Andrew C. RevkinAndrew C. Revkin/ The New York Times
[UPDATED, 3/31 below.]
In early assessments of global warming, most curves were smooth. Rising concentrations of greenhouse gases would raise temperatures. Then glaciologists started finding evidence of extraordinarily abrupt jumps in regional temperatures. Other evidence revealed past eras when seas rose precipitously. The possible shutdown of important Atlantic Ocean currents added to the sense of nonlinear and disruptive risk. A certain best seller propelled the phrase “tipping point” deep into popular discourse. Add that all together and what do you get? The prospect that human-driven warming is poised to push Earth past dangerous tipping points is now a cornerstone of many environmental campaigns.
But what tipping points are well established and which ones remain what Stephen W. Pacala of Princeton University has called “the monsters behind the door”? I have a piece in the Week in Review section exploring these concerns. Given the limits on space in print, I thought it worthwhile to add some additional voices here and encourage further discussion. The bottom line? A growing effort to clarify such risks has yielded what amounts to the same message climate experts have been conveying for more than two decades: More emissions of greenhouse gases raise the odds of trouble. The conclusion is similar to that in the “burning embers” diagrams from the third Intergovernmental Panel on Climate Change report and a recent paper. [UPDATE, 3/31: Daniel Botkin, the ecologist and author, has weighed in with an exploration of tipping points in ecosystems.]
Early signs of scientific discomfort with some allusions to climate-related tipping points came in a 2006 blog post, “Runaway Tipping Points of No Return,” by Gavin Schmidt of Realclimate.org and NASA’s Goddard Institute for Space Studies. His main focus was the use of the term by the media, but he provided a very useful deconstruction of various climate-related risks and whether they had that “little nudge, big effect” quality.
He warned how such terminology could backfire two different ways in the public discourse — leading to both a cavalier and fatalistic outlook (before and after a putative threshold is reached). He also noted the reality that in a complex system with lots of potential tipping points, the end result is in essence a smooth curve of risk, kind of like those early depictions of the climate problem.
A pair of papers published in 2008 and this month in the Proceedings of the National Academy of Sciences have tried to narrow the definition of various “tipping elements” and clarify whether they are poised to tip. In the end, the authors acknowledged that the uncertainties surrounding the various putative tipping points led back to a broader notion of the overall risk. “Basically, it looks like a continuum of increasing likelihood and severity as temperature increases, rather than threshold,” said Jim W. Hall, an author on both papers.
Some physical scientists and biologists who are deeply immersed in climate studies, and convinced that big risks attend an unabated buildup of human-generated greenhouse gases, are pushing back against the use of this loaded term in defining climate risks. And a variety of social scientists and policy specialists warn that such terminology is very likely to backfire.
At the Headwaters Gathering, an environmental conference at Warren Wilson College in Asheville, N.C., Herman Daly, whose focus is “ ecological economics,” spoke through a video hookup Saturday and said that seeking or implying certainty in pursuing climate solutions ignores the reality of the problem — which requires acting in the face of uncertainty. “When you jump out of an airplane, you need a crude parachute more than you need an accurate altimeter,” he said. “Do not wait for exact empirical evidence.”
Here is an incomplete list of some areas of concern. More voices of researchers will be added in the comment thread.