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“The Paris target of limiting global temperature increase to well below 2 degrees Celsius, aspiring to keep the warming even at 1.5 degrees, offers the chance to avoid some of the greatest climate risks – the tipping of critical Earth system elements,” says co-author Ricarda Winkelmann from the Potsdam Institute for Climate Impact Research. “The ice sheets or the Amazon rainforest, for instance, are projected to succumb to disruptive and likely irreversible change once a certain warming threshold range is crossed. These are not isolated processes; they affect the whole planet.”

Necessity: tipping elements and global temperature

Based on the advances made by climate research as a whole over the past two decades, the scientists provide a defining diagram of tipping elements in the context of the global temperature evolution. “We illustrate that for the Earth system, half a degree really matters,” Winkelmann says. Warming of only 1.5 degrees above preindustrial levels will have major consequences, such as threatening the survival of coral reefs worldwide. But the difference to 2 degrees is substantial. In a 1.5-degree warmer world, for example, global sea-level rise could be limited to 1.5 meters by the year 2300, whereas at 2 degrees, 2-3 meters rise by 2300 have been projected, and the Greenland ice sheet may well pass its tipping point.

“Beyond 2 degrees, the course might be set for a long-term complete deglaciation of the Northern Hemisphere,” says Winkelmann. “This would result in sea-level rise that threatens the survival of many major coastal cities including New York, Mumbai and Tokyo. Hence the necessity of the Paris target.”

Feasibility: carbon pricing, divestment movement, wind and solar power

“While the latest IPCC assessment has shattered the infeasibility myth, showing that the 2 degrees guardrail can be respected at relatively low cost with the proper political resolve,” says co-author Stefan Rahmstorf, head of Earth System Analysis at PIK, “almost all IPCC scenarios assume so-called negative emissions – taking CO2 out of the atmosphere and storing it. That is a very long shot. However, the price decrease and the efficiency increase of wind and solar power have been beyond the most optimistic predictions.” A technical explosion of renewables would, once the new technologies reach a market penetration of 15-20 percent, lead to an implosion of the fossil industry, the scientists argue. Currently, India appears to be very serious about implementing its colossal renewables target – an example of self-amplifying developments that have the potential to tip the global market scales.

In addition, a strong climate agreement paves the way towards carbon pricing instruments that will be adopted in more and more countries, the authors say. Last but not least, issues of morality are going to interfere with economics – one case in point is the divestment campaign which aims at pulling assets out of fossil businesses. Already today, key financial market players like the German Allianz insurance, the French company AXA or the legendary US oil dynasty Rockefeller are moving in that direction.

Simplicity: negotiators can turn objectives into action

“Beyond necessity and feasibility, the 2 degrees C guardrail has a comparative advantage over competing targets that cannot be overrated in the world of ‘realpolitik’: it is easy to grasp and communicate,” says lead author Hans Joachim Schellnhuber, director of PIK.  “In fact, the target strikes the optimal balance between concreteness and intelligibility. Now the world of climate action turns around one single number!” In the days and nights of the Paris negotiations, the 2 degrees concept – originating from a 1995 report by the German government advisory council for environmental issues (WBGU) – proved its worth since every national delegation could take a stance for the temperature limit of its choice. This would have been hard to imagine with the more complicated target suggestions made recently – ocean heat content, CO2 equivalent greenhouse gas atmospheric concentrations, or temperature change rate.

“The Paris agreement is a historic achievement and a genuine triumph of reason,” Schellnhuber concludes. “Now the pressure is on to implement that consensus in time, in order to avoid the looming humanitarian tragedy for good.”

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Article: Schellnhuber, H.J., Rahmstorf, S., Winkelmann, R. (2016): Why the right climate target was agreed in Paris. Nature Climate Change

Weblink to the article once it is published: http://www.nature.com/nclimate/index.html

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