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

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Olive Heffernan