CAMBRIDGE: This month the United States Government issued a path-breaking report on the impact of long-term climate change on America's society and environment. We know that human activity is causing serious and complex changes in the global climate, mainly through the effects of burning fossil fuels like oil and gas, and the effects of deforestation. These activities raise the level of carbon dioxide in the atmosphere, which in turn has many effects: a rise in average temperature, a rise in the water level of the oceans, significant changes in the global patterns of rainfall, and an increase in "extreme weather events" such as hurricanes and droughts. The new US study is the first systematic attempt to understand the long-term consequences of these climatic trends in a single country. It is a project of such significance and scope that it should quickly be followed up by similar studies in other parts of the world.

The new report, "Climate Change Impacts on the United States" (available on the website www.gcrio.org) is a remarkable accomplishment, even though it is really only one step in a long-term effort to understand the interactions of climate, environment, and human society. It was produced by a huge team of scientists working in different disciplines and different parts of the United States. If there is one main lesson, it is that long-term climate change is for real, and is likely to have major impacts on US society. If there is a second lesson, it is that those impacts are complex and varied, and depend very much on the characteristics of particular regions. What is good for one part of the country, or indeed one part of the world, might be a disaster for another.

The report stresses that climate change will have several interacting elements. The rise of carbon dioxide in the atmosphere B which is the main cause of the long-term climate change B might actually have some directly beneficial effects, since a higher concentration of carbon dioxide can stimulate the faster growth of some types of forests and some crops. On the other hand, the effects on temperature, rainfall, ocean levels, flooding and droughts, and other climate patterns, will hurt some regions, while even helping some others. Northern regions, for example, could enjoy a longer growing season for some crops, while more Southerly regions, which are already hot, might suffer from the adverse effects of rising temperatures.

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