The Island of Doubt

It's the big question that bedevils climate science and politics: how close are these "tipping points" beyond which things get very bad very fast? Tim Lenton of the Laboratory for Global Marine and Atmospheric Chemistry at the University of East Anglia doesn't have any definitive answers, but he makes a valiant effort to explain what we do know in a new essay, "Tipping points in the Earth system." A properly peer-reviewed version is in progress, but for sheer background info, it's already required reading here on the Island.

Lenton has made a career of exploring some of the more extreme ideas associated with climate change. Last year in Climate Dynamics (Millennial timescale carbon cycle and climate change in an efficient Earth system model, DOI:10.1007/s00382-006-0109-9), he made a stab at calculating what would happen if we burned all the available fossils fuels on Earth, even the really hard-to-get stuff like methane hydrates and shale oil. His conclusion:"CO₂ peaks at 6,000 ppm giving 12.5°C global warming." (The current carbon dioxide level is approx. 382 ppm.)

Now Lenton has turned his considerable analytical talents to tipping points, which he defines as "a critical value ... from which a small perturbation leads to a qualitative change in a crucial feature of the system." In other words, beyond here lay dragons.

We've already warmed the planet by 0.8°C and there's another unavoidable 0.6°C "in the pipeline" thanks to the inertia of the Earth's heat traps, most notably the oceans. So figuring out how much wiggle room with have left until future warming triggers a tipping point is pretty darn important. Lenton takes what little knowledge we have and applies it to seven candidate thresholds:

Greenland Ice Sheet
West Antarctic Ice Sheet
Amazon Rainforest
Boreal Forest
Sahara Desert
Atlantic Thermohaline Circulation
El Niño Southern Oscillation

There is good news and bad and relatively generous prediction ranges. The bad news is:

The tipping element that consistently emerges as having the closest threshold (in terms of global warming) and the least uncertainty in this is (irreversible) melt of the Greenland Ice Sheet ... the corresponding global warming (accounting for polar amplification) is estimated at 1-2 °C. The IPCC (2007) give a more conservative range of 1-4 °C. Others have estimated <1 °C. Their case may be bolstered by observations indicating that the ice sheet is already in net mass loss and the rate of mass loss has accelerated in the last decade.

For the West Antarctic, Lenton's comes up with a threshold estimated at 3-5 °C. Also within 300 years, for another 4-6 m of sea level rise.

Though the melt will probably not proceed in a linear fashion, let's assume it does for lack of anything better to go on. That means that in about 150 years, we could see betweena total of between 4 and 7 metres of sea level rise. (Glacier melt will add a bit, but not enough to make a significant difference.) This isn't that different from Jim Hansen's "dangerous" climate change predictions of "several metres" by the end of this century.

The other five tipping points are further off, according to Lenton. And there's even one that would be very good news, for Africa, anyway:

... a rare example of a potential beneficial tipping element is a greening of the Sahara/Sahel region back toward conditions last seen around 6000 years ago. This suggestion may seem a little odd given recent Sahel drought, and indeed some models project further drought in the 21st century. However, a scenario is conceivable where the West African Monsoon (WAM) effectively collapses, adversely affecting some regions but this leads to increased inflow of moist air from the West to the Sahel, wetting it and promoting vegetation growth.

Overall, though: "Continuing business as usual could threaten all of the tipping elements discussed above. This is, needless to say, an unwise path to follow." And

Perhaps it is better instead to think about the socio-economic system as one that also possesses tipping points. One of these would be a non-linear transition to an economic regime where fossil fuels are replaced by low carbon energy sources as the backstop technology.

Lenton tells me he has had a "lively" reaction to the essay.