Predictions And Evidence Of Global Warming
Most atmospheric scientists assume that the well-documented increase in greenhouse gases will result in an intensification of Earth's naturally occurring greenhouse effect, and to global warming. The exact climatic response to increased concentrations of radiatively active gases, and its potential effects on humans are, however, difficult to measure or predict. However, if global warming were to occur as most scientific studies predict, it would have substantial climatic, ecological, and sociopolitical consequences.
The Earth's surface is surface temperature is extremely 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 CO2concentration 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 1998, more than half of the world's population, some 3.2 billion people, lived with in 120 miles of the ocean. 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 policy-makers, now consider global warming to be a credible threat to the Earth's natural environment and human 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 socioeconomic 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 November 2002, 181 nations had signed, ratified, or acceded to the conditions of the Kyoto protocol. However, the United States, by far the world's largest per-capita producer of greenhouse gases, did not sign the treaty on the grounds that the science of global warming remains inconclusive, and that the economic consequences of action would be too great.
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