Through the study of ancient ice cores from Antarctica it is possible to compare atmospheric concentrations of the dominant greenhouse gas, carbon dioxide (CO2) in the atmosphere with temperature variations over the past 400 thousand years of the earth's history. A visual comparison of the two trends indicates a very tight connection between their performance, with fluctuations in one plot almost exactly mirrored in the other for more than 400 thousand years. as the Industrial Revolution takes off, atmospheric CO2 concentrations begin an unprecedented upward climb, rising rapidly from 280 ppmv (parts per million by volume) in the early 1800s to a current level of 376 ppmv, 77 ppmv above the highest concentrations previously attained in the course of the preceding 400 thousand years.
Noting these trends, and recognizing the potential for
dramatic changes in the climate due to continued unchecked accumulation of
greenhouse gases in the atmosphere, the World Meteorological Organization
(WMO) and the United Nations Environment Program (UNEP) established the
Intergovernmental Panel on Climate Change (IPCC) in 1988. The purpose of
the IPCC was to objectively review existing and developing peer-reviewed
scientific literature to form an objective evaluation about the risk of
human-induced climate change.After years of investigation and in consultation with thousands of
scientists, the IPCC was able to write, in its Second Assessment Report in
1995, that climate has changed over the past century and that the
twentieth century had a mean temperature “at least as warm as any other
century since 1400 A.D.” Their report noted that the dramatic increase in
carbon dioxide concentration in the atmosphere over the past 150 years
(from about 280 parts per million to about 376 parts per million) is
largely due to anthropogenic (human-caused) effects and concluded that
“the balance of evidence suggests a discernible human influence on global
climate.” Their models predicted a rise of 1.8 to 6.3 degrees Fahrenheit
in the global mean surface temperature during the next century, with
sea-levels expected to rise by 6 inches to 3 feet by 2100. (IPCC
1995). The conclusions of the IPCC gained broad support in the world
scientific community and, in the summer of 1997, a letter
signed by 2,600 scientists called for the United States to take a
leadership role in reducing greenhouse gas emissions to diminish the
likelihood of intense, continuous global warming. Improved Models, Growing Confidence
The Third Assessment Report of the IPCC was released in
2001, incorporating new research undertaken in the five years since the
Second Assessment Report. Increased confidence in evolving modeling
techniques lent added weight to the linkage between rising temperatures
and continued greenhouse accumulations.
Other global events are suggestive of climatic changes that are likely to become more prevalent under a changing global climate regime. Glaciers are present on every continent other than Australia and function as reasonably well-distributed indicators of changing global temperatures. Worldwide, glaciers and icefields have been shrinking and receding for at least the last century. The collapse of the 1250 square mile Antarctic Larsen B ice shelf in 2002 was just one of the more spectacular instances of a phenomenon that is likely to become more frequent in a warmer world. Melting of the Antarctic ice sheet is an event long predicted by climate scientists as an indication of a warming atmosphere (Gelbspan 1997). The northern Arctic region appears to be even more vulnerable than the Antarctic (which may actually see increases to its ice sheet due to increased precipitation under a changing climate regime) and in a 2004 report by the Arctic Monitoring and Assessment Programme (AMAP), (Impacts of a Warming Arctic: Arctic Climate Impact Assessment), the list of Arctic change due to warming includes such phenomena as decreases in sea ice, increasing precipitation and river discharge, thawing of glaciers and permafrost, and changes in plant and animal abundances and distributions. While it is impossible to establish a direct causal link between greenhouse gas accumulation and individual, relatively short-term climatic events, it is certain that we have been experiencing increasing numbers of climatic events unprecedented in the human experience.
It is also certain that many of the greenhouse gases, including carbon dioxide and methane, have lengthy residence times in the atmosphere and that we will continue to be affected for years or even centuries to come by the atmospheric burden we are creating today. (For comments on the role of scientific uncertainty in climate change policy, see Dr. John Holdren's address at the White House Conference on Climate Change October 6th, 1997.)
Source: http://www.whrc.org
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| Global warming - Greenhouse effect 
