Streetfilms

350 parts per million. That’s the level of atmospheric carbon dioxide beyond which many scientists warn the earth’s climate may begin to spiral out of control. At higher concentrations, they say, heat-reflecting ice sheets will disappear and permafrost will melt, releasing vast amounts of additional greenhouse gases and driving sea levels higher in a vicious cycle. The earth’s atmosphere is currently at around 380 parts per million, and climbing.

For a young international movement, 350 is a rallying cry, an organizing principle. On October 24^th, climate activists in over 180 countries with the group 350.org staged more than 5,200 demonstrations, pressuring world leaders to take meaningful action on global warming at upcoming United Nations climate talks in Copenhagen. In San Francisco, a ride of 350 cyclists in snorkels and flippers gathered at a downtown rally and traced a route through Bay-side neighborhoods threatened by rising sea levels.

Critics of the movement say the goal of stabilizing the atmosphere is too ambitious, and that even a cap of 450 parts per million would be difficult to achieve with curbs on carbon emissions. But the heated debate on the political possibilities of climate action is up against cold, hard, science.

The head of UN’s Intergovernmental Panel on Climate Change, Rajendra Pachauri, recently endorsed the goal of cutting emissions to 350 parts per million or less. Pachauri, who in 2007 split the Nobel Peace Prize with former Vice President Al Gore, was not able to advocate for any specific goals as chair of the IPCC, “but as  a human being I am fully supportive of that goal. What is happening, and what is likely to happen, convinces me that the world must be really ambitious and very determined at moving toward a 350 target."