composition of the atmosphere by burning fossil fuels and clearing forests for agriculture and other uses for the past 1,000 years. But until about 100 years ago these activities had a minor effect on the global carbon cycle and the climate system (NAST, 2000). Since the late 1800s, increasing emissions associated with human actions have been responsible for a 30 percent increase in the concentration of atmospheric CO2. Many aspects of climate, including warming, have also occurred (Figure 1).

From 1950 to 1995 the developed world accounted for about three-quarters of total CO2 emissions associated with the burning of fossil fuels. In 1995, for example, 73 percent of total CO2 emissions from human activities came from developed countries (OSTP, 1995). The United States was the largest single source, accounting for 22 percent of the total, with carbon emissions per person exceeding 5 tons per year. Elsewhere in the developed world, Western Europe accounted for 17 percent, Eastern Europe and the former Soviet Union for 27 percent, and Asia for 7 percent. China was the largest single source among developing countries, accounting for 11 percent of the total, with carbon emissions per person about one-tenth those of the United States (Figure 2).

In the next few decades as much as 90 percent of the world’s population growth is expected to occur in developing countries (Figure 3), some of which will concurrently undergo rapid economic growth. Per capita energy use in developing countries, which is now only one-tenth to one-twentieth of U.S. energy use, will also rise. If present trends continue, developing countries will account for more than half of total global CO2 emissions by 2035. China—today the second-largest source of CO2 emissions—will become the largest emitter sometime between 2010 and 2015.

As scientists currently understand the global carbon cycle, natural carbon sinks in the ocean and on land eventually absorb between one-half and two-thirds of emissions from human activities. The rest of the emitted carbon remains in the atmosphere. Therefore, to reduce the rate at which CO2 accumulates in the atmosphere, we must either reduce emissions or increase carbon uptake by the land and the oceans.

Both developed and developing countries must eventually be involved in managing the emissions side of the global carbon cycle. Developed nations should lead the way because they are responsible for most of the CO2 that has accumulated in the atmosphere since the late 1800s. Developed countries must also share carbon-efficient, energy-generating technologies with the developing world as soon as possible because developing nations with rapidly growing economies, such as China, are making capital investments now in power plants with lifetimes of at least several decades. It is in everyone’s interest that these power plants be as carbon efficient as possible.

Both land and ocean ecosystems have the capacity to store or sequester carbon. On land vegetation currently stores about 550 billion metric tons of carbon and soil stores another 1,500 billion metric tons. The ocean, which is a

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