Overshooting Climate Targets Increases Risk For Tipping Cascades, Exeter Study

This tipping risk increases even if, in the longer term, the global temperature is stabilised within the Paris range.

Avoiding an overshoot would hence limit the risks, the researchers conclude.

The research team included the Potsdam Institute for Climate Impact Research (PIK) and the University of Exeter.

“We show that the risk for some tipping events could increase very substantially under certain global warming overshoot scenarios,” said PIK scientist Nico Wunderling, lead-author of the study published in Nature Climate Change.

“Even if we would manage to limit global warming to 1.5°C after an overshoot of more than 2°C, this would not be enough as the risk of triggering one or more global tipping points would still be more than 50%. With more warming on the long-term, the risks increase dramatically.”

“To effectively prevent all tipping risks, the global mean temperature increase would need to be limited to no more than 1°C – we are currently already at about 1.2°C,“ said Jonathan Donges, Co-Lead of the FutureLab on Earth Resilience in the Anthropocene at PIK.

“The latest IPCC report is showing that we’re most likely on a path to temporarily overshoot the 1.5°C temperature threshold.”

To arrive at these results, the scientists, together with co-authors from the Earth Commission – a group of leading scientists convened by Future Earth – used different global warming overshoot scenarios with peak temperatures from 2-4°C and applied these to a set of four interacting tipping elements: the Greenland Ice Sheet, the West Antarctic Ice Sheet, the Atlantic meridional overturning circulation AMOC, and the Amazon rainforest.

The researchers applied a risk analysis approach based on millions of model simulations to reflect the uncertainties in relevant parameters such as the uncertainty in critical temperature thresholds as well as interaction strengths and interaction structure.

Such an amount of simulations would be computationally too expensive to do based on fully coupled Earth System Model simulations.

For the different overshoot scenarios, the research team then analysed the risk of crossing critical thresholds and the potential for triggering cascading interactions between the four elements, depending on the magnitude and duration of the overshoot as well as the warming remaining on the long-term.

“We found that the risk for the emergence of at least one tipping event increases with rising peak temperature,” said Nico Wunderling.

“Already at a peak temperature of 3°C, more than one third of all simulations showed a tipping event even when overshoot durations were limited strongly.

“At 4°C peak temperature, this risk extends to more than half of all simulations.”

Ricarda Winkelmann, Earth Commissioner and Co-Lead of the FutureLab on Earth Resilience in the Anthropocene, said: “Especially the Greenland and the West Antarctic ice sheet are at risk of tipping even for small overshoots, underlining that they are among the most vulnerable tipping elements.

“While it would take a long time for the ice loss to fully unfold, the temperature levels at which such changes are triggered could already be reached soon.

“Our action in the coming years can thus decide the future trajectory of the ice sheets for centuries or even millennia to come.”

The other two tipping elements considered in the study, the AMOC and Amazon rainforest, have higher critical temperature thresholds.

Yet, they would react much faster once the tipping process has started.

Therefore, it is much more difficult to stop their tipping process once initiated by a temporary global warming overshoot.

Current mitigation policies are expected to lead to 2-3.6°C of global warming by the end of this century.

“This is not enough,” Ricarda Winkelmann added.

“Even though a temporary temperature overshoot would definitely be better than reaching a peak temperature and remaining there, some of the overshoot impacts may lead to irreversible damages in a high climate risk zone and this is why low-temperature overshoots are key here.

“Every tenth of a degree counts. We must do what we can to limit global warming as quickly as possible.”