‘Low-Probability But Catastrophic’ Climate Scenarios


via Scientist Michael Mann on Climate Scenarios



Michael Mann is a climatologist at Pennsylvania State University, known primarily for his 1999 “hockey stick graph” of global mean temperatures (which shoots up quite dramatically in the 20th century). He has also been steadily involved in the United Nations’ periodic IPCC state-of-the-planet reports, and with Lee Kump has even adapted those reports into a popular, accessible book summarizing the findings. The book, which was updated to reflect the most recent reports, is called Dire Predictions.

Shortly after this week’s cover story was published, Mann took to Facebook to voice some criticism of it — primarily about its framing, which he described as counterproductively “doomist.” Personally, I don’t think we’re doomed, just facing down a very big challenge. But I own up to the alarmism in the story, which I describe as an effort to survey the worst-case-scenario climate landscape. We have suffered from a terrible failure of imagination when it comes to climate change, I argue, and that is in part because most of us do not understand the real risks and horrors that warming can bring, especially with unabated carbon emissions. For the sake of clarity: I do not believe that the planet will become uninhabitable in 2100. As I write in the story, our complacency will surely be shaken before we get there. But I do believe that it is important to contemplate the possibility that parts of the tropics and equator will become cripplingly hot, for instance, or that our agriculture will suffer huge losses, so that we may be motivated to take action before we get to those eventualities. And I do believe that, absent a significant change in human behavior across the globe, they are plausible eventualities.

Mann also took issue with a few particular points of science. He stressed that the danger of the carbon frozen in the arctic permafrost was not a “game-changing arctic methane time bomb” and, separately, he suggested that the recent upward revision to a particular satellite data set on warming was less significant than I made it out to be. My purpose in raising the permafrost issue was to illustrate how uncertain much of our current modeling can be, not to suggest a sudden methane release would be the major cause of devastating warming: I based none of the warming scenarios described in the piece on a dramatic methane release effect but rather on the high end of the IPCC’s business-as-usual estimate, which gave a roughly 5 percent chance of our hitting eight degrees of warming by 2100. Regarding the data set, I grant that the upward revision may have been less meaningful to the scientists close to the data, who understood it as a revision toward expectations, than it was to journalists covering the development from afar, who focused on the fact of the revision itself.