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I know that there been some bitterness in the blogosphere in recent weeks between those who are mad at President Obama, and those who are mad at those who are mad at President Obama.

I want to tell you about an upcoming action — it looks set to turn into the biggest civil disobedience protest in the history of the North American climate movement. It will take place at the White House from August 20-Sept. 3, and we need your help spreading the word. But I want to explain the reasoning behind it in some detail, because for me it helps illustrate how some of the debate about Obama is unproductive.

First, the issue: the Canadians are proposing to build a huge new pipeline from their tar sands in Alberta down to the Gulf of Mexico. It’s disastrous for native lands in the far north (check out this video from the wonderful Cree activist Melina Laboucan) and it will doubtless cause horrible spills much like last week’s disaster on the Yellowstone River.

But there’s a bigger problem here too. Those Alberta tar sands are the biggest carbon bomb on the continent — indeed, on the whole planet, only Saudi Arabia’s oil deposits are bigger. Some of you have followed the work fo 350.org, and know that above 350 parts per million co2 in the atmosphere you can’t have, in the words of NASA climatologist James Hansen, “a planet similar to the one on which civilization evolved and to which life on earth is adapted.” We’re already at 390 ppm, which is why last year, according to Weather Underground’s Jeff Masters, we had the most extreme weather the planet has seen at least since the great volcanic eruption of 1816. But the tar sands of Alberta will make it impossibly worse: if you could burn all that oil at once, you’d add 200 parts per million co2 to the atmosphere, and send the planet’s temperature skyrocketing upwards. Any serious exploitation of the tar sands, says Hansen, means it’s “essentially game over” for the climate. So, high stakes. And don’t think that the Canadians will automatically find some other route to send their oil out to, say, China. Native tribes are doing a great job of blocking a proposed pipe to the Pacific; Alberta’s energy minister said recently that he stays up nights worrying that without Keystone his province will be ‘landlocked in bitumen.’ Without the pipeline, said the business pages of Canada’s biggest paper, Alberta oil faces a ‘choke point.’