Global warming tipping points: how close are we?

New Scientist's Fred Pearce reports today on climatologist Tim Lenton's warnings about climate change "tipping points," some of which may already have been passed, although his story doesn't actually mention any of these past-tense points. Lenton was speaking at a meeting organized by the British Antarctic Survey on "complexity in nature" and it's hard to find anything more complex than the Earth's climate. But it turns out Lenton's nightmare scenarios, which he generates from the University of East Anglia in the UK aren't even as scary as some raised by American climatologists.

I don't know if that's good news or bad.

"We are close to being committed to a collapse of the Greenland ice sheet," Lenton says. "But we don't think we have passed the tipping point yet." The calculations show the Greenland collapse could be triggered by temperatures 1°C warmer than today's, of which 0.7°C is already "in the pipeline", held up by time lags in the system.

But Lenton's research groups puts the timeframe for a complete meltdown of the Greenland ice sheet at 300 years, raising sea levels by 7 metres. NASA's Jim Hansen, in comparison, worries about "several metres" of sea level rise within 100 years. The two predictions aren't entirely incompatible, but Hansen's choice of describing the effects of one century of melt make it seem a lot more frightening.

Among the other seven tipping points are the collapse the infamous thermohaline current (although others believe that's a long ways off), the destruction of the Amazon rainforest and "a near-permanent El Niño, which would hasten a runaway burning of the Amazon rainforest and its disappearance by mid-century," all of are linked, with one setting off another. But again, we're not there yet.

Or are we? Turns out what we don't know is perhaps the most frightening thing of all. What sets Pearce's story above the mainstream cover (see the Guardian, for example) is his inclusion of the meeting's consequent debate over precisely just how useful are these kind of predictions:

Lenny Smith, a statistician at the London School of Economics, warned about the "naive realism" of current climate modelling. "Our models are being over-interpreted and misinterpreted," he said. "They are getting better; I don't want to trash them per se. But as we change our predictions, how do we maintain the credibility of the science?" Over-interpretation of models is already leading to poor financial decision-making, Smith says. "We need to drop the pretence that they are nearly perfect."

He singled out for criticism the British government's UK Climate Impacts Programme and Met Office. He accused both of making detailed climate projections for regions of the UK when global climate models disagree strongly about how climate change will affect the British Isles.

Smith is co-author, with Dave Stainforth of the Tyndall Centre for Climate Change Research in Oxford, of a paper published this week on confidence and uncertainty in climate predictions (Philosophical Transactions of the Royal Society A, DOI: 10.1098/rsta.2007.2074). It is one of several papers on the shortfalls of current climate models.

Some authors say modellers should drop single predictions and instead offer probabilities of different climate futures. But Smith and Stainforth say this approach could be "misleading to the users of climate science in wider society". Borrowing a phrase from former US defence secretary Donald Rumsfeld, Smith told his Cambridge audience that there were "too many unknown unknowns" for such probabilities to be useful.

Policy-makers, he said, "think we know much more than we actually know. We need to be more open about our uncertainties." Meanwhile, the tipping points loom.

Here's more from Smith and Stainforth's paper:

Complex climate models, as predictive tools for many variables and scales, cannot be meaningfully calibrated because they are simulating a never before experienced state of the system; the problem is one of extrapolation. It is therefore inappropriate to apply any of the currently available generic techniques which utilize observations to calibrate or weight models to produce forecast probabilities for the real world. To do so is misleading to the users of climate science in wider society.

But whatever Smith and Stainford think of the weakness of current climate modeling, they do seem to agree that the tipping points are close. They conclude:

There is much to be done but information from today's climate models is already useful. The range of possibilities highlighted for future climate at all scales clearly demonstrates the urgency for climate change mitigation measures and provides non-discountable ranges which can be used by the impacts community.

Seems to me that when even those who aren't keen on the predictive capacity of the best available science tell you things are "urgent," it's time to wake up and smell the bacon.