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EGU 2013: climate change hard to reverse

By Liz Kalaugher

It’s early days, but scientists are developing techniques to remove carbon dioxide from the atmosphere, either directly through technologies such as artificial trees or, less directly, by biomass burning with carbon capture and storage. Even if these methods are implemented, however, the Earth will feel the temperature effects of climate change for centuries to come.

That’s according to Andrew MacDougall of Canada’s University of Victoria, who gave a press conference at the European Geosciences Union’s General Assembly in Vienna. His simulations using the University of Victoria Earth-System Climate Model indicate that without any artificial carbon removal, and assuming that fossil fuels run out, around 60–75% of near-surface warming will remain 10,000 years into the future.

With a middle-of-the-road scenario for carbon-dioxide removal, however, a 20th century-like climate could be restored by the late 24th or early 25th century, MacDougall found. But simulated surface air temperature would still be above the pre-industrial temperature by the end of the 30th century, even for the fastest carbon-removal scenario he modelled, as oceans gradually release their stored heat.

Restoring climate will require removal of more carbon from the atmosphere than was originally emitted by man, MacDougall said. In some scenarios, 115–190% of anthropogenic emissions will need to be sequestered. Currently, land and the oceans remove around half of the carbon dioxide man emits to the atmosphere each year. Once atmospheric carbon levels fall, this stored carbon will start to emerge. In addition, melting of permafrost as temperatures rise has released methane and carbon dioxide to the atmosphere. “There’s no easy process to put this back in,” said MacDougall.

The simulations indicate that it’s much easier to return ocean pH levels to normal than temperatures. But sea-level rise from melting of the Greenland ice sheet seems largely irreversible – while atmospheric carbon levels of less than 350 ppm could stabilize the ice sheet, water from the oceans would only be refrozen into the ice very slowly.

MacDougall simulated carbon concentrations that followed the representative concentration pathways RCP 2.6, 4.5, 6.0 and 8.5 used in the IPCC’s forthcoming fifth assessment until they reached their peak, in 2050, 2150 and 2250, respectively. Then he reduced carbon concentrations at the same rate that they had increased, as well as restoring pasture and croplands to their pre-industrial area.

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