Like so many things in life, it all started with coffee. The polar explorer Martin Siegert drank it with his atmospheric physicist colleague Joanna Haigh. Both work at Imperial College London, but it is not known what the quality of the coffee was or whether it was that terrible stuff from the thermos flask. Anyway, they talked about what happens when humanity eventually stops putting more carbon dioxide into the air than the Earth system can handle. Does this also mean that climate change has ended and everything has stabilized?
For Siegert, the polar explorer, it was clear: nothing would be over then, the ice at the poles would of course continue to melt over centuries because it is a very slow process. But his colleague Haigh looks at the atmosphere. Her colleagues and her colleagues have so far assumed that once emissions have stopped, the rise in temperature should also stop. “We were both surprised,” says Siegert. He hadn’t known that the warming was supposed to stop so quickly – she hadn’t realized what the situation was with the ice masses. And they decided to take a closer look at the matter, not only at the status of the research, but also at the uncertainties. The result is a study published on Tuesday in the journal Frontiers in Science appeared In the end, in addition to Siegert and Haigh, 19 other researchers were involved.
So, does warming stop when emissions stop? “Our best guess is: She does,” says Sofia Palazzo Corner, who was involved in the analysis. But there are significant uncertainties. In both directions, a cooling down after emissions have ended is also entirely possible.
In the most recent report by the Intergovernmental Panel on Climate Change (IPCC), the experts assumed that the temperature should only change up or down by a few tenths of a degree after reaching net zero greenhouse gases. The Palazzo Corner team has now come to the conclusion that even more than 15 percent additional warming or cooling is not that unlikely.
The team looked at 26 processes that still have an effect after the emissions have ended
This would mean that the 1.5 degree limit could be exceeded even if emissions stopped tomorrow. Because the earth has currently warmed up by around 1.3 degrees, 15 percent more if things go wrong, and you end up at around 1.5 degrees. Apart from the fact that air pollution is currently likely to compensate for a few tenths of a degree of warming because particles in the air block sunlight. With this cooling effect, the end of emissions would also be over.
The fact that we don’t even know exactly in which direction the temperature will develop after emissions stop is due to the many processes involved. The ice will continue to melt for a long time, which means fewer bright areas to reflect sunlight – additional warming. The ocean releases some of the heat it absorbs, causing even more warming. But CO₂ continues to be absorbed by the oceans – cooling. The permafrost will continue to thaw and release greenhouse gases – warming. The clouds can change further as a result of warming and retain more or less heat – warming or cooling, you don’t know. Plants and soils on land can absorb additional CO₂ or release it to an increased extent through forest fires, although here too the direction is unclear.
The Palazzo Corner team looked at 26 processes that still have an effect after the emissions have ended. Only about half of them have a clear direction, such as the disappearing ice, which definitely contributes to warming. For many people, the impact itself is very uncertain, but overall you don’t know what the outcome will be. The researchers now recommend conducting targeted research on these processes in order to better integrate them into climate models.
“It’s helpful to clearly frame net-zero emissions as a milestone, not a finish line,” says Declan Finney of the University of Leeds, who was not involved in the study. An important point from the study is that the higher the emissions until the net zero milestone is reached, the greater the uncertainty about the warming that could come afterwards.
From a political point of view, from the researchers’ point of view, there is no change in the fact that emissions must fall as quickly as possible in the coming years. “That should be our top priority,” says Joeri Rogelj, one of the study authors. But he points out: If you want to stay below two degrees in order to limit the consequences of global warming, you should either aim for significantly less warming from the outset in order to have a buffer – or plan to remove carbon dioxide from the atmosphere in the long term. Based on current climate measures The experts from the “Climate Action Tracker” project expect 2.7 degrees of warming. Assuming that all national goals and promises are achieved, it would be exactly two degrees – without any downstream warming effects. If that doesn’t change fundamentally soon, there won’t be much of a buffer in sight.
There remains the possibility of recovering carbon dioxide. In fact, this is already implicit in some climate targets. For example, the EU wants to become climate neutral by 2050 or Germany by 2045, meaning that the bottom line is that it will no longer emit any greenhouse gases. However, since methane emissions from agriculture cannot be completely eliminated, they must be compensated for by storing CO₂, be it in soils, forests or underground storage. Accordingly, the CO₂ emissions would then even be negative and CO₂ would have to be removed from the atmosphere.
With material from the Science Media Center