This theme is coming up a lot lately. I am noticing more and more that some very thoughtful people, like Michael Shermer, are willing to accept that global warming is real and human caused, but neverthless don't think we have to worry about it because it won't be that bad. And now here's Michael Lind of the centrist New America Foundation, writing in Salon.com:
The scenarios with the most catastrophic outcomes of global warming are low probability outcomes -- a fact that explains why the world’s governments in practice treat reducing CO2 emissions as a low priority, despite paying lip service to it....
In one sense, this is obviously true. In another sense, it's completely off base. First, uncertainty cuts both ways, so it makes no sense to be confident that change will be on the low end. This is something about which Kerry Emanuel recently testified:
In soliciting advice, we should be highly skeptical of any expert who claims to be certain of the outcome. I include especially those scientists who express great confidence that the outcome will be benign; the evidence before us simply does not warrant such confidence.
But more generally, you can really only make Lind's argument if 1) you're paying enough attention to global warming to understand that there's a real scientific consensus that it's happening, but 2) you're not paying enough attention to realize what global warming really means for Planet Earth. Fundamentally, global warming means adding more heat to the system. Physics dictates that planet wide changes are then inevitable, and while the speed at which they will occur may be debatable, if you keep adding heat and don't stop then we know what eventually happens--because we know what the Earth was like at earlier points in its history, when temperatures and carbon dioxide concentrations were higher. One of the most obvious things that happens is that land-based ice melts, and sea level rises. This is not only because global warming is adding heat, but because the amount of heat added is amplified at the poles, where the ice is located. And indeed, we know that global warming is already destabilizing Greenland and West Antarctica, although we don't know for sure how much or how fast this is occurring, or how catastrophically it could occur. (For a good discussion of this, see Joe Romm.) But eventually, add enough heat, and the ice melts and goes back into the ocean. It has to. It is ice. And then sea levels rise dramatically, and many places where we have currently established human civilization--we like to live near water, dangerous though it is--become uninhabitable. It's really that simple. We don't know the timeline, but if we don't stop it, we know the eventual outcome--and it is intolerable and unacceptable on any timeline. And that's why any attempt to minimize worry about global warming by citing "uncertainty" about the projections just doesn't make sense.