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The climate changes expected from increased atmospheric concentrations of greenhouse gases are likely to have widespread effects, many of them negative, on ecological systems, human health, and socioeconomic sectors. In general, people in developing countries are more vulnerable to climate change because of limited infrastructure and capital and greater dependence on natural resources. Unless otherwise specified, the impacts and vulnerabilities discussed below are based on scenarios of doubling of current levels of CO2 by 2100 (700 ppm). Beyond such concentrations, impacts appear to worsen, but uncertainty about what will happen increases. In general, uncertainty cuts both ways: outcomes could be less dramatic than expected based on our current understanding, but could just as well be much more severe. Worsening Health Effects - Climate change will impact human health in
a variety of ways. Warmer temperatures increase the risk of mortality from heat
stress. For example, in July 1995, 465 deaths in Chicago were attributed to
a heat wave with temperatures exceeding 90o F day and night. Today,
such events occur about once every 150 years. By 2100, they could become six
times more frequent. The potential increases in the heat index, a calculation
combining temperature and humidity, illustrate the magnitude of this threat.
Washington, D.C. currently has an average July heat index of 85o
F, but if CO2 levels reach 550 ppm (double the preindustrial level), this could
increase to 950 F, and if concentrations quadrupled to 1100 ppm,
it could increase to 110o F. Climate change will also exacerbate
air Diseases that thrive in warmer climates, such as malaria, dengue and yellow fevers, encephalitis, and cholera, are likely to spread due to the expansion of the ranges of mosquitos and other disease-carrying organisms and increased rates of transmission. This could result in 50 million to 80 million additional malaria cases per year worldwide by 2100. Rising Sea Level - Rising sea level erodes beaches and coastal wetlands, inundates low-lying areas, and increases the vulnerability of coastal areas to flooding from storm surges and intense rainfall. By 2100, sea level is expected to rise by 6 to 37 inches. A 20-inch sea level rise will result in substantial loss of coastal land in the United States, especially along the southern Atlantic and Gulf coasts, which are subsiding and are particularly vulnerable. The oceans will continue to expand for several centuries after temperatures stabilize. Because of this, the sea level rise associated with CO2 levels of 550 ppm (double pre-industrial levels) could eventually exceed 40 inches. A CO2 level of 1100 ppm could produce a sea level rise of 80 inches or even more, depending on the extent to which the Greenland and Antarctic ice sheets melt.
Disruption of the Water Cycle - Among the most fundamental effects of climate change are intensification and disruption of the water cycle. Droughts and floods - Intensification of the water cycle will produce more severe droughts in some places and floods in others. Such events are costly. Damages from the Southern Plains drought of 1996 were estimated at $4 billion, the 1993 Mississippi River flood damages at $10 billion to $20 billion, the Pacific Northwest floods in the winter of 1996-1997 at about $3 billion, the 1997 Ohio River flood at about $1 billion, and the 1997 Red River flood in the Northern Plains at about $2 billion. Water quality and quantity - Areas of greatest vulnerability are those
where quality and quantity of water are already problems, such as the arid and
semi-arid Climate change would likely increase water supply problems in several U.S. river basins, such as the Missouri, Arkansas, Texas Gulf, Rio Grande, and Lower Colorado. Water scarcity in the Middle East and Africa is likely to be aggravated by climate change, which could increase international tension among countries that depend on water supplies originating outside their borders. Changing Forests and Natural Areas - Climate change could dramatically alter the geographic distributions of vegetation types. The composition of one-third of the Earth's forests would undergo major changes as a result of climate changes associated with a CO2 level of 700 ppm. Over the next 100 years, the ideal range for some North American forest species will shift by as much as 300 miles to the north, far faster than the forests can migrate naturally. Economically important species, such as the sugar maple, could be lost from New England by the end of the next century. Such changes could have profound effects on the U.S. system of national parks
and refuges, leading to reductions in biological diversity and in the benefits
provided by ecosystems, such as clean water and recreation. Wetlands are particularly
at risk. The wetlands of the prairie pothole region, which support half Challenges to Agriculture and the Food Supply - Climate strongly affects
crop yields. A CO2 concentration of 550 ppm is likely to increase crop yields
in some areas by as much as 30 percent to 40 percent, but it will decrease yields
in other places by similar amounts, even for the same crop. A warmer climate
would reduce flexibility in crop distribution and increase irrigation demands.
Expansions of the ranges of pests could also increase vulnerability and result
in greater use of
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