One of the more terrifying elements of climate change is the uncertainty of it all. You start with the big picture of a warming planet, but as you zoom in you find ever more climatic and geological and biological systems interacting with one another—a complexity unfathomable for the human mind. We’re talking about a crisis that is affecting every organism and every square inch of this planet.
That makes calculating the carbon budget—the amount of greenhouse gases humanity can emit globally while adhering to certain goals—an unenviable task. (The goal of the Paris Agreement was 1.5 degrees Celsius above preindustrial levels; we’re already at 1 degree.) Different teams of researchers have reached wildly diverging conclusions, from “We can emit 1,000 gigatonnes more CO2 before we reach 1.5 degrees” to “Sorry, but we’ve already spent our carbon budget for 1.5.” There is simply too much uncertainty in the models.
But today in the journal Nature, researchers are proposing a new framework that aims to bring clarity to this kind of work, first by reconciling differences in carbon budgets and second by reducing uncertainty going forward. That’s critical, because climate policy hinges on the budget, and it’s climate policy that’ll help us stave off global disaster.
Matt Simon covers cannabis, robots, and climate science for WIRED.
For the past few years, the main challenge with formulating a carbon budget has been definitional. Do you, for instance, only incorporate global air temperatures, or should ocean temperatures be part of the mix too? Also, what greenhouses gases are we even talking about here? The two main warming culprits, CO2 and methane, are very different beasts: Methane is a far more powerful greenhouse gas, but it dissipates much faster in the atmosphere, so is CO2 ultimately of greater concern?
And as the models improve, they grow ever more complex by incorporating ever more planetary factors—what scientists call “unrepresented Earth system feedback mechanisms.” In the polar regions, for instance, permafrost is thawing. When this happens, microbes convert carbon stored in the soil into CO2 and methane, leading to further warming.
If you’re not pitying climate scientists yet, consider just a handful more complicating factors: Melting ice exposes dark land underneath, which absorbs more of the sun’s energy than white snow; plastics might be killing the bacteria that are responsible for 20 percent of carbon capture on Earth; deforestation removes trees that suck CO2 out of the air.
What the Nature researchers have done is reconcile the thinking behind various approaches to the carbon budget. Their holistic calculation includes multiple types of greenhouse gas emissions, including methane from thawing permafrost, and incorporates the most up-to-date estimates of current warming. “It lifts the fog a bit on confusion that was there before,” says lead author Joeri Rogelj, a climate scientist at Imperial College London.
Previously, lots of different carbon budget studies used lots of different methods. “But when it comes to calculating the size of the future carbon budget, the estimates from different methods don’t all agree,” says Phil Goodwin, who studies Earth system dynamics at the University of Southampton and wasn’t involved in this work. “This study does a really good job of explaining the possible reasons why different studies have arrived at different estimates of the future carbon budget.”
The research also provides a more unified framework to compare different methodologies and their carbon budget estimates. And it spits out its own carbon budget, too. Previous estimates have fallen along a spectrum, with some suggesting that the 1.5-degree target is already out of the reach and others assuring us we have 20 years to transition to a low-carbon economy. The good news: The Nature team’s carbon budget sits pretty much in the middle of those extremes.
Still, why not just say, Hey, let’s all cut emissions as quickly as we can? The problem with that, says Concordia University climate scientist Damon Matthews, “is that ‘as quickly as possible’ means many, many different things to different people.” Matthews, who was not involved in the Nature study, added that “we haven’t even started to talk about what might be ‘possible’ and are still mostly arguing about what is ‘feasible without compromising economic growth.’ These are of course extremely different things, and the latter will not get us anywhere near the 1.5 degrees C target.”
Piers Forster, director of the Priestley International Centre for Climate at the University of Leeds and a coauthor on the new paper, agrees with Matthews. “You can’t just tell every country to cut emissions to zero as fast as possible,” he says. When the Intergovernmental Panel on Climate Change was negotiating its most recent report, Forster adds, “we actually spent over 24 hours negotiating the paragraph on the carbon budget, and that’s because it’s a really contentious thing.”
This new framework may well help guide future negotiations. And so researchers continue to chip away at the inherent uncertainties of climate change, one complicated Earth system at a time.