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.
“The most complex Earth system models that provided input to [the IPCC] tend to slightly overestimate historical warming, and at the same time underestimate compatible historical CO2 emissions,” he said by email. “These two small discrepancies accumulate over time and lead to an slight underestimation of the remaining carbon budget. What we did in this study is to reset the uncertainties, starting from where we are today.”
Pierre Friedlingstein, another author of the study and a professor at the University of Exeter in the United Kingdom, added at the news briefing that “the models end up with a warming which is larger than the observed warming for the current emissions. … So, therefore, they derive a budget which is much lower.”
The new research, thus, seems to potentially empower a critique of climate science that has often been leveled by skeptics, doubters and “lukewarmers” who argue that warming is shaping up to be less than climate models have predicted.
But Rahmstorf, for one, finds this to be part of the problem. “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,” he wrote, citing the need to measure the warming of the Arctic properly and account for the effect of aerosols.
In the meantime, the result could be a lot of confusion, says Oliver Geden, who leads the EU Division for the German Institute for International and Security Affairs.
“First, it is quite unusual that scientists say that the state of the climate is better than expected, that a recalculation of the remaining carbon budget gives us more breathing room, not less,” Geden said in an email. “Second, it is far from clear that the authors’ method/results will form a new scientific consensus, given that some researchers are already voicing objections. A significant carbon budget recalculation should not come as a surprise, but for many policymakers it will.”
Rogelj said the study did not explicitly consider whether the carbon budget for 2 degrees Celsius would also be larger, but, nonetheless, it surely rises substantially, too, if the analysis is correct.
Nonetheless, even with the new revision, the latest research finds that keeping warming below 1.5 degrees C will be quite hard. “Even with the largest estimates of the remaining carbon budget, this path is extremely challenging, starting reductions immediately and then reducing emissions to zero over 40 years,” Millar said at the press event.
Overall, the dispute raises questions about how widely the carbon-budget concept has proliferated — and just how much we actually understand it.
“It goes to show, this carbon-budget approach is still much more, let’s say, immature scientifically than what we often assume,” Peters said.
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