Few ideas in climate science have gained greater public attention in the last decade than the concept of the "carbon budget." It's an estimate of how much carbon dioxide can be emitted by humans before temperatures spill over a potentially dangerous threshold.
The concept carries immense weight for international climate policy, because the Paris Agreement calls for keeping global temperatures within at least 2 degrees Celsius, if not a more ambitious 1.5 C, over preindustrial levels. The idea is to have a carbon "budget" to help world leaders develop detailed plans to address warming. Falling into debt carries unknown risks. Numerous studies in recent years have attempted to quantify just how much nations can cumulatively emit without overshooting those goals.
As the concept has grown in prominence, some experts are beginning to argue that it may not be as useful to policymakers as it seems. Some even suggest that confusion about the carbon budget could undermine global climate efforts.
"The carbon budget — you could say it was oversold, it was oversimplified," said Glen Peters, research director at the Centre for International Climate and Environmental Research in Norway. "The focus became on one number, whatever that number was. And so people started to think of the carbon budget as some unique number that's almost like a property of the climate system."
In reality, he notes, the carbon budget is more complex than a magic number. Depending on which study is referenced — and the models it employs, the assumptions it relies on and a vast number of other methodological choices — the number may change.
That's fairly normal to scientists. But to policymakers, the array of different carbon budgets currently debated among researchers may be confusing.
Two separate commentaries last week in Nature Geoscience, one authored by Peters and the other by climate researcher Oliver Geden of the German Institute for International and Security Affairs, have warned of the problems that could arise as a result.
Uncertainties about the value make it difficult to develop policies based around a single budget, they suggest. And the debate about the exact number may actually provide a kind of political flexibility for world leaders, enabling them to endlessly argue that there's still time for action, even while time is actually running out.
"Governments know the practical value of imprecision," Geden, who is also lead author of the Intergovernmental Panel on Climate Change's sixth assessment, wrote in his commentary. "The more precisely a policy objective is defined, the greater its risk of failure."
A quick rise in popularity
The concept of a carbon budget is relatively new. Several high-profile papers brought it to the forefront of scientific attention in 2009. Just a few months later, nations around the world would meet for the U.N. climate change conference, which would ultimately result in the 2009 Copenhagen Accord.
The Copenhagen Accord is perhaps best known for introducing the international agreement to keep global temperatures within 2 C of their preindustrial levels. This threshold, along with the more recent 1.5 C target, has since become the center point of most carbon budget estimates.
Just a few years later, the IPCC would include a carbon budget for 2 C in its fifth assessment report, an estimate that received "quite a high profile" at the time, Peters noted. The report suggested that about a trillion metric tons (or 1,000 billion metric tons) could be emitted while still maintaining at least a 66 percent chance of keeping within the 2 C limit.
Afterward, he noted, the idea became popular among environmentalists who found it an "easy concept to communicate."
"You can say things like, 'The carbon budget is smaller than total amount of oil and gas and coal that we have, so you have to keep it in the ground,'" he said. "The whole idea of 'keep it in the ground,' unburnable carbon, all these catchphrases have really been offshoots of the carbon budget concept."
But as it turns out, the IPCC's trillion-ton estimate has hardly been the last word from scientists. Multiple studies have produced their own estimates for both the 1.5 and 2 C targets, some larger and some smaller. In the last year alone, reassessments of the carbon budget have caused major waves in the scientific sphere.
Last September, a highly publicized paper in Nature Geoscience unexpectedly claimed that the carbon budget for a 1.5 C temperature threshold is substantially larger than previous estimates suggested — allowing for about 700 billion more tons of carbon dioxide emissions, instead of the 200 billion to 400 billion tons some prior studies had suggested. This would indicate that there may be more time to meet the target, which many scientists had previously classified as virtually unattainable (Climatewire, Sept. 20, 2017).
The surprising findings, which were widely debated, illustrates some of the scientific intricacies involved in calculating carbon budgets. As Peters notes in his commentary, a huge variety of factors can influence the final estimate — and tweaking any one of them may result in large changes to the value.
