[This post is from Bill McKibben in Poland. For background on the science behind the 350 target and the challenge it poses see here and here.]

Al Gore gave the international climate talks in Poznan a new set of marching orders this afternoon, declaring that old targets for fighting global warming had been made obsolete by new science and that 350 parts per million co2 was the new standard for which the world must aim.

“Even a goal of 450 parts per million, which seems so difficult today, is inadequate,” said Gore, adding we “need to toughen that goal to 350 parts per million.”

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The number itself is less than a year old — NASA scientist James Hansen first set it as a goal in a scientific paper last winter. But in the months since, a global effort led by 350.org has spread the goal with rallies and demonstrations on every continent.

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“Our efforts reached a new level this afternoon, when Al Gore changed the decade-old goal for a new climate agreement,” said 350.org co-founder Bill McKibben. “The world now has a new target, one that negotiators must figure out how to meet by next year in Copenhagen if those talks are to be a success.”

350.org also used the occasion to announce an international day of action to spread the number next October 24, with events planned from high in the Himalayas to undersea on the Great Barrier Reef. “We need to take this movement for survival to the farthest reaches of the planet,” said Ely Katembo, 350.org organizer from the Democratic Republic of the Congo. “We’re talking to everyone, from wired teenagers in Europe to Masaii tribesman on the plains of Kenya.”

The response to Gore’s remarks highlighted growing international acceptance of the goal — his call for a 350 target drew the longest applause of his speech.

“Actions are already streaming into the 350.org website from Norway, Korea, Ecuador, and more” says Jon Warnow, web strategist for the project. “16 years ago, when the Kyoto protocol was debated, this sort of campaign wouldn’t have been possible. Now, with the internet, we have the tools we need to organize at the scale of the problem we face.”

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A variety of international voices spoke out in support of 350.org‘s call to action. International human rights icon, Desmond Tutu, called the campaign, “an effective way to take action to turn around the climate crisis.” Leading United Kingdom environmental author, George Monbiot wrote, “This is a great initiative, which all those who care about the future of humanity should support.” More “350 Messengers” are displayed on the 350.org website.

“A year ago, nobody had ever heard of 350. But it turns out it’s the most important number on the planet,” said McKibben. “If people around the world know nothing else about global warming, we need them to understand that 350 represents a kind of safety — if we can get that message across, then they’ll demand dramatic action from their leaders.”

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.