Between 1950 and 1998, the world's population grew from 2.5 billion to 5.9 billion human beings. United Nations demographers project world population will gain between 2 and 4 billion more people by 2040 (United Nations 1998). Just as past population increases have influenced greenhouse gas emissions and the composition of the earth's atmosphere in the late 20th century, future demographic trends will affect the earth's climate for centuries to come.Global efforts to reduce fossil fuel use, increase energy efficiency, slow deforestation, and move to carbon-free energy sources are key components of decreasing greenhouse gas emissions. Such efforts, the objective of current negotiations on climate change, are much more likely to have enduring success if world population grows more slowly and eventually stabilizes.The disparities in resource consumption and greenhouse gas emissions between regions, nations, and individuals complicate international efforts to address climate change, since responsibility for the problem varies so broadly. These disparities also cloud the role of population growth in changing climate, since the greatest per capita contributions to climate change are made by populations, such as those of the United States and Europe, that are growing relatively slowly or have stabilized.