Top scientists say the world’s ice sheets are melting more rapidly than expected and that world leaders must ramp up their climate ambitions to avoid a catastrophic rise in sea levels.

A report released Thursday from the International Cryosphere Climate Initiative, a network of policy experts and researchers, pleads with world leaders to heed their warnings as they gather for the United Nations’ COP28 climate conference later this month. The report says if global average temperatures settle at 2 degrees Celsius above the preindustrial baseline, the planet could be committed to more than 40 feet of sea-level rise — a melt that would take centuries and reshape societies across the globe.

The collapse of ice sheets and ice shelves has been a major point of uncertainty within the climate science community. But a flurry of new research suggests that dangerous tipping points are nearer than once thought and that there is likely less room in Earth’s carbon budget than expected.

“We might be reaching these temperature thresholds that we’ve been talking about for a long time sooner than we were thinking about years ago,” said Rob DeConto, the director of the University of Massachusetts Amherst School of Earth & Sustainability and an author of the report. “And it may be that the thresholds for some of these processes that can drive really rapid ice loss are lower than we were thinking just a few years ago.”

Without a dramatic turn in the pace of climate action, those factors could leave humanity “facing rates of sea-level rise way outside the range of adaptability,” DeConto said.

In the report, the scientists argue a rise in global temperatures of 2 degrees Celsius would force many to flee coastal communities.

“We’re displacing millions of people with the decisions being made now,” said report author Julie Brigham-Grette, a geosciences professor at the University of Massachusetts Amherst.

More than 60 scientists contributed to the report. Many are experts in their specialties, and some have worked on past reports for the U.N.’s Intergovernmental Panel on Climate Change, the world’s leading body on assessing the climate crisis.

In the IPCC’s 2021 report, scientists estimated that sea level will rise about 0.9 to 3.3 feet (0.28 to 1.01 meters) by 2100, but also said those numbers didn’t factor in uncertainties around ice sheets like the ones scientists have been probing more deeply in the past few years.

New studies suggest that melting ice sheets are a greater cause for concern than the IPCC had considered.