New climate change calculations could buy the Earth some time — if they’re right – Washington Post

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Participants look at a screen showing a world map with climate anomalies during the U.N. Climate Change Conference in Le Bourget, near Paris, on Dec. 8, 2015. (Stephane Mahe/Reuters)

A group of prominent scientists on Monday created a potential whiplash moment for climate policy, suggesting that humanity could have considerably more time than previously thought to avoid a “dangerous” level of global warming.

The upward revision to the planet’s influential “carbon budget” was published by a number of researchers who have been deeply involved in studying the concept, making it all the more unexpected. But other outside researchers raised questions about the work, leaving it unclear whether the new analysis — which, if correct, would have very large implications — will stick.

In a study published in the journal Nature Geoscience, a team of 10 researchers, led by Richard Millar of the University of Oxford, recalculated the carbon budget for limiting the Earth’s warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above temperatures seen in the late 19th century. It had been widely assumed that this stringent target would prove unachievable — but the new study would appear to give us much more time to get our act together if we want to stay below it.

“What this paper means is that keeping warming to 1.5 degrees C still remains a geophysical possibility, contrary to quite widespread belief,” Millar said in a news briefing. He conducted the research with scientists from Britain, Canada, New Zealand, Austria, Switzerland and Norway.

But the new calculation diverged so much from what had gone before that other experts were still trying to figure out what to make of it.

“When it’s such a substantial difference, you really need to sit back and ponder what that actually means,” Glen Peters, an expert on climate and emissions trajectories at the Center for International Climate Research in Oslo, said of the paper. He was not involved in the research.

“The implications are pretty profound,” Peters continued. “But because of that, you’re going to have some extra eyes really scrutinizing that this is a robust result.”

That may have already begun, with at least one prominent climate scientist confessing he had a hard time believing the result.

“It is very hard to see how we could still have a substantial CO2 emissions budget left for 1.5 °C, given we’re already at 1 °C, thermal inertia means we’ll catch up with some more warming even without increased radiative forcing, and any CO2 emissions reductions inevitably comes with reduced aerosol load as well, the latter reduction causing some further warming,” Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany said by email.

Any substantial revision to the carbon budget would have major implications, changing our ideas of how rapidly countries will need to ratchet down their greenhouse gas emissions in coming years and, thus, the very workings of global climate policymaking.

Limiting the Earth’s warming to 1.5 degrees Celsius above preindustrial temperatures was the most ambitious goal cited in the 2015 Paris climate agreement. It is of particular importance to vulnerable developing nations and small island states, which fear that they could be submerged by rising seas unless warming remains this modest.

Discussion up until now, however, has largely focused on how to avoid the more lenient but still-quite-difficult target of 2 degrees Celsius (3.6 degrees Fahrenheit). That is both because 1.5 degrees C was widely viewed as infeasible and because considerably less research had focused on studying the achievability of the target.

In 2013, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) calculated that humanity could emit about 1,000 more gigatons, or billion tons, of carbon dioxide from 2011 onward if it wanted a good chance of limiting warming to 2 degrees C — launching the highly influential concept of the “carbon budget.”

The allowable emissions or budget for 1.5 degrees C would, naturally, be lower. One 2015 study found they were 200 billion to 400 billion tons. And we currently emit about 41 billion tons per year, so every three years, more than 100 billion tons are gone.

No wonder a recent study put the chance of limiting warming to 1.5 degrees C at 1 percent. Peters said that according to the prior paradigm, we basically would have used up the carbon budget for 1.5 degrees Celsius by the year 2022.

That’s what makes the new result so surprising: It finds that we have more than 700 billion tons left to emit to keep warming within 1.5 degrees Celsius, with a two-thirds probability of success. “That’s about 20 years at present-day emissions,” Millar said at the news briefing.

“These remaining budgets are substantially greater than the budgets that might have been inferred from the” IPCC, he added.

The recalculation emerges, said study co-author Joeri Rogelj of the International Institute for Applied Systems Analysis in Austria, because warming has been somewhat less than forecast by climate models — and because emissions have been somewhat more than expected.

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