Chapter 11, Image 13

Caption

Over tens of thousands of years. The most recent ice age began about 115,000 years ago and ended about 11,500 years ago. Then came a dramatic warm-up, which lasted until about 3000 BC. Since then, Earth's temperature has changed relatively little, with a very slight cooling interrupted by warmer periods and punctuated by the last century's sharp temperature rise. More than a thousand years from now, after humans have exhausted (or stopped burning) fossil fuels and much of the resulting greenhouse gas has left the atmosphere through natural processes (mostly through slow absorption by the ocean), we may return to cooler times. If the length of the last interglacial is any guide, we're due for another ice age in the next few thousand years, although some scientists believe it's more likely to take 20,000 years or more. It's even possible that a mix of polar melting, land-use changes, and slowly waning greenhouse gas concentrations could postpone the next ice age for an undetermined period.