‘Tipping Points’ and the Climate Challenge

[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.

Arctic Methane. Many scientists and environmental campaigners have focused recently on work hinting that the warming Arctic tundra could release vast quantities of methane, a potent greenhouse gas.

Chris Field, the director of the department of global ecology for the Carnegie Institution, was widely cited for warning last month that emissions of greenhouse gases were already exceeding recent projections by the Intergovernmental Panel on Climate Change, of which he was a member. But Dr. Field, a specialist in the flow of greenhouse gases to and from tundra and other ecosystems, said there was little understanding of whether releases of methane from warming soil could move from an amplifier of warming to an overwhelming torrent. He said the gas release clearly amounts to an amplifying effect, or positive feedback, but there’s not even close to a consensus yet on whether it could take on its own momentum.

Unraveling ecosystems. James E. Hansen of NASA and many other prominent figures pressing for cuts in greenhouse gases put the potential for a cascading unraveling of ecosystems in lists of climatic tipping points.

Some scientists examining past extinction patterns for clues related to what may happen in coming decades said they were concerned on two fronts about depictions of climate-driven changes in extinction rates as a tipping point. In interviews, several biologists said past spasms of extinctions appear to have been triggered by a host of shocks, including sharp cooling. At least one past global hot spell widely attributed to a natural spike in greenhouse gases, the Paleocene-Eocene Thermal Maximum 55.8 million years ago, appeared to cause a mass die-off of some marine plankton, but other forms thrived, as did mammals and other terrestrial species, specialists on that period say.

Basically, there is a lot still to be learned, and the downside potential from a big sustained warming is real, but hard to nail down, said David Jablonski, a specialist in mass extinctions at the University of Chicago.

“It’s difficult to pinpoint extinction cascades in the fossil record, and more difficult to attribute them directly to climate change,” he said in an e-mail. “That said, Hansen’s statement isn’t pulled out of thin air (so to speak): some of the major extinctions (especially the end-Cretaceous and the end Permian) have climate signals suggesting a sudden warming episode. The idea is that runaway greenhouse effects derived from the (end-Permian) eruption of the Siberian Traps and the (end-Cretaceous) asteroid impact and/or eruption of the Deccan Traps contributed to these extinctions which evidently did remove >50% of species. The problem is that other mass extinctions lack a greenhouse signal, and other strong greenhouses (like the Paleocene-Eocene Thermal Maximum) lack extensive extinctions. Speaking more generally, food webs probably do get restructured during some of the big mass extinctions, but whether that’s due to (or causes) cascading extinctions isn’t at all clear.”

Dr. Jablonski added, “There’s a key difference between the potential impact of climate change now versus in the past. Most groups used to survive quite handily by shifting their distributions to track temperatures (very clearly seen in plants, insects, mammals, and marine inverts from clams to reef corals). But that’s increasingly disallowed by the human environmental footprint: too many malls, highways, agribusinesses, marinas, sewage outfalls, etc.”

Fast-rising seas. One of the biggest potential impacts on human affairs from sustained warming is coastal inundation as warming seas swell and fill with water flowing from melting ice sheets. Dr. Hansen, particularly, has asserted that more than two yards of sea-level rise is almost assured under a “business as usual” track for emissions.

Robert Bindschadler of NASA and Tad Pfeffer at the University of Colorado, both glacier specialists, told me that they saw scant evidence that a yards-per-century rise in seas could be produced from the ice sheets that currently cloak Greenland and West Antarctica, which are very different than what existed in past periods of fast sea-level changes. But they said it’s already clear that a sustained and disruptive rise in seas was probably inevitable under further warming, providing ample justification for concern.

Tipping points in the climate debate. The phrase has become boilerplate of late in arguments for action by environmental groups and some scientists. Organizations have created names around thresholds, including onehundredmonths.org and 350.org.

Robert Brulle, a sociologist at Drexel University, saw little evidence that such efforts could succeed. “It seems to me that a discussion of tipping points is useful in terms of discussing the science of nonlinear processes, but this is hardly the type of language that will increase issue saliency in the public,” he said. “When you set a specific goal, or say if we go over some threshold the risk will increase dramatically, you are then linking action to a specific scientific claim that can be disputed. Since we aren’t yet past the tipping points, or if we are past them, the evidence has not emerged unambiguously from the background variance, it is difficult to maintain the truth of dire predictions in the future.”

Some veteran climate scientists, while eager for cuts in heat-trapping emissions, warn that descriptions of climatic tipping points or thresholds could backfire in the absence of evidence, leading to less public support for action. “There are real tipping points out there, and they might be politically useful at first, but if you’re too specific about particular thresholds that’s a quick trip to lost credibility,” said Stephen H. Schneider, a climatologist at Stanford University who said he could point back to some overly precise climate predictions of his own in the 1970’s as evidence.

Pieter Tans of the National Oceanic and Atmospheric Administration stressed the persistent uncertainty in the range of warming expected from a buildup of greenhouse gases as cutting against the idea of specific thresholds: “Our biggest science problem is that we do not know how strong the climate feedbacks are, or even whether we know all of the ones that are important on decadal and longer time scales,” he said in an e-mail. “Especially the latter are intrinsically very hard to figure out because they are (by definition) slow, and they could all be working simultaneously, some positive, some negative. Basically, we are playing Russian roulette, with the revolver pointed at the generation of our children and grandchildren.”