The drought- and flood-stricken summer of 2022 has shown the impact of 1.1° Celsius of global warming — the amount that’s already occurred since pre-industrial times. Now a major scientific reassessment finds that several critical planetary systems are at risk of breaking beyond repair even if nations restrain warming to 1.5°C, the lower threshold stipulated by the Paris Agreement. 

At that level of warming, coral reefs may die off, ice sheets in Greenland and the West Antarctic may melt and permafrost may abruptly thaw, according to a new paper in the journal Science. 

The paper compiles evidence that major changes in the climate system, with massive environmental and societal consequences, are likely to occur at lower temperatures changes that previously assumed. It was written by a team of international scientists led by David Armstrong McKay of Stockholm University in Sweden and the University of Exeter in the UK. 

“With this paper we show clearly that 1.5°C is not a climate limit to take lightly,” said Johan Rockström, one of the authors and director of the Potsdam Institute for Climate Impact Research in Germany. “Exceed it, and we are likely to trigger several tipping points.” The current trajectory of planetary warming is estimated to reach about 2.6°C. 

Rockström and colleagues analyzed global and regional “tipping points”— thresholds beyond which climatic changes become self-perpetuating. The authors break them down by sensitivity to warming and offer confidence levels of low, medium and high in estimating the temperatures that will trigger them and the timescales in which they may happen. 

Crossing these thresholds isn’t the planetary equivalent of suddenly driving off a cliff, from safety to danger. Rather, every increment of warming raises the odds of changes that become self-perpetuating. “Every tenth of a degree counts,” Rockström said. 

At about 1.5°C some tipping points may be reached, including for the Greenland and West Antarctic ice sheets, accelerated thawing of boreal permafrost, and die-off of tropical coral reefs. But the authors “cannot rule out” that ice-sheet tipping points have already been passed and that some other tipping elements have minimum thresholds in range of 1.1°C to 1.5°C of warming. 

With further warming, the list grows: Winter sea ice in the Barents Sea, north of Russia, may surpass a threshold at 1.6°C; Alpine glaciers at 2°C; and the ocean currents in the North Atlantic at 1.8°C. The Sahel zone could experience significant disruption at 2.8°C, with a possible drying of West Africa or greening of the Sahara Desert, and the sub-glacial basins in the East Antarctic may become unstable at 3°C. 

The likelihood of setting off dieback of the Amazon rainforest becomes “non-negligible” at ~2°C and increases at ~3°C, the authors write. 

“Our assessment provides strong scientific evidence for urgent action to mitigate climate change,” the scientists write in a summary. “We show that even the Paris Agreement goal of limiting warming to well below 2°C and preferably 1.5°C is not safe as 1.5°C and above risks crossing multiple tipping points [CTPs]. Crossing these CTPs can generate positive feedbacks that increase the likelihood of crossing other CTPs.”

The possible cascading effects of tipping points need to be taken into account, says co-author Ricarda Winkelmann, also based at the Potsdam Institute. As an example, she notes that substantial melt from the Greenland ice sheet would release freshwater into the ocean, which could slow down the Atlantic overturning circulation that carries heat from the tropics to the mid-latitudes and polar regions. “This in turn can lead to net warming in the Southern Ocean, and hence might in the long-run destabilize parts of the Antarctic ice sheet,” she said. 

The new paper builds on work going back to 2008, when a group of researchers first identified tipping points in the Earth’s climate system. Years of further research into ancient climates, more real-world data and more sophisticated models led the authors of the new paper to expand the list of threatened systems from nine to 16, with seven additional impacts that would manifest regionally rather than globally. These include the West African monsoon and coral reefs around the Equator.

Compared to the 2008 assessment, the set of global “core” tipping factors has changed as well. It now includes collapse of convection in the Labrador Sea and of sub-glacial basins in the East Antarctic. No longer included are Arctic summer sea ice and the weather phenomenon El Niño-Southern Oscillation, due to lack of evidence of tipping dynamics. 

Marine biologist and deep-sea researcher Antje Boetius, who was not involved in the new study, called it timely and important. “It highlights early-warning signs of nature that must be heard,” said Boetius, director of the Alfred Wegener Institute for Polar and Marine Research in Germany. “The current 1.1°C warming already comes with shocking extremes in terms of floods, droughts, storms [and] fires, disrupting livelihoods worldwide.”

–With assistance from Eric Roston.

To contact the author of this story:
Fritz Habekuss in New York at fhabekuss@bloomberg.net

To contact the editor responsible for this story:
Amanda Kolson Hurley at ahurley21@bloomberg.net