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Why the tipping point is two degrees ahead

The goal of international climate policy is to prevent global warming from exceeding two degrees Celsius. But what actually happens if it does so is still unclear – and that’s exactly the problem.

Whenever the issue of global warming comes up, one figure is always described as something like a magic limit, the number which may not be passed: two degrees Celsius. Since as long ago as 1996, the institutions of the European Union have been pleading for global warming not to be allowed to exceed two degrees Celsius. The two-degrees target was acknowledged at UN level for the first time at the 2009 Climate Change Conference in Copenhagen, and not exceeding the two-degrees mark was made an official target at the 2010 Climate Change Conference in Cancún. But what is the science behind this ‘magic number’ of two degrees Celsius actually based upon?

“Two degrees Celsius is not just an arbitrary crossbar picked out of the air and agreed upon in political discussions,” explains Helmut Haberl, a Professor in Human Ecology at the Alpen-Adria University in Klagenfurt. It is actually “a figure at which the risk of triggering what we refer to as ‘tipping points’ increases exponentially.” A tipping point is what we call a turning point, the moment when a trend suddenly breaks away, changes direction or accelerates sharply, when the balance is ‘tipped’ and there is no going back to old ways of being, like the moment when general discontent is surprisingly transformed into revolution.

In the case of climate models, tipping points are moments when drastic climate change is caused within a short time. “These are points in the Earth system when massive qualitative upheavals in the system occur,” Helmut Haberl explains, “in other words, when the effects of tipping arise in natural systems.” The Amazon rainforest is one example. Most of the precipitation in the Amazon Basin originally comes from the evaporated water of the forest itself. The effects of global warming and deforestation can cause the ground to rapidly dehydrate, however, accelerating the disappearance of the rainforest further – with disastrous consequences for the global climate in turn.

The Greenland ice sheet provides another, even more vivid, example of a tipping point: if Greenland’s ice melts away completely as a result of global warming, then not only will sea levels rise by several metres, but the cooling effect of the ice sheet will be lost, because ice strongly reflects sunlight, while water and the ground both absorb the vast majority of the heat.

“In recent years, we have identified a number of these tipping points in the Earth system,” says Helmut Haberl, who works, amongst other things, as lead author of the Intergovernmental Panel on Climate Change (IPCC). Some of these tipping points “have a very high potential impact”.

Because while the human ecologist concedes that we don’t yet know exactly where many of these tipping points are, “as we get closer to exceeding the two-degrees mark, the risk increases massively that we are going to pass one or more of these points of no return.” So what would happen if we were to do nothing to counter this? “We just don’t know exactly. And that’s the really worrying thing.”