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.

So far, I've written about the carbon budget that Greta Thunberg talked about in her speech. But in SR15 there is more than one carbon budget for 1.5˚C of warming. The horizontal dotted lines in Figure 1 show two of the others. They are calculated with different assumptions, which illustrates what I wrote at the beginning of the post. Namely, the carbon budget is also very complicated. I then linked to the Carbon Brief post A new approach for understanding the remaining carbon budget written by Dr Joeri Rogelj and professor Piers Forster. They are both co-authors of the climate panel's SR15 report. Zeke Hausfather wrote in oktober 2018 the Carbon Brief post Why the IPCC 1.5C report expanded the carbon budget. These posts provide a good introduction to the complexities surrounding the carbon budget, and I will now give a summary of this as I understand these two posts and SR15.

The SR15 has larger carbon budgets than the AR5

The carbon budgets in SR15 are larger than they are in AR5. The main reason is that we now have better knowledge of the historical emissions than we had five years ago. In AR5, the historical emissions were calculated with models, and they calculated slightly less emissions than The Global Carbon Project now does. AR5 therefore estimates that a certain amount of CO₂ emissions causes more warming than SR15 does, which causes the carbon budgets in AR5 to be smaller than they are in SR15.

Other emissions than CO₂

A carbon budget sets an upper limit for the cumulative CO₂ emissions. But other greenhouse gases, such as methane, also contribute to global warming. Measurements show that the methane concentrations in the atmosphere are higher than expected. When AR5 and SR15 calculated the carbon budgets, they had to estimate the amount of greenhouse gases other than CO₂ that will be emitted in the future.

According to the Carbon Brief article by Joeri Rogelj and Piers Forster, SR15 estimated additional non-CO₂ warming of about 0.1˚C in the future. This estimate is uncertain.

Several feedbacks are not included in the carbon budgets

Large amounts of carbon are bound in the permafrost and in the Arctic seabed. If this thaws to a large extent, it will emit large amounts of methane and CO₂. It is uncertain how much will thaw, and such additional emissions were not included when the carbon budgets were calculated. Also some other uncertain feedbacks and tipping points were not included. SR15 suggests that such additional emissions could reduce the carbon budgets with up to 100 gigatonnes of CO₂

Less air pollution in the future

Some air pollutants, such as aerosols, block incoming sunlight. It is uncertain how much this has slowed the global warming in the past and how much it will do so in the future.

It is likely that we in the coming decades will manage to reduce the air pollution, with the consequence that the cooling effect of the pollutants will be reduced. This is not included in the carbon budget calculations.

Several ways to calculate the temperature rise

The Paris Agreement states that we should keep the temperature rise since pre-industrial times well below 2˚C and try to limit it to 1.5˚C. But the Paris Agreement does not define how pre-industrial temperature should be calculated, nor does it define how temperature rise should be calculated.

The major global temperature series have measurements since 1880. The industrial revolution began before that, but we have no reliable knowledge of the temperature then. Therefore, it is common to say that pre-industrial temperature is the average temperature in the period 1880 to 1909.

The large temperature series calculate global temperature on the basis of the air temperature over land and ice and the surface temperature in the oceans. This temperature is called Global Mean Surface Temperature (GMST). The climate models calculate the air temperature over land, ice and oceans. This temperature is called SAT (Surface Air Temperature). The SAT has risen about 0.1˚C more since pre-industrial times than the GMST has. The Paris Agreement does not specify whether the limits for future temperature rise apply to GMST or SAT.

Probability of reaching the 1.5˚C target

The likelihood of keeping future temperature rise below a certain level has a major impact on the carbon budget calculated for that level. The 67 percent probability of reaching the 1.5˚C target has a significantly lower carbon budget than if the probability is reduced to 50 percent. The Paris agreement does not specify whether the probability should be 67 or 50 percent. Based on the wording well below two degrees it is reasonable to apply 67 percent for the 2˚C target. But it is not obvious for the 1.5˚C target. The wording try to limit to 1.5˚C indicates that 50 percent may be as relevant as 67 percent.

Since the Paris agreement is unclear on the points mentioned above, SR15 calculated several carbon budgets for the 1.5˚C target. The relevant ones are:

Table 1: Four carbon budgets as of January 1, 2018, in SR15 for the 1.5˚C target

Three of these carbon budgets are shown in Figure 1. The smallest one, 420 GtCO₂, is drawn with a horizontal solid black line. The two largest ones, 580 and 770 GtCO₂, are drawn with horizontal dotted black lines. The vertical lines in the figure hits the x-axis when the carbon budgets are gone with today's emissions.

Greta Thunberg chose to use the smallest carbon budget in Table 1 in her speech. SR15 gives her coverage to do so, because it focuses on that budget.

There are good reasons not to accept less than 67 percent probability of reaching the target. That is to say 33 percent for not reaching the target, and in most other contexts we are not willing to accept higher risk than that.

There are also good reasons to use the surface air temperature (SAT), because it is SAT that affects humans, agriculture and melting of ice on land.

Negative emissions

As discussed at the beginning of the post, Greta Thunberg warned against exceeding the carbon budget. If we do so, we have to assume that we or future generations will be able to extract CO₂ from the atmosphere with technologies that do not exist in the scale required and maybe never will.

But several emission scenarios for the 1.5˚C target do just that. In addition to assuming that we figure out how to do it in the scale required, those scenarios presume that the countries of the World will have the resources to do so and that they will agree on who should pay for it.

The scope of the job of extracting CO₂ from the atmosphere is illustrated by a few examples. One kg of coal gives about 3.6 kg of CO₂ when burned, and one kg of crude oil gives about 3.4 kg of CO₂ when burned. Coal and oil are easy to handle. CO₂ is a gas, and it is more difficult and more energy intensive to handle.

In his Carbon Bief post, Zeke Hausfather referred to a study showing that if we exceed the carbon budget by a certain amount of CO₂, we must later extract more CO₂ than that amount to reach the target.

Greta Thunberg has good reasons to argument against making us dependent on future negative emissions.