About 1.8 million years ago, great mountains of ice began to form, blanketing the planet. In this grip of continuous cold, long periods of time passed when the annual snowfall didn't melt. As more snow fell, the deeper layers compacted into ice and the ice sheets grew thicker. The planet's climate began to warm 8,000 years ago, melting the ice at lower latitudes, but in the Arctic and in Antarctica, ancient ice still remains. In those places, scientists drill to depths of up to two miles to extract long cores of ice dating back 740,000 years.
By carefully analyzing the oxygen atoms bound up in the ice, scientists can determine the temperature of the air when those atoms fell as snow. Two types of oxygen atoms are common on earth; one has a heavier nucleus than the other. Water molecules containing light oxygen evaporate more readily than heavy ones. But as the air temperature rises, more heavy molecules are able to make the jump from ocean to atmosphere. When the water vapor falls as snow and accumulates on polar ice sheets, each year's snowfall serves as a built-in temperature gauge: Layers that are relatively abundant in heavy oxygen formed during warmer periods than those with very little. Scientists measure the ratio of light to heavy oxygen at regular intervals along an ice core, and from these data they can create a time line of climate change. Their findings reveal that climate varies over long periods, but at no point has the warming been as abrupt as it has been since the Industrial Revolution. Pockets of air trapped within the ice tell a similar story: CO2 concentrations have risen most rapidly since we started burning fossil fuels.