The warming of the Earth has been the subject of intense debate and concern for many scientists, policy-makers, and citizens.
Surface temperature measurements recorded daily at hundreds of locations for more than 100 years indicate that the Earth’s surface has warmed by about 1 degree Fahrenheit in the past century. This warming has been particularly strong during the last 20 years, and has been accompanied by retreating glaciers, thinning arctic ice, rising sea levels, lengthening of growing seasons for some, and earlier arrival of migratory birds. In addition, several other data support that conclusion.
Part of the debate over global warming centres on disparities between the surface temperature and upper-air temperature. While the Earth’s surface temperature has risen, data collected by satellites and balloon-borne instruments since 1979 indicate little if any warming of the low-to mid- troposphere.
Concentrations of greenhouse gases will accelerate and conservative assumptions about how the climate will react to that, computer models suggest that average global surface temperatures will rise between 2.5 and 10.4 degrees Fahrenheit and 1.4 and 5.8 degrees Celsius by the end of this century. Projecting how the world’s climate will change in the future requires estimating the amount of greenhouse gases produced by burning fossil fuels and other human activities. These gases often remain in the atmosphere for many years, trapping radiation that would naturally escape into the atmosphere. Several components of climate change identified that are highly uncertain and make it difficult to predict future changes.
Global warming in the last 50 years is likely the result of increases in greenhouse gases, which accurately reflects the current thinking of the scientific community, the committee said. However, it also cautioned that uncertainties about this conclusion remain because of the level of natural variability inherent in the climate on time scales from decades to centuries, the questionable ability of models to simulate natural variability on such long time scales, and the degree of confidence that can be placed on estimates of temperatures going back thousands of years based on evidence from tree rings or ice cores.
Almost all of the major greenhouse gases—with the (CFCs) — have both natural and human-induced sources. For example, carbon dioxide is not only formed by the decay in plant matter, but also by the burning of coal, oil, natural gas, and wood. And atmospheric methane can be formed by growing rice, raising cattle, coal mining, using land-fills, and handling natural gas. Both carbon dioxide and methane are more abundant in the Earth’s atmosphere now than at any time during the past 400,000 years, the report says. Carbon dioxide is probably the single most important agent contributing to climate changes today, the report says. In addition, the other greenhouse gases combined contribute to climate changes approximately equal to that of carbon dioxide.
The cooling trend in the Earth’s stratosphere— documented by satellite data since 1979—is so pronounced that it would be difficult to explain through natural variability alone. The cooling is believed to be partially a result of the buildup of greenhouse gases and the depletion of stratospheric ozone, which warms the atmosphere at low levels but cools it at high levels. The use of CFCs—which were employed in a variety of industrial applications including refrigeration, air conditioning, and aerosols— was banned in 1996 by the Montreal Protocol. This cooling could delay or perhaps temporarily reverse the recovery of the stratospheric ozone layer, which was the intended goal of banning the use of CFCs.
Climate change and its consequences are likely to have strong regional effects, but regional projections can vary substantially from climate model to climate model, NACCI says. Some models predict increased tendency for drought in some regions and higher rainfall in others. In the near term, agriculture and forestry may benefit, but hotter and drier conditions increase the potential for crop distributions to change. Areas around the country that already have water shortages or water quality problems could see these problems exacerbated.
The impact of climate change on human health is the subject of intense debate.
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