[00:08] Since the beginning of the industrial age, humans have been affecting Earth's climate in ways we don't fully understand.

[00:15] Is there evidence that burning fossil fuels, for example, causes global warming?

[00:19] Scientists use several ways to learn how Earth's climate has changed since prehistoric times. One method is to study changes recorded in the growth rings of ancient trees.

[00:28] When the climate is warm, and water plentiful, the growth rings of trees near timberline grow wider. When the climate cools, the rings narrow.

[00:36] Scientists at Columbia University's Lamont Doherty Earth Observatory have found evidence from tree rings that temperatures have risen steadily since the late 1800's.

[00:46] By coring the trunks of 500 year old Siberian pines botanists can study growth rings without damaging the trees.

[00:53] Researchers found that the 20th century has been warmer compared to the last five centuries.

[00:57] What tales do tree rings tell us? For one thing, the rings give us a long term view of climate. The narrowest ring growth occurred between 1830 and 1870. The widest growth began in the 1900's and increases as the century progresses.

[01:12] By comparing the rings of trees around the world, along with other evidence, we now know that the 20th century is the warmest century in a thousand years.

[01:20] For Our Changing Planet, I'm Jane Joyce. And I'm Lou Varricchio. To learn more about Our Changing Planet visit us on the web at www.umac.org/ocp.