Tipping points: we know enough to know better

Let's return now to climate change "tipping points," or as a group led by Tim Lenton of the University of East Anglia has renamed them, "tipping elements." They're important because if we can nail these down with a fair degree of confidence, we'll finally have some tangible policy advice to offer the powers that be.

Lenton et al have turned a 2005 meeting on the subject into a paper just published in the Proceedings of the National Academy of Science. It's full of predictions about just how far away we might be from triggering dramatic shifts in critical features of the Earth's climate. But what I find fascinating are the authors' references to shortcomings in the IPCC reports, and their ability to generate predictions anyway.

The notion that the Intergovernmental Panel on Climate Change is behind the curve is nothing new. For one thing, the need to allow sufficient time for hundreds of reviewers to do their jobs meant the cutoff date for research effectively eliminated a lot of recent and relevant work. But the Lenton paper, "Tipping elements in the Earth's climate system" (Vol 5, No. 6: 1786-1793), offers a splendid overview of just how out of date the IPCC really is.

That wasn't their primary objective, of course. For that, the authors whittled down a longer list of potential tipping elements, which they define as anything that results in a "qualitative change" in a climate feature that also meets a list of "policy-relevant" issues. They came up with nine, which are: Arctic sea ice, the Greenland and West Antarctic ice sheets, thermohaline circulation (deep over conveyor), the El Niño-Southern Oscillation, the Indian summer and West African monsoons, and the Amazonian and boreal forests. To each they assigned a likelihood of a critical threshold being reached and a timeframe for same.

Arctic sea ice, for example, will probably reach a tipping point within a decade, while the Greenland and West Antarctic ice sheets each may take at least 300 years to melt beyond the point of no foreseeable return. Most predictions will be familiar to anyone following the literature and are subject to various degrees of debate. But as I said, what piqued my curiosity was the references to the inadequacies of the models on which the IPCC (and other) reports are based. To wit:

Arctic sea ice:
"Given that the IPCC models significantly underestimate the observed rate of Arctic sea-ice decline, a summer ice-loss threshold, if not already passed, may be very close and a transition could occur well within this century."

GIS:
"...existing ice-sheet models are unable to explain the speed of recent changes. These changes include melting and thinning of the coastal margins and surging of outlet glaciers, contributed to by the intrusion of warming ocean waters."

THC:
"The most complex models have yet to be systematically tested because of excessive computational cost."

ENSO:
"Now that numerous OAGCMs [ocean-atmosphere general circulation models] have been intercompared, there is no consistent trend in their transient response and only a small collective probability of a shift toward more persistent or frequent El Niño conditions."

ISM:
"We differ from IPCC on the basis of past apparent threshold behavior of the ISM and because brown haze and land-use-change forcing are poorly captured in the models."

West African Monsoon:
"Future 21st century projections differ; in two AOGCMs, the WAM collapses, but in one this leads
to further drying of the Sahel, whereas in the other it causes wetting due to increased inflow from the West. The latter response is more mechanistically reasonable, but it requires a 3°C warming of SSTs in the Gulf of Guinea (76). A third AOGCM with the most realistic present-day WAM predicts no large trend in mean rainfall but a doubling of the number of anomalously dry years by the end of the century."

Amazon rainforest:
"Different vegetation models driven with similar climate projections also show Amazon dieback, but other global climate models project smaller reductions (or increases) of precipitation and, therefore, do not produce dieback."

Boreal forest:
"Studies suggest a threshold for boreal forest dieback of 3°C global warming, but limitations in existing models and physiological understanding make this highly uncertain."

Despite all those shortcomings, Lenton et al. were able to produce specific predictions about how close we've put the planet to the danger zone (except for the West African Monsoon, which might actually bring net positive change). This is yet another reminder of how robust the models are. Even though there is plenty we don't know, what we do know is more than sufficient for the generation of sound policy advice to our governments.

Some will dismiss the exercise as statistical trickery. But as IPPC lead author Richard C. J. Somerville writes in the latest Bulletin of the Atomic Scientists:

Climate science is now able to provide significant predictive power, meaning the IPCC report can link specific concentrations of greenhouse gases with the associated climatic consequences. How much sea level will rise or how much temperatures will change does depend on how much humanity allows greenhouse gas amounts to increase.

And that's just using IPCC numbers, which, as we've seen, are far from the most useful at hand. Why there persists a significant community of pseudoskeptics who continue to cast aspersions on the IPCC and its alleged alarmist bias, I'll never know. For an amusing, but also depressing, summary of one example of how hard it is to convince those who just won't listen, read about Joe Romm's efforts at Climate Progress.

Seems that some people just can't get it through their heads that science can produce meaningful policy advice in the absence of that mythical of monsters, absolute proof. Unless and until we can learn to communicate how science works to those responsible for leading society out of it messes, we're just going to plow right on past the tipping points. And given how much science we have available, we're going to look awfully stupid from the perspective of 2050.

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