A Closer Look at Global Warming
A Closer Look at Global Warming
The warming of the Earth has been the subject of intense debate and concern for many scientists, policy-makers, and citizens for at least the past decade. Climate Change Science: An Analysis of Some Key Questions, a new report by a committee of the National Research Council, characterizes the global warming trend over the last 100 years, and examines what may be in store for the 21st century and the extent to which warming may be attributable to human activity. The committee was made up of 11 of the nation's top climate scientists, including seven members of the National Academy of Sciences, one of whom is a Nobel Prize winner.
The Evidence for Warming
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, the report says.
Part of the debate over global warming centers 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. The report concurs with a previous Research Council report that said despite these differences, "The warming trend in the global mean surface temperature observations during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming in the 20th century."
Read about a scientist's efforts to study disappearing glaciers in Alaska.
Learn how scientists use data from tree rings, ice cores, and other sources to reconstruct climate and temperature before the advent of climate-observing networks from the National Oceanic and Atmospheric Administration's paleoclimatology program.
Uncertainties in Climate Predictions
Based on assumptions that 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 ...
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Read about a scientist's efforts to study disappearing glaciers in Alaska.
Learn how scientists use data from tree rings, ice cores, and other sources to reconstruct climate and temperature before the advent of climate-observing networks from the National Oceanic and Atmospheric Administration's paleoclimatology program.
Uncertainties in Climate Predictions
Based on assumptions that 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. The report identifies several components of climate change that are highly uncertain and make it difficult to predict future changes.
The New Scientist discusses many of the uncertainties at the heart of the scientific debate on global warming.
The Intergovernmental Panel on Climate Change (IPCC) concluded that 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.
The report urges the establishment of a vigorous program of basic research to reduce uncertainties in future climate projections. In addition, a global observing system that monitors long-term climate predictions is needed.
Are We Changing the Climate?
The Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, and surface temperatures have risen at a substantially greater rate than average in the past two decades. The changes observed over the last several decades are likely because of human activities, for the most part. But it is not known how much of the temperature rise to date is the result of human activities, the report says. Climate models do not adequately represent all the processes that contribute to variability of the climate system. A Research Council report, Improving the Effectiveness of Climate Modeling, identifies the lack of a coherent national climate modeling program and sufficient computing resources and suggests areas for improvement.
Almost all of the major greenhouse gases -- with the exception of chlorofluorocarbons (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 Environmental Protection Agency (EPA) examines several greenhouse gases in detail, along with their potential to contribute to global warming.
The report notes that 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.
The Possible Consequences
The U.S. National Assessment of Climate Change Impacts (NACCI) summarizes the potential consequences of climate change. The NACCI concludes that the United States is likely to be able to adapt to most of the climate change impacts on human systems, but these adaptations may come with significant cost for some regions.
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. Will a warmer climate change the amount and distribution of mosquitoes, ticks, rodents, and other agents that carry infectious diseases? Or will worsening air quality lead to more asthma or lung disease? Under the Weather: Exploring the Linkages Among Climate, Ecosystems, and Infectious Disease, a recent Research Council report, concluded that the United States appears to be protected against adverse health outcomes by a strong public health system, a high standard of living, and high levels of public awareness.
The International Human Dimensions Programme on Global Environmental Change studies the effects of environmental change on society. The Research Council's Committee on Global Change Research provides links to a number of international organizations tracking climate change.
Back to Basics
Can't tell the difference between greenhouse gases and the greenhouse effect? EPA provides a complete glossary of terms frequently used in climate change science.
Looking for a concise summary of the issues surrounding climate change? The New Scientist and EPA answer frequently asked questions.
Get an overview of the issues from PBS.
Figure out how much you contribute to greenhouse gas emissions and what you can do to reduce them.
The Research Council's committees on global change research and climate research examine a number of issues related to climate change.
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