A much-anticipated report from the world’s leading authorities on climate change has reignited a debate over the usefulness of the “carbon budget.”

The concept refers to how much carbon dioxide can be emitted before temperatures rise beyond a given threshold. The idea is that informing world leaders about how much carbon will cause a tipping point can help design policies that will prevent the globe from crossing that threshold. And it also helps scientists keep track of how quickly the threshold is approaching.

But some experts suggest that, although scientifically useful, the carbon budget may not actually be promoting climate action among policymakers.

They say the international community should focus less on how much carbon can still be emitted and more on setting concrete timelines for transitioning to a net-zero carbon world. This would be a shift from the current terms of the Paris Agreement, which includes a global temperature target but no specific timeline for collectively cutting emissions.

The new report, released late Sunday night by the U.N. Intergovernmental Panel on Climate Change, explores the steps that must be taken to keep the planet from warming more than 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above preindustrial temperatures. A key question in the months preceding the report’s release was whether the formidable target is still feasible at all. In the last few years, some experts have suggested that overshooting the 1.5 C threshold is likely inevitable.

The final report indicates that it’s technically possible to meet the 1.5 C target—but nations would need to collectively bring carbon emissions down to zero within the next 30 years, requiring a staggering global effort. Whether the political will exists to make it happen remains up to the governments, authors of the report cautioned during a press conference announcing the findings.

“There are signs that mitigation is going on, but if this is to be achieved, there is an urgent need to accelerate,” said Valérie Masson-Delmotte, co-chair of the IPCC’s Working Group I and a co-author of the report.

In its last assessment report, released in 2014, the IPCC suggested that about a trillion metric tons of carbon dioxide could be emitted before the world tipped beyond a 2 C threshold, the major climate goal at the time the report was published. In the years since the last assessment report, numerous independent studies have attempted to refine the carbon budget for both the 1.5 and 2 C targets, some arriving at larger budgets and others at smaller ones.

To date, there’s no overarching consensus on what the budget should be—which creates uncertainty about how much time is left, under current emissions trajectories, before the clock runs out on the Paris climate targets.

The new IPCC report underscores these uncertainties. An early leaked draft of the report, published by Climate Home News in February, reported that emissions should be limited to 580 billion tons of carbon dioxide for only a 50 percent chance of meeting the 1.5 C goal. And an even smaller carbon budget, which would allow a 66 percent chance of hitting the target, was declared likely out of reach already.

The final version includes a revised, slightly larger carbon budget calculated using updated methods, according to authors of the report. Depending on the way that global average temperatures are measured, the carbon budget could be as small as 580 billion tons for a 50 percent chance of success, or as large as 770 billion tons.

A two-thirds shot of success would call for a carbon budget of 420 billion tons on the low end or 570 billion tons on the high end. That’s about 10 to 14 years left at current emissions levels.

It’s a higher budget, though, than would be expected based on the methods used in the previous IPCC assessment report—akin to resetting the doomsday clock to “five minutes to midnight,” according to Oliver Geden, a climate researcher at the German Institute for International and Security Affairs.

But these new values aren’t watertight, either. The report points out that carbon budgets are subject to high levels of uncertainty, related to gaps in the scientific understanding of climate system feedbacks and responses.

All in all, the carbon budget is a tricky concept, and one that even scientists are still working to refine. Geden suggests that the updated carbon budget is likely to inspire “a huge debate” among scientists about the methods used to calculate it. It’s an issue that he says will need to be addressed again in the next IPCC assessment report, which is a work in progress.

And as a tool for policymakers, Geden and other experts have suggested that the carbon budget may not be especially useful—it is confusing, is subject to constant revision and may give world leaders a false sense of security about how much time they have left to continue emitting carbon dioxide, they’ve noted (Climatewire, May 22).

As a result, some experts have begun to advocate for a different kind of global target—not just a temperature goal, but a concrete timeline to reach net-zero carbon emissions, regardless of the ongoing debates about the exact carbon budget.

The new IPCC report generally takes this approach. While it includes a discussion of the carbon budget, it maintains that the 1.5 C target requires a shift to net-zero emissions by 2050, no matter what. This timeline should be strict enough to work with any range of carbon budgets thought to be consistent with the 1.5 C goal.

When asked about the revised carbon budget, co-author Masson-Delmotte said that “if you would like to stabilize global warming to 1.5 degrees Celsius, the key message is that net CO2 emissions must reach zero by 2050, and that’s the most important finding of the report.”

Glen Peters, research director at the Centre for International Climate and Environmental Research in Norway, added that the updated carbon budget is “largely of academic interest.”

“What really matters is the pathways, how fast do emissions need to go down, when do they get to zero, and how much negative emissions do we need,” he said in an email to E&E News. “Those pathways, at least in the special report, do not change with the updated carbon budget, as the calculations were done before the carbon budget was revised.”

That said, even a net-zero target carries its own challenges and decisions. The new report notes that reaching net zero by 2050 will likely require a certain amount of “negative emissions”—using vegetation or other forms of technology to suck carbon dioxide back out of the atmosphere. Exactly how much negative emission will be required depends on how quickly world leaders can cut their carbon output in the coming years. That’s a concern, because some of the technologies proposed for negative emissions have yet to be proved on a large scale.

Still, a net-zero target gives policymakers a timeline for specific climate action beyond simply setting a temperature goal or adopting a carbon budget. And the idea may become increasingly relevant as world leaders prepare to meet in Poland this December, where they will finalize the Paris Agreement’s rulebook—a set of guidelines for how individual nations should outline their climate action plans and report their progress.

According to Geden, the new report will likely inspire debate on new long-term targets. It would also be useful for policymakers to start thinking about short-term targets, as well, he added, since the report calls for significant emissions reductions by 2030, on the way to net zero by 2050.

“Every industrialized country and every emerging economy has to strengthen midterm climate targets considerably,” he noted.

Whether the concept of a net-zero target will reappear in the IPCC’s next assessment report—or in the international discussions in Poland this December—remains to be seen. But it’s an idea that experts say should stay on the burner.

“We want to stop going over the cliff by applying the brakes now,” Peters told E&E News. “It is more interesting to know how hard to apply the brakes as opposed to how long it takes to hit the cliff at the current speed.”

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