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Lower limit for future climate emissions needed, research says

The world can emit even less greenhouse gases than previously estimated if global warming is to be kept under control, a new study co-authored by a University of Exeter mathematician has found.

University of Exeter

The world can emit even less greenhouse gases than previously estimated if global warming is to be kept under control, a new study co-authored by a University of Exeter mathematician has found.

Research, published this week in the journal Nature Climate Change and carried out by a team of researchers which includes Professor Pierre Friedlingstein, Chair of Mathematical Modelling of Climate Systems in the College of Engineering, Mathematics and Physical Sciences, lends further urgency to the need to address climate change.

The study found that future greenhouse gas emissions, or our annual carbon budget, should be limited to 590-1240 billion tons of carbon dioxide from 2015 onwards, in order to keep warming to below 2°C, a temperature target set by scientists if the planet is to avoid the most dangerous impacts of climate change.

Various previous estimates, including those from the International Panel on Climate Change's Fifth Assessment Report, estimated the carbon budget for holding global warming below 2°C relative to pre-industrial levels ranged between 590 to 1620 billion tonnes of carbon dioxide.

The new paper assesses the methodologies behind a number of these carbon budget and estimated that the figures to be most in line with the 2°C warming limit is in the lower range of this spectrum compared to these previous predictions.

Professor Friedlingstein said: "It is imperative that we have a clear and concise understanding of the remaining carbon budget and its sources of uncertainty. We all have a responsibility to do what we can to reduce carbon emissions, and this needs to be something that we do now, not decades in the future. Regardless on uncertainties on the total amount, the fact remains that the lower the carbon emissions on a global scale, the better it will be for the environment both now, and for the generations to come."

Estimates for a carbon budget consistent with the 2°C target have varied widely. The new study provides a comprehensive analysis of these differences. 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.

The proposed budget accounts for warming of all human activities and greenhouse gases and is based on detailed scenarios that simulate low-carbon futures.

"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. Scientifically we wanted to understand what causes these differences, and provide some clarity on the issue for policymakers and the public," said the paper's lead author Joeri Rogelj, of the International Institute for Applied Systems Analysis.

"We now better understand what the carbon budget is 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 now figured out that this budget is at the low end of what studies indicated before, and if we don't start reducing our emissions soon, we will blow it in a few decades."

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Differences between carbon budget estimates unravelled by Joeri Rogelj, Michiel Schaeffer,Pierre Friedlingstein,Nathan P. Gillett,Detlef P. van Vuuren,Keywan Riahi,Myles Allen and Reto Knutti is published in Nature Climate Change.

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