Greta Thunberg and the carbon budget

Joeri Rogelj and Piers Forster wrote on CarbonBrief that the concept of a “carbon budget” has proved to be both elegantly simple and stubbornly complicated. I start with the elegantly simple.

The theory behind the concept says that the maximum global warming is determined by the sum of the CO₂ emissions that we have emitted so far and will emit in the future, ie by the cumulative CO₂ emissions. The climate panel's fifth assessment report AR5 from 2013 states that there is a linear relationship between the cumulative emissions and the rise in temperature. The AR5 estimated the carbon budgets for both 1.5˚C and 2˚C of warming. These estimates were updated in 2018 in the Climate Panel's special report on 1.5˚C warming SR15.

In her speeches, Greta Thunberg uses a carbon budget calculated in SR15 to explain why it is urgent to reduce the emissions. She does this in an elegant and simple way. On December 11, 2019, at COP25 in Madrid, she said:

• In chapter two, on page 108 in the SR 1.5 IPCC report that came out last year, it says that if we are to have a 67 percent chance of limiting the global temperature rise to below 1.5˚C, we had on January 1, 2018, 420 gigatons of CO₂ left to emit in that budget. And of course, that number is much lower today as we emit about 42 gigatons of CO₂ every year including land use. With today’s emission levels, that remaining budget will be gone within about eight years. These numbers aren’t anyone’s opinions or political views. This is the current best available science.

A gigaton is one billion tons. Later in the same speech she said:

• Most models assume, however, that future generations will somehow be able to suck hundreds of billions of tons of CO₂ out of the air with technologies that do not exist in the scale required and maybe never will. The approximate 67 percent chance budget is the one with the highest odds given by the IPCC. And now we have less than 340 gigatons of CO₂ left to emit in that budget to share fairly.

The message is clear and simple. We must reduce emissions so that the sum of all future emissions remains within what is left of the carbon budget. Furthermore, we cannot trust that future generations will be able to clean up after us.

She correctly retrieves and interprets the information in the Climate Panel's Special Report SR15. The presentation of the speech indicates that she understands the topic well.

The Global Carbon Project (GCP) publishes emissions data every year. On December 5, 2019, they published data up to and including 2018. Global emissions in 2018 were 42.1 billion tonnes of CO₂. It includes land use change. This is in line with what Greta Thunberg said in her speech.

Emissions in 2018 were 2.2 percent higher than they were the year before. Greta Thunberg has evidence to say that politicians do not take climate problems seriously.

Figure 1: Cumulative CO₂ emissions. The three horizontal lines show three different carbon budgets for 1.5˚C global warming. Greta Thunberg applies the smallest one in her speech.

I downloaded the GCP emission data and plotted the cumulative emissions through 2018. They are shown with the blue curve in Figure 1. The dotted blue line shows future cumulative emissions if the emissions continue as they were in 2018. The horizontal solid black line shows the carbon budget that Greta Thunberg talked about in her speech. It hits the dotted blue line in December 2027. This graphically illustrates what Greta Thunberg said in her speech. With today's emissions, this carbon budget will be gone in about eight years.