World leaders have about two decades before overshooting their most ambitious climate goal of preventing temperatures from rising more than 1.5 degrees Celsius. It doesn’t sound like much time to turn things around, but it’s substantially more wiggle room than some previous estimates have suggested.

A study in yesterday’s Nature Geoscience seems to support a blockbuster paper from last September challenging a widely held belief that the world is about to blow by the 1.5-degree goal (Climatewire, Sept. 20, 2017).

Previous research has suggested that the world can only emit another 200 billion to 400 billion metric tons of carbon dioxide before overshooting the target. At the current rate of global emissions, that budget would be spent within a decade. The September paper revised the budget to more than 700 billion tons—nearly 20 years of current emissions—and caused major ripples among climate scientists in the process, with some contending that the new estimate is likely too high.

The September paper, like most carbon budgets before it, used models to arrive at its estimates—in fact, part of the debate about the budget has stemmed from disagreements about how accurately some models represent Earth’s current warming. But the new paper arrived at its conclusions with a different approach, without using models.

The researchers, led by Nicholas Leach of the University of Oxford, present a metric that relies on observations of the amount of warming the planet has experienced so far, and the rate at which it’s continuing to heat up. The ratio between these two key observations provides “a remarkably accurate prediction of remaining emission budgets” to the 1.5-degree threshold.

“That’s the key message we wanted to take away from this—that it’s not just where we are, but where we’re pointing, that will determine the remaining carbon budget,” said Leach, who just completed his undergraduate studies and is preparing to start a doctorate program at Oxford.

The new metric is best suited for the stringent 1.5-degree target, the researchers note. Beyond this level of warming, other uncertainties begin to come into play—including potential changes in non-CO2 greenhouse gas emissions and the potential for certain climate feedbacks—that are best addressed with models. But for a near-term goal, they say the new approach is a useful alternative.

Using this metric, scientists suggest that about 22 years at present-day global emission rates—around 40 billion tons of carbon dioxide each year, currently—likely remain before there’s at least a 50 percent chance of crossing the 1.5-degree threshold. At current rates, that’s roughly 900 billion tons of carbon dioxide equivalent.

To phase out greenhouse gases fast enough to hit the target, emissions would need to fall by about 4 percent every year, the study suggests—starting last year. Because carbon emissions actually rose in 2017, for the first time in three years, that means the planet is technically already behind on that trajectory (Climatewire, March 23). As a result, even if there’s a bit more leeway than previous estimates have suggested, the 1.5-degree target is still far from certain and requires immediate reductions in global emissions.

Remaining uncertainties

Recently, some experts have begun to question the usefulness of the carbon budget from a policy standpoint, suggesting that the ongoing uncertainties and revisions may either confuse policymakers or provide them with too much flexibility to continue emitting greenhouse gases (Climatewire, May 22).

But from a scientific standpoint, continued research of the budget remains undeniably important, providing “valuable information regarding the current state of the climate system,” writes climate scientist Katarzyna Tokarska of the University of Edinburgh, in a comment also published Monday in Nature Geoscience.

And among climate scientists, it remains a hot topic. Since last September, researchers have debated whether an upward expansion of the carbon budget was justified, some of them challenging particular methods the researchers used to arrive at the adjusted budget.

More broadly, though, the study opened up a larger discussion about the myriad factors that may influence a carbon budget calculation, many of which are still being debated among scientists. For instance, not all researchers agree on the exact amount of warming the Earth has experienced since the Industrial Revolution—a critical value in estimates of how close a given temperature threshold may be.

The new study’s novel approach bypasses some of the debate about the models, by not using them. But it highlights another factor the authors say needs more attention from researchers—the rate at which the planet is currently warming.

Discussions about how much the planet has warmed so far—the metric’s other key component—have “received lots of attention” since last fall, said Richard Millar of Oxford, who led the September paper and co-authored the new study, as well.

“What has not received so much focus thus far is constraining how fast the climate is currently warming up in response to human-induced drivers of warming,” he pointed out in an email to E&E News. “This paper highlights the need to constrain both of these variables to derive accurate estimates of the remaining carbon budget for very ambitious mitigation goals.”

The study applies one commonly used estimate of the current rate of warming to its final calculations. But the researchers also demonstrate that using a slightly different data set would result in a slightly different budget. So the new paper may reinforce the idea of a slightly larger carbon budget—but it also supports the discussion of the uncertainties still remaining.

“We wanted to highlight the rate of warming,” Leach said. “And the more research that’s done about it, the better it might be constrained, and therefore, we could constrain the carbon budget.”

Millar pointed out that the new paper isn’t the only one to support the September study’s expanded budget. Several other studies in the last few months have also suggested that previous estimates may have been too grim.

It’s expected that the Intergovernmental Panel on Climate Change may also include an updated budget in a much-anticipated upcoming report on the 1.5-degree target, slated for release this October. A leaked draft of the report, obtained by Reuters, reportedly states that the threshold will pass by 2040 if emissions continue at their current rates. That’s 22 years from now, the same timeline outlined in the new paper.

It may be, then, that recent research is converging on similar budgets. But while they suggest that the world’s most ambitious target is still within reach, the message remains that time is swiftly running out.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.