TIME TRIP: Blowing Hot and Cold

To cross the Sahara today, you'd probably want a camel and bottles of water. But about 6,000 years ago, you would have needed a boat to traverse parts of it — and you'd have had to keep an eye out for crocodiles Lurking in the swamps. Earth is no stranger to climate change. The planet's 1 billion-year history has included frigid ice ages and tropical warming periods.

About 60 million years ago, trees grew in Arctic and Antarctic regions, and Earth was virtually ice-free. About 20,000 years ago, the last major ice age was at its height. Agriculture in Europe blossomed during the Medieval Warm Period, which lasted from 1000 to 1300 AD. That was followed by a cold period known as the Little Ice Age, which didn't begin to thaw until about 1890.

Typically, temperature changes occur gradually. Yet volcanic eruptions have sometimes changed Earth's climate virtually overnight. Volcanoes spew sun-reflecting sulfur dioxide into Earth's stratosphere. When Indonesia's Mount Tambora erupted in 1815, it blanketed the world with volcanic ash, changing the climate so dramatically that snow was reported in July in New England and Northern Europe. The year 1816 became known as the year without a summer.

Current Events, 4.23.2007