Talk at Cancun is of 2 degrees Celsius – even 1.5 degrees would have effects lasting centuries

This is due to a change in ocean-atmosphere heat exchange, scientists of the Potsdam Institute for Climate Impact Research discovered. Heat in lower oceanic layers is trapped by a cooling of upper layers, says a study which is about to be published in the journal Earth System Dynamics.

Even if it would work to cool down the planet by extracting CO2 from the atmosphere, according to the calculations this cooling would be ten times slower than the currently observed temperature rise by greenhouse gases. Steric sea level rise under this scenario would continue for 200 years after the peak in surface air temperatures.

At the climate summit in Cancún, Mexico, it is being debated whether and how climate change of more than two degrees compared to the preindustrial age can be prevented. Pledges by many states to reduce their emissions of CO2 would still lead to an increase of global mean temperatures by 3 to 4 degrees. One year ago, in Copenhagen, after pressure by mainly small island states it was decided that a more ambitious scenario of emission reduction should also be investigated, with a 1.5 degrees limit. Research on this up to now has been sparse. PIK researchers have now simulated several scenarios with high-performance computers.

“The good news is that a limitation of global warming really could be achieved if emissions declined after the year 2015 and if from 2070 considerable amounts of CO2 would be extracted from the atmosphere,“ says lead-author Jacob Schewe. To achieve this, however, fossil fuels would probably have to be substituted by the massive use of biomass plus carbon capture and storage. These two technologies imply risks. But the bad news is, Schewe says: “Even a mean temperature rise of less than two degrees would have far-reaching consequences – though less severe ones than in the case of stronger global warming.“

Thermal oceanic expansion alone would yield, under the 1.5 degrees scenario, a mean sea level rise of 30 centimeters in 2250. “This would already have serious effects on many coastal regions,“ says Anders Levermann, co-author and professor of dynamics of the climate system. “If we just keep on emitting greenhouse gases, in a business-as-usual scenario, steric sea level rise wouldn't stop even in 2500, resulting in a mean sea level rise of 200 centimeters.“ In addition to this, there would be contributions to sea level rise from melting ice sheets. Monsoon rainfall in Asia could also be considerably affected, even under the 1.5 degrees scenario. All of this is significant for the issue of adaptation to climate change, which is on the agenda in Cancún.

Most relevant are changes of temperature in the oceans, the simulation run by the PIK researchers shows. In the northern Atlantic, for instance, there could be abnormal warming for a long time. This would be provoked not just by the known inertia but by the newly discovered, still stronger mechanism whereby cooling on the surface interferes with regular ocean-atmosphere heat exchange. This could affect marine ecosystems. The ice-shelves of Antarctica could melt. And prolonged deep ocean warming could trigger the dissociation of methane hydrates in ocean sediments and thereby release additional amounts of greenhouse gas into the atmosphere.

Article: Schewe, J., Levermann, A., Meinshausen, M.: Climate change under a scenario near 1.5 C under a scenario of global warming: Monsoon intensification, ocean warming and steric sea level rise. Earth System Dynamics Discussions 1 (2010), 297-324 [doi:10.5194/esdd-1-297-2010]

Link: http://www.pik-potsdam.de/%7Eanders/publications/schewe_levermann10b.pdf

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