350 ppm

The most important number in the world? It’s 350, as in parts per million carbon dioxide in the atmosphere.

Two years ago, NASA scientist James Hansen—the man who first testified before Congress 20 years ago that the activity of human beings was causing global warming—put a finite, bottom-line number on the climate-change crisis. If there is carbon dioxide in the atmosphere greater than 350 parts per million, he said, it will lead inexorably to a condition that would no longer be “compatible with the planet on which civilization developed or to which life on Earth is adapted.”

The statement was a wake-up call.

The Earth’s number is already slightly above 350 ppm and, though many scientists didn’t predict global warming would have dangerous impacts until the turn of the century, we’re already seeing alarming trends that indicate the crisis may manifest much sooner. The Arctic sea ice is melting at a disturbing rate and the Earth’s oceans are becoming acidified. Glaciers are in rapid retreat and even slightly hotter temperatures have brought on wrenching drought in some places and massive flooding in others.

All of which leads to the number 350.

Author and environmentalist Bill McKibben and others have called for an international day of protest around this number, and it’s set for this coming Saturday, October 24. They hope to use that day to pressure world governments to get much more aggressive about attacking global warming. (For more information on this global day of protest and its local offshoots, see McKibben’s “Meltdown is imminent” SN&R Green Days.)

Why this Saturday? Because in about a month, the United Nations convenes its concluding international climate talks in Copenhagen, Denmark. Our hopes for an ambitious, fair and binding post-Kyoto treaty on global warming all hang on what happens in Copenhagen. Unfortunately, those hopes have dimmed somewhat because of the failure of industrialized nations like the United States to offer major emission cuts or to help finance developing countries—like India and China—in cutting their emissions.

Climate-change has become a standoff between rich and poor nations. We can only hope a diplomatic bridge can be reached in Copenhagen this December. The 350 protests on October 24 hope to focus the world on this desperate need.

Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change, recently went public in agreeing with Hansen that 350 is the clear and unequivocable number of importance when it comes to global warming. This gives enormous new credibility to the 350 movement since Pachauri was a former Indian Oil Corporation man, the one handpicked by former President George W. Bush to replace a chairman Bush’s administration thought too “alarmist” about climate change.

Now, the nonalarmist has agreed with Hansen and McKibben that it’s more than time to sound the alarm. We need dramatic action on global warming now, not later. Our future depends on it.