Differences between carbon budget estimates unraveled (Abstract, Joeri Rogelj, Michiel Schaeffer, Pierre Friedlingstein, Nathan P. Gillett, Detlef P. van Vuuren, Keywan Riahi, Myles Allen & Reto Knutti, Nature Climate Perspective, Feb. 24, 2016)
Also discussed here: A lower limit for future climate emissions (International Institute for Applied Systems Analysis (IIASA) News, Feb. 24, 2016) And here: A second look at the two-degree target (International Institute for Applied Systems Analysis (IIASA) News, Dec. 7, 2015)
Today we review research at the International Institute for Applied Systems Analysis near Vienna which examines the conditions necessary to keep global warming within the 2 deg C target accepted by the United Nations Conference COP21 at Paris in the fall of 2015. The limit to the amount of carbon which can be burned (or the available carbon budget) in the future after 2015 has been estimated at varying amounts from 590 to 1240 billion tonnes, a difference of almost a billion tonnes. Differences arise because of assumptions made in various scenarios as well as the contributions by other greenhouse gases than CO2. The possibility of overestimating the budget by up to a billion tonnes is a major concern in terms of the urgency to reduce carbon emissions in action plans for the immediate future.
"researchers propose a limit to future greenhouse gas emissions—or carbon budget—of 590-1240 billion tons of carbon dioxide from 2015 onwards, as the most appropriate estimate for keeping warming to below 2°C, a temperature target which aims to avoid the most dangerous impacts of climate change”
“A global climate target will always be a political goal, not a scientific one…Even at two degrees of warming there will be consequences and impacts, and science alone cannot say what level of climate change should be considered safe by society.”
“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…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,”
“The researchers identified that the variation in carbon budgets stemmed from differences in scenarios and methods, and the inclusion of other human activities that can affect the climate, for example the release of other greenhouse gases like methane. Previous research suggested that the varying contribution of other human activities would be the main reason for carbon budget variations, but surprisingly, the study now finds that methodological differences contribute at least as much”