Climate Feedback

Could tipping happen any time soon?

I wrote here yesterday that ‘I don’t think that anyone knows for sure how close we are to reaching tipping points in the climate system’. As it so happens, a pair of articles published in Proceedings of the National Academy of Sciences this week illustrates this point nicely.

The first is a Perspective by atmospheric scientist V Ramanathan and postdoctoral researcher Yan Feng from the Scripps Institution of Oceanography at the University of California, San Diego, who argue that the Earth is now committed to a 2.4°C rise in temperature above pre-industrial levels.

Anything above a 2°C increase is generally considered to be ‘dangerous’ climate change and would likely trigger several of the Earth’s tipping points, such as the complete loss of Arctic summer sea ice and melting of the Greenland ice sheet. And according to the IPCC, a rise in global temperature by 1-3°C will commit the planet to widespread loss of biodiversity, widespread deglaciation of the Greenland Ice Sheet, and a major reduction of area and volume of Hindu-Kush-Himalaya-Tibetan glaciers, which provide the head-waters for most major river systems of Asia.

The authors argue that for atmospheric greenhouse gas concentrations to remain constant at 2005 levels for the rest of the century, aggressive emissions reductions would be required – yet emissions are rising.

Currently, the warming effect of greenhouse gases in the atmosphere is being masked by the cooling effect of other air pollutants – such as smoke from cooking and agricultural waste burning – that create a dimming effect at the Earth’s surface.

Assuming policies to reduce these air pollutants are successful, the full warming potential of greenhouse gases will soon be realized. So as air pollution measures become effective (and much headway is being made here), the need for reducing carbon dioxide emissions becomes even more urgent, say Ramanathan and Feng.

Weighing in with a separate Commentary, Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research, agrees that we are headed towards a dangerous level of warming, but Schellnhuber is rather more optimistic that policy can bail us out.

He challenges the assumption that the dimming effect of aerosols will soon vanish, arguing that in the real world, some aerosols will be harder to reduce than others. He also disputes that climate policy won’t do any better than freezing emissions at 2005 levels. Instead, he takes a scenario where, in Copenhagen in 2009, the world adopts the goal set out at the G8 summit in Japan of halving global GHG emissions on 2000 levels by 2050 and then phases out carbon emissions completely by 2100. If this were the case, he says we would have “a fair chance to hold the 2°C line, yet the race between climate dynamics and climate policy will be a close one”.

Though Schellnhuber’s scenario is somewhat less daunting, it could still constitute dangerous climate change, so either way, it’s not good news. What is less clear is when this threshold will be reached – and this will inevitably depend on national and international air pollution control and climate policies.

Ramanthan and Feng point out that stalling efforts to curb other air pollutants should not be seen as an option, though, because of their negative health effects. Over on BioPact, Ramanathan is quoted as saying:

This paper demonstrates the major challenges society will have to face in dealing with a problem that now seems unavoidable. We hope that governments will not be forced to consider trade-offs between air pollution abatement and mitigation of greenhouse gas emissions


Olive Heffernan


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    Karen Nyhus said:

    The commentary here unfortunately makes a common error: that of confusing emissions (rate of GHG release) with levels (atmospheric concentrations). (“Schnellnhuber… disputes that climate policy won’t do any better than freezing emissions at 2005 levels”). Indeed, Schnellnhuber is talking about long-term emissions targets and timelines, the standard currency of UNFCCC talks (“50% reductions by 2050,” etc.), and holds out some hope for us. However, as a response to the Ramanthan paper, it must be restated (as you do correctly in summarizing it at first) that the Ramanathan and Feng paper argues that 2.4 degrees is committed if GHG concentrations are held at their fixed 2005 level. That would mean that emissions, far from holding steady at their 2005 rate, would have drastically decreased in 2005 such that they (plus other sources) matched total sinks, leaving us with steady-state atmospheric concentrations. Even if emissions had magically and instantaneously leveled off at their 2005 levels, the atmosphere would still be accumulating GHG at around 2 ppm/yr. Ramanathan and Feng posit this magically instantaneous steady-state atmospheric concentration as a hypothetical example to illustrate how ‘locked-in’ the 2.4 of additional warming is, as Schnellnhuber correctly states. With few exceptions (such as, EcoEquity, James Hansen, and yes, Schnellnhuber himself, who recently called for stabilization at 280 ppm, a new record low target – UK Guardian 9/15/08) no one is talking about reducing concentrations or even leveling them off, they’re talking about limiting the increase to 550 ppm, 450 ppm, etc. Just a point of clarification; I suspect it was a small oversight, but thought it deserved a comment.

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    Olive Heffernan said:

    Thanks for the comment, Karen. Yes, I meant atmospheric greenhouse gas concentrations rather than emissions and that’s an important point of clarification.

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    Jack Lacton said:

    Scientists have tried to prove that pollutants have enough cooling effect to counter any GHG induced warming and drawn a big, fat blank. It’s a major problem for climate science to explain the large gap between the increase in GHG concentration and fall in temperature over the last decade. One of the major problems with models is that they vastly over-estimate the feedback effect that a rise in CO2 could have, especially with regard to water vapour. Any rise in temperature will be much less than models predict and so we should expect another 0.7C through to the end of the 21st century. Certainly nothing to be worried about.

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