“In order to have a reasonable chance of keeping global warming below 2°C, we can only emit a certain amount of carbon dioxide, ever. That’s our carbon budget,” says researcher Joeri Rogelj, from the International Institute for Applied Systems Analysis, who led the study. “This has been known for about a decade and the physics behind this concept are well-understood, but many different factors can lead to carbon budgets that are either slightly smaller or slightly larger. We wanted to understand these differences, and provide clarity on the issue for policymakers and the public.”
“This study shows that in some cases we have been overestimating the available budget by 50 to more than 200%. At the high end, this is a difference of more than 1000 billion tons of carbon dioxide,” says Rogelj.
Rogelj says, “We now better understand the carbon budget for keeping global warming below 2 degrees. This carbon budget is very important to know because it defines how much carbon dioxide we are allowed to release into the atmosphere, ever. We have figured out that this budget is at the low end of what studies indicated before, and if we don’t start reducing our emissions immediately, we will blow it in a few decades.”
In addition to the radical action needed, a lever in the short-term could be action on short-lived greenhouse gases and atmospheric pollutants–including methane, hydrofluorocarbons (HFCs), black carbon or soot, and sulfates– which are emitted by human activities and also contribute to climate change, but remain in the atmosphere for a much shorter time than carbon dioxide (CO2). Some sources emit both , for example diesel engines which emit both CO2 and black carbon. Yet, the other way around, air pollution measures alone would not reduce carbon dioxide emissions.
Joeri Rogelj, Michiel Schaeffer, Pierre Friedlingstein, Nathan P. Gillett, Detlef P. van Vuuren, Keywan Riahi, Myles Allen, Reto Knutti. Differences between carbon budget estimates unravelled. Nature Climate Change, 2016; 6 (3): 245 DOI: 10.1038/NCLIMATE2868