Earth's axis rotates slowly around an imaginary center line, like a wobble in a spinning top. The main cycle of this process--known as precession--takes about 26,000 years, and it shifts the dates of perihelion and aphelion forward by about one day every 70 years. Thus, in about 13,000 years, Earth will be closest to the sun in July instead of January. This will intensify the seasonal changes in solar energy across the Northern Hemisphere and weaken them in the south. The stronger summer sun is likely to bolster the African and Asian monsoons, as was the case around 10,000 years ago (see p. 80).

Earth's tilt goes up and down, ranging from about 21.8° to 24.4° and back over about 41,000 years (the range widens or narrows a bit with each cycle). The tilt is now just over 23.4° and on the decrease. When the tilt is most pronounced, it allows for stronger summer sun and weaker winter sun, especially at high latitudes. Ice ages often set in as the tilt decreases, because the progressively cooler summers can't melt the past winter's snow. At the other extreme, increasing tilt produces warmer summers that can help end an ice age.

Mount Pinatubo erupts on June 12, 1991. (U.S. Geological Survey/Cascades Volcano Observatory)

The Earth, circa 250 million years ago, showing the supercontinent Pangaea.