A British newspaper obtained a secret U.S. Defense Department report that they said predicted catastrophic climate change in the next 20 years. This climate change would cause rioting and nuclear war, and would give Britain a "Siberian" climate. The story quickly spread on the Internet.

However, the report isn't very secret (you can read it here), and the authors write that it describes a "plausible" but "not very likely" scenario.

This Oakland Tribune article explains how all this happened.