Global warming | Exceed the target…and come back?

What if global warming exceeded 1.5 degrees? More and more studies are looking at what will happen if the target set out in the Paris Agreement is exceeded, then restored after a few decades. However, this type of research also provokes a torrent of criticism.

Greenland

A recent study on Greenland, published at the end of October in the journal Nature, for once includes some good climate news. Scandinavian researchers estimate that if humanity misses its current climate targets, but manages to return to them before 2300, the Danish territory’s glaciers will likely remain in place.

“There are more and more studies on overtaking [overshoot] », indicates the main author of the study of Nature on Greenland, Nils Bochow, of the Norwegian Arctic University. “They postulate that we will be able to remove greenhouse gases from the atmosphere to bring the temperature back to current targets, after having exceeded them. But these studies are complicated. We were surprised by the results for Greenland. »

Uncertainty

Studying what would happen in the event of a temporary exceedance is important, but should not diminish the urgency of acting, believes Patrick Bonin, head of the climate-energy campaign at Greenpeace Canada.

The central message of a September webinar on overshooting from Climate Analytics, an international think tank, was “an immediate peak in emissions,” Mr. Bonin notes. The first speaker of this webinar stated that the uncertainties in overshoot studies are so great that it is impossible to use them to make decisions.

“Certainly studies on overshoot span several centuries, which increases uncertainty,” says Bochow. Then, it is possible that uncontrollable feedback phenomena may occur which make temporary overshooting impossible. And there is the problem of returning to a lower temperature: the technologies for capturing greenhouse gases are not yet ready. »

Studies on overshooting will be inevitable soon anyway, observes Sabine Fuss, a climatologist from Humbolt University who discussed carbon capture technologies during the same webinar. “At the current rate, we only have a few years before reaching the 1.5 degree target. And the Paris Agreement [traité climatique de 2015] evoked something of an overstep by talking about limiting warming to 1.5 degrees before the end of the century. »

Exceedance studies sometimes consider the 1.5 degree target, and sometimes the 2 degree target.

Permafrost

One of the uncontrollable feedback phenomena most often cited by critics of overshoot studies is a melting of permafrost, which would release enormous amounts of carbon.

“We saw this kind of uncontrollable feedback locally, in regions of Siberia affected by a heat wave in 2019,” says Joe Melton, a climate chemist at the University of Victoria and Environment Canada. “But at the moment, we do not see such large-scale changes. This is extremely difficult to model. » A project monitoring “thaw-backslides,” occasional landslides in permafrost, is underway in the Northwest Territories to see if they are linked to feedback loops.

Modeling the melting of permafrost during a temporary exceedance of current climate targets is in its infancy, according to Mr. Melton.

In the shorter term, permafrost, by releasing frozen nitrogen, could stimulate plant growth, therefore removing CO₂ of the atmosphere, notes the chemist.

Ecosystems

About a quarter of living species could not recover from a temporary exceedance of climate targets, according to a study published last year in the Philosophical Transactions of the Royal Society by biologist Andreas Meyer, from the University of Cape Town, South Africa.

“Essentially, the temperature remains higher in some regions, despite an average drop in atmospheric temperature,” says Meyer. There are also species that are affected in the longer term by ecosystem losses. If a forest burns to the ground during the overrun, some species could be displaced or become extinct. Same thing if an island disappears. »

In other words, the temperature could remain high in certain microclimates even if the global average temperature drops after temporarily exceeding the target by 1.5 degrees. And if ecosystems disappear, species could be affected in the long term. For example, if a forest burns to the ground, or an island disappears, the species that live there could disappear.

The ecosystems themselves are affected for longer than the exceedance period, according to another study, published last October in the journal Communications Earth & Environment. “With an excess of 50 years, terrestrial ecosystems are affected for several decades later and marine ecosystems for even longer,” explains the lead author of this study, Yeray Santana-Falcon, of the University of Toulouse. “There is inertia in the systems. The oceans, for example, are heat reservoirs, slow to warm and slow to cool. Changes in vegetation also constitute another form of inertia. »

The Commission

An international commission on overshooting, the Overshoot Commission, was created in 2022 to study how exceeding climate targets could be temporary.

“The overarching objective we are proposing is to meet the targets by slashing emissions,” explains former Canadian Prime Minister Kim Campbell, who is part of the commission. “But we also need to study carbon capture technologies in case there is an overrun. Following this type of work has been personally helpful in reducing my climate anxiety. »

The commission is also working on geoengineering, technologies for directly cooling the atmosphere, for example with giant parasols in orbit or the spreading of soot to imitate the cooling caused by volcanoes. “It’s a question of intergenerational justice,” says M.^me Campbell. Our grandchildren should not need geoengineering, but cannot use it because we have banned the studies. »