That's especially true for budgets designed around the 1.5 C target, noted Zeke Hausfather, a climate scientist and data expert at the University of California, Berkeley. For such a stringent target, "the remaining allowable emissions are small enough that relatively minor differences in approaches or assumptions ... can have a relatively large impact on the results," he told E&E News in an email.
Different models may produce different results. How the budget itself is defined — as the emissions it would take to cross a given temperature threshold or to keep temperatures beneath it — makes a big difference, as well. Assumptions about other future greenhouse gas emissions, such as methane, can also affect the budget.
And the concept of negative emissions — the idea that future technology may allow humans to pull carbon dioxide back out of the atmosphere after it's already been emitted — can have a huge influence on the carbon budget. Assuming nations are able to deploy negative emissions in the future suggests more emissions can be allowed for the time being.
Much of the debate revolves not so much around whether a given estimate is "right" or "wrong," but on whether its underlying assumptions and methods are the best way to think about the problem. But that may be less clear to policymakers than scientists, and certainly less clear to skeptics of accepted climate science.
In the aftermath of the September paper, for instance, some right-wing publications seized on the new — and larger — carbon budget as evidence that climate science is unreliable or that global warming is less of a problem than it was previously made out to be. The Daily Mail — which has come under fire for its climate reporting in the past — covered the study with the headline "Fear of global warming is exaggerated, say scientists."
A new communication
This is one of the risks of communicating about the carbon budget as though it's a single, defined number that scientists will one day perfect, Peters suggests.
"Instead of oversimplifying, the scientific community should seek to discuss and emphasize the persistent uncertainties," he wrote — making it clear, instead, that any given estimate includes a wide range of assumptions about all kinds of factors, and that these are all part and parcel of any given budget.
Geden, too, suggests a different approach to communicating about carbon budgets — one emphasizing that climate targets are not possible without specific action, rather than suggesting that there's still time to spare.
"Instead of saying 'yes, meeting the 1.5 °C target is still feasible, but only if A, B and C happens,' the core message would be 'no, meeting the 1.5 °C target is currently not plausible, unless governments implement A, B and C,'" he wrote.
This may help to reduce the risk that policymakers will see the carbon budget not as a ticking clock, but as proof that there's still time left to delay on climate action, Geden suggested. For many, "it seems always to be five minutes to midnight," he told E&E News.
From a scientific standpoint, the carbon budget is still an immensely valuable field of research. Exploration of the carbon budget is closely related to many other critical scientific questions, such as how sensitive the climate system is to future greenhouse gas emissions.
But both Peters and Geden ultimately suggest that it's not the best concept to push on world leaders as a policy tool. That's mainly because of the uncertainties surrounding any single concrete value for the carbon budget. But even if a magic number did exist, it would be extremely difficult to translate it into national climate policy, Peters notes.
The carbon budget, by definition, is a global value — it estimates the total cumulative emissions the entire world can produce. Translating it into something useful under the terms of the Paris climate agreement, for instance, would require dividing up the carbon budget among all participating nations and assigning everyone a concrete allowance for all their future emissions. This might be a useful idea from a scientific standpoint, but it would probably be a "non-starter" in political negotiations, according to Peters.
"Countries aren't going to be willing to share out a pie," he said.
Instead, he and Geden — as well as Hausfather, the Berkeley climate scientist — all suggest that it's more useful for countries to continue setting specific timelines for a transition to net-zero carbon emissions. Pledging to be at zero by the year 2050 or 2080, for example, is more actionable than getting hung up on exactly how much carbon can be emitted in the meantime, and potentially delaying climate action in the process.
And that action plan, in a roundabout way, would be in keeping with the scientific concept of the carbon budget anyway, Peters notes. The whole point of a budget is that once those emissions have been spent, the world can't produce any more — at all.
"What the concept really says is that you have to stop emitting," Peters said. "So the carbon budget really means that you have to go to zero emissions."