How might we stabilize the composition of the atmosphere? That question looms large in the eyes of scientists and political leaders as the levels of carbon dioxide grow. Carbon dioxide and methane (甲烷) have long lives in the atmosphere and, once they are there, the earth may be destined to become warmer. If we find that the climate is becoming too warm, there is no easy or rapid way to remove the gases and return to an earlier climate pattern. We may be able to control fossil fuel use and rates of deforestation (砍伐森林), but there is no direct way to control the acceleration of decay (腐烂) except by stopping the warming. To stabilize the composition of the atmosphere immediately, we would have to cut present releases by about 4 billion tons of carbon annually. It is not now possible to accomplish this without reducing both deforestation and our consumption of fossil fuels. Most scientists belive that if immediate global action is not taken, the rapid increases of atmospheric carbon from decay will exceed the reductions made possible through control of fossil fuel use and management of forests. In 1990, the United Nations gathered many scientists from around the world to review these issues. This group, the Intergovernmental Panel on Climate Change, concluded that an immediate 60 percent reduction in fossil fuel use would be necessary. But by April 1992, no nation had accepted the goal, though several had recognized a need for reducing emmisions by 20 percent. At the Earth Summit in June 1992, leaders from most industrialized nations agreed in principle to return to earlier levels of carbon dioxide emissions — though opposition from the United States prevented them from agreeing to specific targets for emissions, as many scientists had hoped. The leaders also agreed to assist developing nations in limiting their release of greenhouse gases. Human beings are only one of millions of organisms sufficiently different from one another to be recognized as species. Until the 1980s, scientists estimated that there were between 3 million and 10 million species on the earth. Then, scientists began to examine populations of insects living in the high foliage (叶子) of trees in tropical forest, and the experience caused them to increase their estimate to 30 million species. After scientists began to consider the populations of microorganisms in the tropics, the upper limit of their estimates rose to 100 million species. We shall probably never have an accurate count of the different kinds of organisms that share out planet. We do know, however, that these species — plants and animal together — keep the planet functioning as a habitat suitable for all. We also know that human activities are reducing both the numbers of species on the earth and the potential amount of land and water for supporting them. This process is commonly called biotic impoverishment (贫穷) — the loss of the rich biological potential of the earth.