part countering the effect of increased CO2. The 1991 eruption of Mount Pinatubo, for example, caused slightly cooler-than-average global temperatures for about a year.1 Despite the uncertainties and feedbacks, a doubling of CO2 from fossil fuel burning is now predicted to increase the mean surface temperature 2°C to 4.5°C by about the middle of this century (IPCC, 2007a).
As the period of time under consideration lengthens, more diverse processes that can affect climate come into play. Over thousands of years, variations in Earth’s orbit around the Sun (Milankovitch forcing) affect how solar energy is distributed around the globe and lead to changes in mean annual temperature, precipitation, and seasonality. Earth’s orbital cycles are responsible in part for the oscillations between ice ages and interglacial periods that characterize the past 3 million years of Earth history (Figure 3.2). Over thousands of years the oceans are important as well; for example, excess CO2 in the atmosphere should dissolve into the oceans after about 1,000 years. And in glacial times the increased ice cover on Earth changes the albedo. If ice caps start to grow as a result of cooling over thousands of years, they can reflect more sunlight and enhance cooling.
The role of tectonic processes (volcanoes, mountain building, continental drift) becomes dominant at timescales of a million years or longer (Figure 3.3). Volcanoes, for example, tend to move CO2 from the deep Earth to the atmosphere, whereas erosion of mountain ranges and the associated chemical weathering of minerals tend to remove CO2 from the atmosphere and ocean and bury it as calcite and organic matter in sediments on the ocean floor. Plate motions, which rearrange the continents and oceans, affect atmospheric