At this week's G-8 get-together in Italy, the world's 17 biggest polluters agreed to work together to limit world temperature increases to no more than 2C (3.6F) above pre-industrial levels. It was the first time the United States had agreed to that goal. To put that number in context, the Earth has already warmed 0.8C in the past century, and the carbon pollution we've thrown up in the air has locked in another 0.6C or so. Not much wiggle room left.

One basic question is, why 2C? This usually gets reported as a "scientifically recommended limit" for global warming, and that's somewhat accurate, insofar as it's grounded in solid science. Lou Grinzo over at Energy Collective wrote a post looking at the history of how the 2C figure became so widely accepted—it's actually been floating around since 1989. Just this past March, the synthesis report from a major climate-science conference in Copenhagen—the most comprehensive update since the 2007 IPCC report—declared:

Temperature rises above 2C will be difficult for contemporary societies to cope with, and are likely to cause major societal and environmental disruptions through the rest of the century and beyond.

Still, there's no shortage of scientists who will tell you that a sharp line like this can obscure some of the subtleties at work. Andy Revkin has an indispensible post rounding up some of the caveats and warning labels that tend to get slapped on this 2C number. Here, for instance, are the scientists who run Real Climate:

We feel compelled to note that even a “moderate” warming of 2C stands a strong chance of provoking drought and storm responses that could challenge civilized society, leading potentially to the conflict and suffering that go with failed states and mass migrations. Global warming of 2C would leave the Earth warmer than it has been in millions of years, a disruption of climate conditions that have been stable for longer than the history of human agriculture. Given the drought that already afflicts Australia, the crumbling of the sea ice in the Arctic, and the increasing storm damage after only 0.8C of warming so far, calling 2C a danger limit seems to us pretty cavalier.

A number of climate scientists seem to view things this way. Setting a "limit" for global warming at 2C is a scientifically informed political judgment, but it's still a political judgment—it has to be, in the face of ambiguity. It might be the case that we could go slightly above 2C and muddle through. Most conservatives are ready to gamble on this—a habit they've become awfully fond of. But on the other hand, even staying at 2C might turn out to be unmanageable. Revkin quotes climatologist Stephen Schneider of Stanford, who sums up the uncertainty well:

I like to use the analogy to a kids skateboard park, where the ramp starts up slowly, and gets non-linearly steeper until it is vertical at the top and the kid jumps and the parents hide their eyes! In other words, there are many such threshold tipping points in the bio-geophysical-social system, but the problem is we don’t know precisely where they are—ergo the need to frame it probabilistically and my skateboard ramp is an analogy to the steepening threats as we add warming. The warmer we get the more systems there are at risk and the deeper the impacts.

--Bradford Plumer