by Rebecca Solnit from Truth Dispatch. You might want to skip the introduction by scrolling down to the article which is mostly a commentary on Bill McKibben's book entitled, "Eaarth: Making a Life on a Tough New Planet".
A word about that number, 350. For a long time, McKibben relates, the premise, or pretense, was that the parts per million of atmospheric carbon we needed to worry about was 550, double the historic concentration. As it turns out, it was also a random figure, easy to calculate, not too alarming. We weren’t anywhere near there yet, which is why we could frame global warming as some terrible thing that was going to happen way down the road -- the grandchildren theory of climate change.
Then the scientists got more data and so more precision about where peril lay: in December of 2007, NASA climatologist James Hansen announced at the American Geophysical Union that 350 was about the upper limit at which life on Earth as we know and like it was likely to continue.
We’re now at about 390.