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Looking ahead, it's possible the U.S. Southwest will suffer [-----1-----] drought. The region has a history of drought. Global warming may increase this trend.

Edward Cook: Over the past thousand years in the Westsouth state, there have been profound periods of drought conditions lasting again decades to a century or more. So we know that the natural climate system, independent of human activities, is capable of locking itself into long-term dry [-----2-----].

You're listening to Edward Cook, of the Lamont-Doherty Earth Observatory. Cook reconstructed the Southwest's past climate using tree ring records. Scientists can determine a dry or wet period by measuring the size and [-----3-----] of tree rings.

Edward Cook: So we can actually tie certain variability in the tree ring records, that indicate periods of drought happening, with changes in early culture.

Cook said that tree ring records link periods of severe drought with the [-----4-----] of certain Native American civilizations. Today's American cities are better insulated from the immediate impacts of drought, but a drier climate has major implications for regional water supplies.

Edward Cook: [-----5-----]

Today we're observing The International Year of Planet Earth. Thanks to the National Science Foundation and US Geological Survey.

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