The vast frozen terrain of arctic permafrost has thawed across North America several times over the past 1 million years, when the global climate was not much warmer than it is today, researchers from the United States report and Canada in today’s edition of Scientific progress.
Arctic permafrost contains twice as much carbon as the atmosphere. But the researchers found that the thaws – which expel stores of carbon dioxide sequestered deep in frozen vegetation – were not accompanied by increased CO levels.2 in the air. This surprising finding flies in the face of predictions that, as the planet warms, the volume of these natural carbon stores can dramatically increase CO.2 produced by human activity, a combination that could increase the climate balance of greenhouse gases.
The team of researchers explored caves in Canada to look for clues left in speleothems – mineral deposits accumulated over thousands of years – that could help track when in the past Canadian permafrost thawed and how much the climate was warmer, said Boston College Associate. Professor of Earth and Environmental Sciences Jeremy Shakun, co-author of the study.
The team was following up on a 2020 study that dated Siberian cave samples. This research revealed evidence of thawing permafrost up to about 400,000 years ago, but not much since. Since the study looked at only one region, the researchers sought to broaden the search for a more representative view of the arctic region, said Shakun, a paleoclimatologist.
Over the course of two years, researchers dated 73 cave deposits from several now frozen caves in Canada. The deposits offer revealing clues to climatological history because they only form when the ground is thawed and water drips into a cave. By dating the age of the speleothems, scientists were able to determine when in the past regions had thawed.
Shakun said the results are very similar to the previous Siberian study, suggesting that arctic permafrost has become more stable during the Ice Age cycles of the past two million years.
But he said the team was surprised to find that many high arctic speleothems turned out to be much younger than expected. Their relatively young age means thawing permafrost formed mineral deposits when the world was not much warmer than it is today.
Sediment cores from the Arctic Ocean suggest what might have happened then.
“Summers were ice free before 400,000 years ago,” Shakun said. “It would have heated the land more in the summer and insulated it under deeper snow in the winter, causing the ground to thaw.
This theory is concerning if correct, he added. “Half of the Arctic sea ice has disappeared since I was born, which perhaps makes the permafrost even more vulnerable.”
Second, records from the old atmosphere show that greenhouse gas levels were not higher during the past permafrost thaw intervals that we have identified – this is surprising because the standard opinion is that Massive amounts of carbon are expected to be released into the atmosphere when permafrost thaws. .
Shakun said the findings call for further research to understand what allowed permafrost to thaw at times in the past when it wasn’t much warmer, and why there is little evidence of a significant release of carbon at these times.
“These results do not easily square with typical forecasts of global warming for the future,” Shakun said. “This may mean that scientists have overlooked the processes that will prevent thawing permafrost from causing a significant spike in CO.”2 go forward. On the other hand, it may be that the gradual thaw events in the past have been slow enough that the CO2 they released could be taken up by oceans or plants elsewhere – a situation that may not apply to today’s much faster warming. “
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Material provided by Boston college. Original written by Ed Hayward. Note: Content can be changed for style and length.