[00:09] About 18-thousand years ago, much of the northern hemisphere was locked in a brutal winter. In fact, this North Dakota drift prairie I'm standing on was buried a mile deep under an ice sheet.

[00:21] The marching ice and cold temperatures annihilated many plants and animals that couldn't move or adapt to the changing environment.

[00:28] Geologic records show Earth has experienced ice ages, or glacial periods many times in its history.

[00:34] During the last two million years, our climate has alternated between the cold climate of glacial periods and the warmer, more benign interglacial climate to which modern life has adapted.

[00:46] What makes an ice age? While many factors contribute to the advance and retreat of glaciers, a fundamental control of Earth's climate is the amount and distribution of heat on the planet's surface. This will change if the shape and angle of the Earth's orbit around the sun changes.

[01:02] While orbital variations can trigger an ice age, these changes occur gradually over thousands of years. Geologists now know that some ice ages developed in just a few decades.

[01:13] Such rapid climate change is probably due to interactions within Earth's climate system such as the exchange of heat between the oceans and atmosphere.

[01:22] Understanding how ice ages occur helps scientists predict how the climate might respond in the future to both natural and human induced changes.