse gases vary greatly in their ability to absorb infrared radiation. On a per-molecule basis, methane is about 25-40 times more absorptive than carbon dioxide, nitrous oxide is 200-270 times, and CFCs are 3-15 thousand times. CO₂, however, has by far the largest atmospheric concentration, 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.

Predictions and Evidence of Global Warming

Most atmospheric scientists today, with few exceptions, are convinced that the well-documented increase in greenhouse gases is resulting in an intensification of Earth’s naturally occurring greenhouse effect, with resulting climate change. The exact climatic response to increased concentrations of radia-tively active gases, and its potential effects on humans are, however, difficult to predict. If global climate change proceeds as most recent scientific studies forecast, it will have substantial and disastrous climatic, ecological, and sociopolitical consequences.

The Earth’s surface is 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 extremely complex; cause-and-effect relationships between changes in one system, the atmosphere in this case, and results in another, global climate, are very difficult to predict, observe, and “prove.” In spite of these scientific challenges, there is significant evidence that the Earth has warmed significantly during the past 150 years or so, and that global climate has responded to the temperature increase. Climate records show a 1°F increase in the average temperature of the Earth’s oceans, atmosphere, and solid surface since the late 1900s. Geologic and historical studies document dramatic thinning and shrinkage of the polar ice caps, and retreat of Earth’s alpine glaciers. Less conclusive, but still suggestive, data supporting anthropogenic global warming include a several centimeter increase in global sea-level since 1900, and alterations in large-scale weather phenomena like the southeast Indian monsoon, Atlantic hurricane season, El Niño Southern Oscillation, and North African drought cycle.

The empirical, or observed, data listed above generally agree with predictions computed by mathematical models of global climate processes. These “virtual experiments,” called three-dimensional general circulation models (GCMs), simulate the complex movements of energy and mass involved in the global circulation of the atmosphere and oceans. Scientists use GCMs to predict the effects of a change in a specific variable, like the concentration of atmospheric 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 had somewhat variable results. However, most experiments do suggest that the increased concentration of atmospheric greenhouse gases has 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 rise in global temperature, and suggests that the warming would be 2-3 times more intense at high latitudes than in the tropics.

Other predicted consequences of warming include large-scale shifts in atmospheric and oceanographic circulation patterns, melting of the polar ice caps, global sea-level rise, reorganization of the Earth’s climatic zones, and establishment of new large-scale weather patterns. Such changes in the distribution of heat, precipitation, and weather phenomena like storms and floods would affect the productivity and distribution of natural and managed vegetation. Animals and microorganisms would experience dramatic changes in their habitats, and perhaps face much higher rates of species extinction. Most ecologists consider that global warming, if were it to occur as predicted, would represent a serious threat to biodiversity and to the health of ecosystems worldwide.

The predicted climatic and biological changes associated with anthropogenic global warming could have potentially disastrous outcomes for the Earth’s human population. In 2006, about two thirds of the world’s population, some 4 billion people, lived within 120 miles (400 km) of the coast. Even small increases in global sea level, and in the intensity of coastal storms and floods, would threaten the lives and property of large numbers of people. Changes in regional temperature, precipitation, and weather, as well as biological health, would affect the managed agriculture, fishing, and forestry that provide food and shelter for the Earth’s burgeoning human population.

Most scientists, and many international policymakers, now consider global warming to be a credible threat to the Earth’s natural environment and human

KEY TERMS

Global warming —A projected increase in Earth’s surface temperature caused by an increase in the concentration of greenhouse gases, which absorb infrared energy emitted by Earth’s surface, thereby slowing its rate of cooling.

Greenhouse effect —The warmingof Earth’s atmosphere as a result of the capture of heat re-radiated from the Earth by certain gases present in the atmosphere.

population. However, because the specific consequences of global warming are difficult to predict, and in some cases unknown, the scientific community remains divided about the potential effects of the phenomenon. Attempts to prevent anthropogenic global warming, especially measures that require socioeco-nomic sacrifice, have therefore been extremely controversial. The 1992 United Nations Framework Convention on Climate Change (UNCCC),also called the Kyoto Protocol, acknowledges that human activities can alter global climate, and requires signatory nations to reduce greenhouse gas emissions. As of October 2006, 166 nations had ratified the Kyoto protocol. The United States, by far the world’s largest net producer of greenhouse gases and one of the biggest per capita, had not signed the treaty. Neither had Australia, the world’s largest per capita producer of greenhouse gasses as of 2005. These governments argued that the science of global warming remains inconclusive, and that the economic consequences of action would be too great.