As a number, 350 may not mean much to you, but it’s extremely important to those following the climate-change crisis.

According to the Citizens Climate Lobby, 350 parts per million (ppm) is the carbon-dioxide limit each nation must achieve to keep global temperatures from rising to catastrophic levels. The current atmospheric carbon-dioxide concentration is 389 ppm—a number that’s climbing 2 parts a year.

Rep. Bob Filner (D-CA) has asked President Obama to reach agreement on this goal with other world leaders at next month’s United Nations Climate Change Conference in Copenhagen. He and Rep. Dennis Kucinich (D-OH) are circulating a letter in the House that calls upon Obama to take action. Click here to read the full text.

“Global leaders can negotiate with one another, but humanity cannot negotiate with the earth’s climate,” says Citizens Climate Lobby Founder and President Marshall Saunders. “It will not compromise. We must yield or face the wrath of nature.”

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