This was the apex of the last ice age, so, of course, the world was much cooler, probably between 3 and 5 degrees colderthan today. Glacial ice sheets reached down from the Arctic, extending as far south as Germany in Europe and New York City in North America. (And much further south, many mountains were capped with glaciers, too.) But a slight shift in the tilt of the earth's axis meant that more sunlight began to reach the poles. The world slowly began to warm, causing glaciers to retreat.

After an extended period of increasing temperatures, the climate cooled slightly. It stayed cool for roughly a thousand years. (The name of the period comes from the white dryas, an Arctic wildflower that, found buried in ancient sediment, has helped us understand past climatic conditions.)

Scientists believe the Younger Dryas may have been launched by shifts in ocean currents. In some places, the drop in temperature was large—as much as 14°C. On other parts of the globe, though, temperature increased. These sorts of complex changes are typical; indeed, after the Younger Dryas it becomes difficult to generalize about global climate change.

After an extended warm period, the climate cooled and glaciers began to advance again. The Neoglacial period lasted roughly a thousand years, though its precise borders are hard to define, in part because there were still substantial regional oscillations in temperature and precipitation.

After that nice period of warmth, the planet cools again. (Note that the Medieval Warm Period and the Little Ice Age overlap: that, again, is because the climate's behavior has varied from place to place.)

The existence of both the Medieval Warm Period and the subsequent Little Ice Age has sparked controversy, in part because a naturally warmer era has been cited as evidence that current climate change might not be human caused. There is one difference between these events and the current warming, though: they tended to be scattered, happening in different times in different places, rather than the globally synchronized increase in temperature we're seeing today.

After a series of warm summers, 3,250 square kilometers of an Antarctic ice sheet disintegrated in a quick 35-day collapse. It was the largest such collapse ever observed, setting off a wave of news coverage.