In the past, changes in the composition of the atmosphere have been a major cause of climate differences. Already, observations of the global climate indicate that average temperatures are starting to warm, most likely because of increasing concentrations of greenhouse gases (especially carbon dioxide) in spite of the apparent cooling influence of the increased concentration of sulfate aerosols, primarily from sulfur dioxide emissions.
The US Global Change Research Program supported a major portion of climate change research, the results of which have been endorsed by the international scientific community and summarized in scientific assessments by the IPCC.