1.5C global warming target still possible, say scientists

By Mark Lynas

Image courtsey Nature Geoscience

It's not often you get good news on climate change. New research published yesterday in the journal Nature Geoscience indicates that — contrary to previous more pessimistic projections by scientists — humanity still just about has time to limit the globe's end-of-century temperature rise to 1.5 degrees C, the ambitious target agreed at the 2015 Paris summit.

An analysis of updated computer models of the earth system by an international team of 10 climate experts suggests that the threshold for 1.5C will not be crossed for another 20 years at current emissions rates, rather than the 4 to 5 years assumed previously. The revision has come about because the Earth has not warmed quite as much as was previously assumed was likely for the cumulative stock of carbon that has been emitted so far into the atmosphere.

The new results — which aim to update the previous IPCC projections issued back in 2013 — suggest that "1.5C is not yet a geophysical impossibility," according to Dr. Richard Millar, a postdoctoral research fellow at Oxford University and lead author of the study. This contradicts earlier work suggesting that the chance of staying within 1.5C is already vanishingly unlikely, or only about 1 percent.

The news triggered a rash of media headlines implying that global warming had been exaggerated. "Climate change not as threatening as previously thought," read a headline in the right-leaning UK newspaper the Daily Telegraph. "We were wrong — worst effects of climate change can be avoided, say experts" was the spin from the Times, with the sub-head that scientists had been forced to "admit" that "the world is warming more slowly than predicted."

But these takes rather miss the point. Staying within a carbon budget that — according to the models — yields a two-thirds chance of staying within 1.5C by 2100 is still incredibly difficult. The extra 20 years of emissions does not mean we can afford to spend the next two decades doing nothing. Rather, it means that we have just two more remaining decades until the entire carbon budget has been exhausted.

As the researchers point out in the paper, the new estimates yield a carbon budget that is equivalent to a 40-year linear decrease in annual CO2 emissions from 2015, with the world reaching net-zero carbon emissions by the year 2055. The scale and rapidity of emissions decreases still demanded in the updated projections therefore has only ever been seen in times of warfare or economic collapse.

Some scientists who were not involved in the new study are worried that even this picture seems too rosy. "They appear to have adjusted the budget upward based on the idea that there has been less observed warming than suggested by the climate models, but that is not actually true if you do the comparison properly," complained the German climate scientist Stefan Rahmstorf to the Washington Post.

Technicalities aside, the new study is surely a win for science in general because it shows that even polarized debates can be shifted when new evidence arises. As one of the authors, University College London professor Michael Grubb told reporters, "When the facts change, I change my mind, as Keynes said. It’s still likely to be very difficult to achieve these kind of changes quickly enough but we are in a better place than I thought." Grubb had previously been quoted as saying that emissions reductions necessary to achieve 1.5C were so extreme as to be "incompatible with democracy."

So the upshot is this: If a carbon budget sufficient to keep the planet on a relatively tolerable 1.5C trajectory is now compatible with democracy, then surely that is something to be welcomed. It does not mean we can afford to ease off on the carbon brake, but it does mean that we have a higher chance of success for the emissions reductions we do manage to achieve in years to come.

New hope should not be a bad thing. As Professor Pierre Friedlingstein of the University of Exeter concluded: "This is very good news for the achievability of the Paris targets."

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