“Climate change is real. It is happening right now,” DiCaprio continued. “It is the most urgent threat facing our entire species, and we need to work collectively together and stop procrastinating.” (You can watch the full speech here.)
A little-noticed scientific study that emerged last week not only bears this out — but it also suggests that climate change could be a more urgent problem than we all assumed.
At least since 2013, it has been common to claim that the world has a limited carbon “budget” to emit if we still want good odds of keeping global warming below 2 degrees Celsius above pre-industrial levels, a widely accepted international target.
Because scientists can calculate the relationship between how much carbon there is in the atmosphere and how much temperatures are expected to rise, this concept of a “budget” implies a number beyond which emissions must cease entirely (or beyond which we must find some way of pulling carbon dioxide out of the atmosphere).
In 2013, the United Nation’s Intergovernmental Panel on Climate Change outlined such a budget in its highly influential “Summary for Policymakers for Working Group I” (as this particular, widely read document is called). The panel laid out the math to let readers reach their own conclusions. But the gist was that, taking into account how much we’ve already emitted and the role of gases other than carbon dioxide, humanity can’t emit more than about 1,000 billion tons, or gigatons, of carbon dioxide if we want a 66 percent chance or better of staying below 2 degrees.
Since then, the 1,000 gigaton figure has been quite influential. The U.N. Environment Program, for instance, puts it this way: “The IPCC in its fifth assessment report concluded that to limit global warming to below 2 degrees C, the remaining cumulative CO2 emissions — the so-called carbon budget — are in the order of 1,000 GtCO2.” The program is just one of many parties that have often cited the IPCC’s calculations.
But in a new study in Nature Climate Change last week, Joeri Rogelj of the International Institute for Applied Systems Analysis in Austria and an international team of colleagues interrogate this notion of a carbon “budget” and provide some reasons for thinking we could actually have even less wiggle room than that.
“We shaved off the higher end of the estimates, and we showed that the lower end of the previous range is actually what we should be aiming at,” Rogelj says.
First of all, the IPCC’s budget numbers were up through 2011 — and the world, Rogeli and his colleagues say, has been emitting about 40 gigatons per year. So already the budget is much narrower.
On top of that, though, Rogelj and his fellow researchers looked at different ways of computing the carbon budget — including different approaches taken by other parts of the IPCC itself that differ from the estimate cited above. “Already in the IPCC, there were many carbon budget estimates that all used different methodologies and showed something different,” Rogelj says.
And for a variety of technical reasons, Rogelj and his team don’t opt for the aforementioned 1,000 gigaton budget as the top choice. Rather, they say, a budget of between 590 and 1,240 gigatons, as of last year, is the “most appropriate” to use, if we want a better than two-thirds chance of keeping global warming below 2 degrees. That translates, the study says, into between 15 to 30 years of emissions at the current rate.
Note, by the way, two key assumptions here (which these authors would fully acknowledge): that 2 degrees is somehow “safe” (there are many reasons to question that assumption) and that a mere 66 percent chance of being right about the fate of the planet is acceptable (also highly questionable).
The reason for adopting the different and somewhat lower budget, Rogelj says, turns on the issue of thresholds and the lag time between when emissions happen and when they have their effect on temperature. “By taking the budget until the moment that you exceed a certain temperature threshold, then the emissions of the last five years to a decade have not yet been accounted for in the warming,” he says. “So you basically always overestimate the budget by a slight amount if you use that methodology.”
“Different estimates of the carbon budget either ignored the climate effects of non-CO2 gases, ignored co-emissions, were based on a smaller sample of emissions scenarios, or used a different threshold of probability for holding warming below 2 degrees C,” explains Climate Analytics, whose director Michiel Schaeffer was one of the co-authors of the new paper.
The prominent figure of 1,000 gigatons — now reduced to about 850 as of 2015, Rogelj says — still lies within the 590 to 1,240 gigaton range. It’s just that the danger zone could be breached with a considerably lower level of emissions, based on this new analysis. And of course, if we want to hold warming to just 1.5 degrees — or, if we want a higher level of certainty than 66 percent — the budget gets even narrower.
In his Oscar’s speech, DiCaprio concluded, “Let us not take this planet for granted. I do not take tonight for granted.” The math of the carbon budget implies that if we want to do something about climate change, the window for action could be even narrower than we thought.
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