Nature Climate Change | Climate Feedback

AGU 2008: Uncertainty and overshooting 2°C

As speculation grows that agreeing a global deal on climate change may extend well beyond the 2009 deadline, the risk of overshooting the EU’s target to limit the increase in global temperature to 2°C over pre-industrial levels looks increasingly likely.

The target is based on scientific evidence that below a 2°C increase, some of the worst impacts of climate change would be avoided. In its fourth assessment report, the IPCC calculated that limiting warming to that extent would mean stabilizing atmospheric concentrations at roughly 450 ppm CO2-equivalents.

But it’s clear at this year’s AGU that much uncertainty, and disagreement, remains on whether 2°C is an appropriate target and on the atmospheric greenhouse gas concentrations that equate to a given amount of warming.

Speaking here yesterday, James Hansen, director of the NASA Goddard Institute of Space Studies, called for atmospheric greenhouse gas concentrations to be restricted to 350ppm. Hansen also recently advised the UK government’s Environmental Audit Committee that the 2°C target should be revised to a 1°C increase above pre-industrial levels. Meanwhile, many of the scientists presenting work here are wrangling with whether stabilizing at 450ppm – or higher – is economically and technologically feasible.

The debate on where to stabilize is largely due to what Stanford’s Stephen Schneider dubs ‘double-barrelled uncertainty’ a term that encompasses the unknown factor in how far we can reduce emissions and in how the climate system will respond to any reductions we make.

The latter uncertainty – that inherent in the system – is measured in terms of climate sensitivity i.e. how much warming would occur if atmospheric GHG concentrations were doubled. This is currently estimated at 2-4.5°C, but Schneider said here on Tuesday that it’s more important for society to know the uncertainty around this range i.e. the chance that we will see a much larger degree of warming than anticipated for various atmospheric concentrations.

Given the fact that a massive reduction in emissions in the coming decades it is very unlikely, he says, and the fact that we could overshoot our target anyway, we need to take a serious look at the consequences. Schneider suggests that the IPCC should take it on board to evaluate climate scenarios for overshooting to 600 ppm and then subsequently reducing atmospheric concentrations to say 500ppm or 450ppm.

The subsequent reductions would undoubtedly be difficult; that’s one reason why many people believe Hansen’s 350ppm target is unachievable. But according to David Keith of the University of Calgary in Canada, air capture could make it feasible. Even though the cost would be high, said Keith here on Tuesday, it might make sense to remove greenhouse gases from the atmosphere irreversibly later on than to mitigate now at a lower cost. It would be much faster and allow us to remove that portion of atmospheric carbon dioxide than would otherwise hang around for millennia, says Keith.

What is striking about the discussions on climate stabilization here this week is the overwhelming acceptance that we’ll overshoot even the 2°C target.

Olive Heffernan


  1. Report this comment

    Hank Roberts said:

    Schneider “double barreled uncertainty” – did not match any documents (Google).

    Have you a pointer to where he uses this phrase?

  2. Report this comment

    Olive Heffernan said:


    Good point; I’ve updated the link to point directly to the abstract from AGU, but I also couldn’t find anywhere else on the web where Schneider has used this terminology