Earth's polar ice sheets can't withstand 1.5°C warming target

Holding global warming to the Paris Agreement’s 1.5°C threshold may not be enough to protect Greenland and Antarctic ice locked in place.

A new synthesis finds the “safe” temperature limit for polar ice sheets is lower than previously thought. It’s likely closer to the early 1990s level – about 1°C above pre-industrial levels – rather than the 1.5°C political target.

The analysis comes from an international team led by Durham University and including specialists from the University of Bristol in the United Kingdom and the Universities of Wisconsin-Madison and Massachusetts Amherst in the United States.

The authors drew on satellite data showing current ice loss at 370 billion tons yearly – four times faster than in the 1990s. They compared today’s trends with ancient warm periods and models projecting future ice loss over coming centuries.

Sea-level rise locked in

Taken together, the lines of evidence suggest that stabilizing at 1.5°C would still initiate several meters of sea-level rise over the coming centuries as air and ocean temperatures continue to destabilize glaciers.

Greenland and Antarctica jointly store enough frozen water to lift global seas by almost 65 meters. Even partial ice loss threatens 230 million people living within one meter of today’s shoreline in coastal cities and island nations.

“There is a growing body of evidence that 1.5°C is too high for the ice sheets in Greenland and Antarctica,” said lead author Chris Stokes, a professor at Durham University.

“We’ve known for a long time that some sea-level rise is inevitable over the next few decades to centuries, but recent observations of ice sheet loss are alarming, even under current climate conditions.”

Every degree matters for ice

The study stresses that halting warming sooner rather than later will make it easier to return to safer conditions.

“Limiting warming to 1.5°C would be a major achievement and this should absolutely be our focus,” Stokes said. Even if targets hold, sea levels could still rise 1 cm yearly – posing major challenges for today’s younger generations.

“We are not necessarily saying that all is lost at 1.5°C, but we are saying that every fraction of a degree really matters for the ice sheets – and the sooner we can halt the warming the better, because this makes it far easier to return to safer levels further down the line,” he added.

Lessons from the past

Paleoclimate archives show that the planet’s polar ice sheets were healthier only a few decades ago. “Put another way, and perhaps it is a reason for hope, we only have to go back to the early 1990s to find a time when the ice sheets looked far healthier,” Stokes noted.

At that time, global temperatures were around 1°C above pre-industrial levels. Atmospheric carbon dioxide stood near 350 ppm, a level many scientists consider safer than today’s 424 ppm.

“Evidence recovered from past warm periods suggests that several meters of sea-level rise – or more – can be expected when global mean temperature reaches 1.5°C or higher,” said co-author Andrea Dutton, a geoscientist at the University of Wisconsin-Madison.

She also pointed out that the longer warm temperatures persist, the greater the ice melt and resulting sea-level rise.

Ice sheet loss outpaces models

For decades, computer models struggled to reproduce the rapid ice-sheet losses now captured by satellites.

“Recent satellite-based observations of ice-sheet mass loss have been a huge wake-up call,” said co-author Jonathan Bamber from the University of Bristol.

Models haven’t captured the rapid polar ice sheet changes seen in real-world observations over the past 30 years.

“It is important to stress that these accelerating changes in the ice sheets and their contributions to sea level should be considered permanent on multi-generational timescales,” said senior author Robert DeConto, a professor of geosciences at UMass Amherst.

“Even if the Earth returns to its pre-industrial temperature, it will still take hundreds to perhaps thousands of years for the ice sheets to recover.”

If too much ice melts, sea-level rise could become irreversible for millennia – underscoring the urgent need to limit global warming.

Urgent climate questions remain

The authors stress that policymakers must grapple with the possibility that the Paris ceiling may be insufficient to secure coastlines. They call for more precise research to pin down a truly safe threshold, likely nearer 1°C, and urge rapid emission cuts that can eventually pull temperatures back below today’s level.

Ambassador Carlos Fuller of Belize is a veteran climate negotiator for a country already retreating from rising seas.

“Findings such as these only sharpen the need to remain within the 1.5°C Paris Agreement limit, or as close as possible, so we can return to lower temperatures and protect our coastal cities,” said Fuller.

The study’s central message is stark yet actionable. Every tenth of a degree averted this century could spare future generations from centimeters – or meters – of relentless sea-level rise.

The study is published in the journal Nature Communications: Earth and Environment.

Image Credit: Richard Jones

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