3.3 Dealing with climate: Carbon budget

Author: Lloyd Alter

ABSTRACT: The carbon budget is a simplified method of estimating the maximum emissions that can be added to the atmosphere to keep under a given temperature rise.

Image credit: Carbon Brief


The Carbon Budget is a target that starts with the “simple idea”, as Zeke Hausfather of Carbon Brief calls it, that “the amount of global surface temperature warming tends to increase proportionately with the total cumulative emissions of CO2.” The challenge was to calculate how much could be emitted before the specified temperature rise was likely to occur. It was based on calculations of how much carbon had been added to the atmosphere since the mid-1800s, which has caused a temperature rise of 1 degree, and extrapolating this into the future. This was again complicated science, given that the oceans and forests absorb so much carbon and there are so many variables that have to be separated out to figure out the quantity of anthropogenic emissions. As Bard Lahn notes, it is “a concept explicitly aimed at mediating between scientific knowledge and policymaking.” He continues in his History of the Global Carbon Budget:

Alongside its scientific merits, therefore, the main strength of the carbon budget concept was seen by scientists to lie in its ability to simplify and accentuate certain choices and challenges facing policymakers. It was based on this line of reasoning that the IPCC AR5 report unequivocally concluded that “the simplicity of the concept of a cumulative carbon emission budget makes it attractive for policy.”

Bard Lahn

It might be a simple idea, but it is controversial and hard to calculate. At the end of 2018 the IPCC estimated it to be 420 gigatonnes of CO2 equivalents to have a 66% chance of keeping the temperature rise below 1.5 degrees; by the beginning of 2020 it was down to 235 tonnes. In 2019 emissions totalled 33 gigatonnes, so the balance was shrinking quickly. 2020 was lower by about 7 percent thanks to the pandemic, which is about the amount we have to keep reducing emissions every year.

There are many different calculations of the carbon budget, but ultimately it is a communications tool, a target that is easy to understand, a way of conveying the scale of the problem.

Personal Carbon Budget

The 1.5 degree lifestyle project is based on every person on earth having a personal carbon budget, which you get by dividing the budget by the population, which comes out to 30.13 tonnes based on a 235 tonne total budget. I wrote on Treehugger: “am not saying that everyone should or can live their entire lives counting their cumulative carbon budget, but that is what we have to do collectively, so it is a useful tool to keep in mind.”


Alter, Lloyd. (2020). What is your Lifetime Carbon Budget and Why Does it Matter? Retrieved from: https://www.treehugger.com/lifetime-carbon-budget-why-it-matters-5092738

Hausfather, Zeke. (2018). Why the IPCC 1.5C Report expanded the carbon budget. Carbon Brief. https://www.carbonbrief.org/analysis-why-the-ipcc-1-5c-report-expanded-the-carbon-budget/

Lahn, Bard. (2020). A history of the global carbon budget. Retrieved from: https://wires.onlinelibrary.wiley.com/doi/full/10.1002/wcc.636

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