Reading Past Climate: Ice Cores, Tree Rings and Fossils

0:00Nobody measured the weather thousands of years ago, so how do we know what Earth's climate was like?

0:07Nature keeps its own records, and scientists have learned to read them.

0:12In very cold places, each year's snow is pressed into a new layer of ice.

0:18Scientists drill out a long ice core, where each layer is one year and deeper layers are older.

0:25Tiny bubbles trap old air, so we can measure how much carbon dioxide it once held.

0:32A tree grows one new ring every year.

0:37A wide ring means a warm, wet year, and a narrow ring means a cold or dry year.

0:44Fossils show what lived in a place long ago.

0:48A fossil of a warm-climate plant, found where it is cold today, means that place was once much warmer.

0:55Tree rings reach back thousands of years, ice cores about eight hundred thousand years, and fossils many millions of years.

1:04Together, the records show that Earth's climate has changed many times, slowly, over long ages.

1:12But in the last 150 years Earth has warmed much faster, as people burn coal, oil and gas.

1:21Ice cores, tree rings and fossils are Earth's climate diary, and it shows that today's warming is unusually fast.