Earth may already be too hot for the survival of polar ice sheets
Ten years ago, policymakers and nation states set the world's most important climate goal: limiting planetary warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit). If the Earth could stay below that threshold, a climate catastrophe and major rise in sea levels might be staved off.
But a group of scientists have demonstrated that if the world stays on course to warm up to 1.5 degrees — or even stays at its current level of 1.2 degrees above preindustrial levels — polar ice sheets will probably continue to quickly melt, causing seas to rise and displacing coastal communities, according to a study published Tuesday in Communications Earth and Environment.
'There was a kind of misunderstanding that 1.5 was going to solve all our problems,' said Chris Stokes, a professor at Durham University in England who focuses on glaciers and ice sheets, and an author of the study. Now, the team surmised that limit is closer to around 1 degree Celsius, though more research is needed to come to an official conclusion.
The team focused on Greenland and Antarctica, behemoth ice sheets that together could raise global sea levels by more than 210 feet if they melted. They are losing around 370 billion metric tons of ice each year at a rate that has quadrupled since the 1990s.
To come to their analyses, scientists pored over more than 150 research papers and focused on three aspects of sea-level rise: recent observations of rapidly melting ice sheets, modeling that uses equations to predict how temperatures could affect the rates of ice melting and past sea-level change tens of thousands of years ago.
To help gauge how high sea levels could rise over the coming centuries, scientists have looked back at what happened the last time the Earth was as warm as it is now: roughly 125,000 years ago, during a period known to scientists as the Last Interglacial.
Back then, research shows, a wobble in Earth's orbit had changed how much sunlight hit the northern hemisphere, raising global temperatures. The warmer conditions allowed Neanderthals to venture into northern Europe. Mammoths and giant ground sloths migrated poleward. And the ice caps covering the Arctic and Antarctica began to melt, raising sea levels around the world.
A vast array of ancient evidence — including ice cores, fossils, deep sea sediments and even octopus DNA — allowed the researchers to reconstruct how this sea-level rise unfolded. For example, ancient coral reefs found 25 feet above the current sea surface mark where the water once reached. Bits of bedrock uncovered in the middle of the ocean reveal how icebergs calved off disintegrating glaciers and then drifted across the sea.
This research into Earth's ancient climate has revealed that ice sheet collapse depends on complex processes and can happen at surprising speed. Pulses of sudden sea-level rise, when the ocean surface may have risen multiple feet in less than a century, indicated that the ice sheets could have crossed temperature thresholds that caused them to shed mass all at once.
The scientists then fed their findings into computer models of the Earth system, allowing them to confirm that the models' outputs matched what actually occurred. This gave them confidence in the models' forecasts for the future, and the results were sobering.
'Every fraction of a degree matters,' said Andrea Dutton, a research professor at the University of Wisconsin at Madison, who was a co-author of the study. 'We can't just adapt to this type of sea-level rise. We can't just engineer our way out of this.'
Around 230 million people live within about three feet of sea level, the researchers noted. Over the coming centuries, if the Earth stays at the same temperature, the sea could rise several meters, displacing entire cities and even states.
Because of gravitational effects, said Stokes, places closer to the equator, including Pacific islands like Micronesia and some Caribbean islands, will experience more sea-level rise.
'It's an existential threat,' he said. 'Some of these entire states are going to be underwater in a few centuries.'
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