The carbon dioxide budget: what if humanity overcounts?

Carbon budgeting is one of the fundamental concepts in climate science and at the same time it is constantly forgotten. In fact everyone knows this principle – on their own account. For example, if you are saving for a vacation, you can set a budget, that is, the amount of money you want to spend each month. If you exhaust your budget at the end of the month and there is no money left, you won’t be able to buy anything anymore.

In principle, it works in a similar way with atmospheric carbon dioxide. Money is CO₂. Instead of expending it, people release it when they burn coal, gas or oil. But the more carbon dioxide enters the atmosphere, the warmer the planet gets. In 2015, the countries of the world decided in the Paris Agreement to limit global warming to less than 2 degrees, and if possible 1.5 degrees. In doing so, they set themselves a budget: specifically the amount of carbon dioxide that can still be contained in the atmosphere before 1.5 (or even 2) degrees of global warming is reached.

What is often overlooked is that if climate change is to stop, it is not just a question of when the world will become climate neutral. It’s about all the greenhouse gases that still end up in the atmosphere.

Just how big is this amount? Researchers are now facing this question Study in The nature of climate change He answered again. We have come to the following conclusion: The remaining carbon dioxide budget has been almost exhausted – and what remains in the world is even smaller than previously assumed. From January 2023, 250 gigatonnes of carbon dioxide will remain until the 1.5 degree limit is reached. If the world continues as it was before, this budget will be exhausted in just six years (of which one year is almost gone).

The budget has shrunk dramatically

Recently, the Intergovernmental Panel on Climate Change (IPCC) outlined the remaining carbon dioxide budget in 2021. The new study can be seen as an update to these calculations; Some IPCC authors also participate. In 2021, it has been said that humanity could produce an additional 500 gigatonnes of carbon dioxide to achieve a maximum temperature rise of 1.5 degrees. But since then the budget has been significantly reduced. With 250 gigatonnes of carbon dioxide, only about half remains today.

Even for the 2 degree mark, the remaining budget is not really generous: according to the study, there is still 1,200 gigatonnes of CO2 to meet this limit with a 50 percent probability. If you want to be on the safe side, much less.

The main reason why the budget is shrinking so quickly is simply how quickly humanity is currently spending its budget. It has been three years since the Intergovernmental Panel on Climate Change did its calculations. Global carbon dioxide emissions were at record levels during this time. With about 40 gigatonnes of CO2 per year, another 120 Gigatonnes of CO2 has been melted from the budget. “Because budget estimates for the 1.5 degree target were already very tight, every year that emissions don’t fall makes a big difference,” said Gabriel Abrahau, a research fellow at the Potsdam Institute for Climate Impact Research, Germany’s Center for Science Media. .

At the same time, understanding of the climate system has also improved. This is the second reason for the low result of the calculation: the researchers have modified how the climate changes by factors other than carbon dioxide. The role of aerosols is now more precisely known. These tiny particles, which are also emitted when fossil fuels are burned, block sunlight, darken clouds and thus have a cooling effect. If the world gave up fossil fuels, it would save carbon dioxide. However, this (smaller) cooling effect is also lost. Because climate science now assumes that this effect is stronger than previously thought, the calculations have been modified.

Has humanity already exceeded its reckoning?

This adjustment alone shows that the remaining CO2 budget can be determined, but uncertainties remain: How much will the climate warm per ton of CO2? How many degrees has it actually gotten warmer so far? Are there feedbacks and interactions in the climate system? How long will global warming continue if emissions are at zero? What are the effects of other greenhouse gases – and can the world stop them?

Precisely because the remaining budget is so small, such uncertainties have a significant impact. In addition to their budget estimate of 250 gigatonnes of CO2, the researchers also identified an uncertain range: from -170 to 840 gigatonnes of CO2. In other words: you cannot rule out that the 1.5 degree calculation is already exposed.

“The scenarios in which we are very likely to limit global warming to 1.5 degrees are over, and have long been over,” Guiri Rogelj, a climate scientist at Imperial College London and one of the study’s authors, said at a news conference. conference.

However, one thing is certain: it will be tight. “The remaining CO2 budget to meet the Paris Agreement goals is very small – no matter which method you use,” says Karl Friedrich Schleussner, who teaches at Humboldt University in Berlin. “There is a very real possibility that we will exceed the 1.5 degree target early this decade.”

Just a loan?

But what if the budget is exhausted? As with saving for a vacation, all is not lost. Some people might take out a loan: first overdraft the CO2 account to the point where the temperature temporarily rises more than 1.5 degrees, then remove the CO2 from the atmosphere again, whether through natural sinks such as forests And swamps or through carbon capture – investments. Such an overshoot, as it is called in research, is actually envisioned in most climate scenarios. However, as is usual with loans, paying them off ultimately results in a much larger cost.