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The U.S. Global Climate Response Index (GCRI) is comprised of five different indicators of weather conditions over the United States. It can be used as a tool to determine whether U.S. weather patterns have changed in a manner consistent with global greenhouse warming. The expected value for the GCRI in any given year under stable conditions is 10%. As depicted in Figure 4.26, the GCRI has averaged 13.3% since 1980; this is 3.3% above the average from previous years in the century. Although there is a 5 to 10% chance that this trend could occur under stable conditions, the tendency toward increased values of the U.S. GCRI over the past two decades suggests that the climate of the United States is changing in ways that would be expected with global greenhouse warming.
The Intergovernmental Panel on Climate Change (established jointly by the United Nations Development Program and the World Meteorological Organization) has stated that observations suggest “a discernible human influence on global climate.” Since the turn of the century, the average daily temperature in the United States (for the 48 conterminous states) has risen by about 0.3o Centigrade (0.7o Fahrenheit), and most of the increase has occurred in the last 30 years. This trend is consistent with the best estimates for the Earth as a whole over the same period of time. Changes in global climate could lead to changing weather patterns and rising sea levels and have major impacts on ecosystems, agriculture, transportation, water resources management, and communities.
If greenhouse warming is indeed occurring, we would expect to see changes in local weather patterns over a period of years. The United States has some of the most complete and continuous meteorological records in the world, and we can examine these records to see if weather patterns have changed as we would expect with greenhouse warming. Although the United States covers only 2% of the Earth’s surface, weather trends we detect in the United States could also be a bellweather of global change.
It should be noted that weather patterns can change due to random events or to human-induced or natural changes over the long term. The challenge for scientists is to separate the background noise from meaningful signals. By examining a number of weather variables together over a period of years, we can begin to see the whole picture.
The Greenhouse Climate Response Index (GCRI) combines the following five climate indicators into a single index. The weight of each indicator increases in proportion to our confidence in the projected changes in weather and climate due to rising atmospheric concentrations of greenhouse gases. The five indicators are:
(1) the percent of the United States with much above normal temperature (i.e., temperatures falling in the upper tenth percentile of the local, century-long record)
(2) the percent of the United States with much above normal precipitation (i.e., precipitation in the upper tenth percentile of the local, century-long record) during the cold season (October through April);
(3) the percent of the United States in extreme or severe drought (i.e., precipitation in the lower tenth percentile of the local, century-long record) during the warm season (May through September);
(4) the percent of the United States with a much greater than normal proportion of precipitation derived from heavy rainfall events (over 2 inches or 50.8 mm); and
(5) the percent of the United States with much below normal day-to-day temperature differences.
As stated above, the expected value for the GCRI in any given year under stable conditions is 10%. Since 1980, the GCRI has averaged 13.3%, suggesting that U.S. climate is changing in a manner consistent with global greenhouse warming.
Reference:
Link(s) to be added, when feasible, to data at level of detail suitable
for use at the community level.
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http://www.sdi.gov/indicators/lc_ghg.htm Last Modified: May 13, 2002 |