Science study: Earth's ice and snow have passed safe planetary limits

What the study found

A study published in Science and led by researchers at the Stockholm Resilience Centre concludes that the cryosphere has already moved beyond its safe operating space. The cryosphere covers the frozen parts of the planet: ice sheets, mountain glaciers, sea ice, snow cover and permafrost. The authors measured four indicators (land ice mass, sea ice area, snow cover and permafrost temperature) against reference values drawn from the Holocene, the roughly 11,700 years of relatively stable conditions in which human civilizations developed. More than 30 scientists from a dozen countries took part, and the work was presented at the 31st UN Climate Change Conference (COP31) in Antalya, Turkey. The team proposes adding the cryosphere as a possible tenth planetary boundary alongside the nine already in the scientific framework.

Where the four indicators stand

The Potsdam Institute for Climate Impact Research (PIK) says seven of the nine existing planetary boundaries are already exceeded. Polar sea ice currently averages 16.5 million square kilometres, short of a safe threshold of 19.8 million. Annual-mean global snow cover measured 35.9 million square kilometres, against a safe level of 38.8 million. Permafrost covers 13.7 million square kilometres, below its limit of 14.8 million, and its mean annual ground temperature sits above the threshold of minus 6.6 degrees. Land ice is the exception so far: it stands at 27.63 quadrillion tonnes against a safe level of 27.48, but a further loss of more than 0.64 quadrillion tonnes is considered unavoidable. The authors call their assessment provisional because cryosphere data are patchy and carry large uncertainties.

Sea ice safe limit19.8

Sea ice current average16.5

Snow cover safe limit38.8

Snow cover current35.9

Permafrost limit14.8

Permafrost current13.7

Sea ice safe limit

19.8 million km²

Sea ice current average

16.5 million km²

Snow cover safe limit

38.8 million km²

Snow cover current

35.9 million km²

Permafrost limit

14.8 million km²

Permafrost current

13.7 million km²

Data from the article text

Feedback loops and risks

Bright ice and snow reflect sunlight back to space, so their loss lets the surface absorb more energy and warm faster. Thawing permafrost can release carbon and methane, which in turn add to warming. Estimates examined in the study put the extra warming from permafrost-thaw emissions at between 0.05 and 0.7 degrees by 2100. The West Antarctic Ice Sheet holds enough ice to raise sea levels by about 11 feet (3 meters), and permafrost holds trillions of tons of carbon dioxide. Greenland's melting land ice could also slow or even collapse the ocean conveyor belt that helps keep Europe temperate. Albert Noström, a climate researcher at the Stockholm Resilience Center who co-authored the paper, stressed how far the changes reach.

“The consequences of a changing cryosphere extend beyond regions where ice and snow are disappearing.”

What it means

Johan Rockström of the Potsdam Institute said the risks linked to the cryosphere are already visible, pointing to recent floods in Nepal.

“The risks linked to the cryosphere are already clearly evident today, most recently in the floods in Nepal.”

Crossing a boundary does not mean an immediate tipping point. It signals a much higher risk of severe and partly irreversible environmental change. Ice sheets and permafrost respond to today's conditions only over decades to millennia, so the effects unfold over long timescales. The authors say the work gives policymakers a guide for weighing these changes at local and global scales. The land-ice threshold is meant as a reference for strategies against irreversible ice loss, sea-level rise, climate instability and freshwater scarcity.