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Improved Estimates for Paris Agreement Carbon Emission Goals

A group of researchers from Imperial College, the Met Office, and the University of Exeter has developed a novel method for estimating total carbon emissions that adheres to the 1.5 °C and 2 °C global warming objectives outlined in the Paris Climate Agreement.

Symbol for limiting global warming. Wooden blocks with word

Image Credit:  Dmitry Demidovich      /   

The Paris targets refer to the average warming over 10 or more years, though 2023’s unusually warm weather came dangerously near to breaking the 1.5 °C threshold. As such, the research team set out to investigate how much carbon will remain before the Paris goals are surpassed.

Fifteen years ago, climate scientists discovered a beneficial piece of information regarding climate change.

Despite the tremendous complexity of the climate system, total carbon dioxide emissions since pre-industrial times account for the great majority of global warming.

The notion of Net Zero—the point at which global warming virtually stops—was made possible by this finding, which also made it possible to define total carbon budgets that are compliant with the Paris targets.

The issue is that there is a significant discrepancy in the Earth System Models used to estimate climate change on the amount of global warming that would result from one billion tons of carbon emissions.

This dilemma is resolved by the new study, which demonstrates that observed carbon emissions up to the present day and observed global warming are very good indicators of how much carbon emissions remain until the Paris climate targets are passed.

The authors do this by using a concept they refer to as an “Emergent Constraint,” a fancy term for a rather basic idea.

In essence, they examine the outputs of every Earth System Model that is now available, which relate emissions per °C of global warming to the present and emissions for a specific amount of future global warming in a lovely straight line.

This implies that the most accurate projections of emissions and global warming up to this point can be easily translated into projections of the overall carbon budget needed to meet the Paris climate targets.

The new research predicts emissions budgets that are at least 10% higher than the average figure for the models, which is good news.

The bad news is that, even with decade-mean warming, there will only be a little over ten years left if carbon emissions by humans continue at their current rate until the Paris 1.5 °C threshold is exceeded.

This emergent constraint is elegant and powerful. It both uses observations to narrow the possible range of future emissions, but also lets us consider other greenhouse gases than just CO2. In this way the remaining carbon budget is made much more policy relevant.

Chris Jones, Study Co-Author, Met Office

Study lead author Peter Cox, Director of the Global Systems Institute at the University of Exeter, stated, “Our study clarifies the climate problem that needs to be solved, and we hope that it will stimulate greater efforts to reduce our emissions to Net Zero.

Journal Reference:

Cox, P. M., et. al. (2024) Emergent constraints on carbon budgets as a function of global warming. Nature Communications. doi:10.1038/s41467-024-46137-7.


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