What do we know about global climate change?

We know that the earth has become warmer over the last century. The Intergovernmental Panel on Climate Change (IPCC), a group established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), reports that the average surface temperature of the earth has increased during the twentieth century by about 0.6° ± 0.2°C. (The ± 0.2°C means that the increase might be as small as 0.4°C or as great as 0.8°C.) This may seem like a small shift, but although regional and short-term temperatures do fluctuate over a wide range, global temperatures are generally quite stable. In fact, the difference between today’s average global temperature and the average global temperature during the last Ice Age is only about 5 degrees C. Indeed, it’s warmer today around the world than at any time during the past 1000 years, and the warmest years of the previous century have occurred within the past decade.

We also know that human activities—primarily the burning of fossil fuels—have increased the greenhouse gas content of the earth’s atmosphere significantly over the same period. Carbon dioxide is one of the most important greenhouse gases, which trap heat near the planet’s surface. The vast majority of climate researchers agree with these overall findings. The scientific disagreements that do still exist primarily concern detailed aspects of the processes that make up these largely accepted general themes. Red, orange, and brown coloring indicate areas where temperatures measured in 2000 are warmer than the average temperature from 1951 to 1980. The scale represents degrees in Celsius. Negative numbers represent cooling, and positive numbers depict warming.

You can think of this web site as a window into the world of scientific research. In this primer, you’ll find a general discussion of the physical processes underlying the earth’s climate, an outline of the kinds of data that may shed light on how the climate is changing—and the role of human activity in these changes —and a description of some of the questions and uncertainties that researchers continue to explore. This primer is organized into four interconnected sections: the Atmosphere; the Hydrosphere (the earth’s oceans and water); the Cryosphere (the areas of the planet covered by snow and ice); and the Biosphere (the living organisms inhabiting all these domains).