It should be simple. When temperatures on Earth warm, huge amounts of water ice trapped in giant glaciers begin to thaw, releasing water into the oceans and causing sea levels to rise. It is the story of our lives.
In contrast, when global temperatures plummet, which occurs during ice ages, sea levels drop, as water content withdraws from the ocean and freezes once more into massive ice sheets on land. inside.
This epic and continuous cycle of ebb and flow of ice, the transitions from glaciers to interglaciers, has been going on since time immemorial. But there’s a problem.
For years, scientists who follow these cycles have suggested that there is a problem of “lost ice”: a mysterious discrepancy between the very low levels of the sea about 20,000 years ago and the volume of ice stored in the glaciers at the same time.
Ultimately, the problem is this. During the peak of Earth’s last ice age, the Last Glacial Maximum (LGM), which ended approximately 20,000 years ago, the sea level is believed to have been about 130 meters (427 feet) lower than it is today, based on evidence from ancient coral sediments. .
But the models suggest that the volume of ice on the glaciers at this time was not large enough to explain such a low sea level. So how can we explain this ‘lost’ ice?
In a new study led by geophysicist Evan Gowan of the Alfred Wegener Institute in Germany, researchers appear to have found a solution.
With a new reconstruction called PaleoMIST 1.0, the researchers were able to model the evolution of global ice sheets in the past, much further back even than the LGM.
“We seem to have found a new way to reconstruct the past 80,000 years ago,” says Gowan.
The model results suggest that the anomaly in our data is not a case of lack of ice, but rather erroneous inferences about how low sea level actually fell during the LGM.
According to the PaleoMIST 1.0 ice physics model, the sea level did not drop more than 116 meters below where the waves lap today, with a volume of ice (which is fully accounted for) somewhere around of 42.2 × 106km3.
“Therefore, we did not find any basis for the problem of lost ice, since our reconstruction of LGM is compatible with the existing limitations at sea level,” the researchers explain in their study.
According to the team, the misdirection of the lost ice argument is due to a couple of factors: First, over-reliance on far-field indicators (evidence of coral sediments from places in other parts of the world), which may not accurately representing mean global sea levels as we once thought they did.
Another problem is a long established but apparently flawed method used to estimate glacier masses, oxygen isotope ratio cycles, which appears to produce discrepancies when reconciling sea level height and masses. of glaciers from the LGM, at least.
“The isotope model has been widely used for years to determine the volume of ice in glaciers up to many millions of years before our time,” says one of the team members, geophysicist Paolo Stocchi of the Royal Institute of Marine Research in the Netherlands.
“Our work now raises questions about the reliability of this method.”
While the mystery of the lost ice appears to be solved, the researchers don’t expect theirs to be the last word on this issue.
After all, the incompatibility of their own solution with cycle-based reconstructions of the ratio of oxygen isotopes has, in a way, “created a new lost ice problem,” the team admits.
Whether that new uncertainty can be resolved, and how it is a challenge for another day, in future research that may yield even clearer glimpses of ice sheet evolution in the distant past.
Findings are reported in Communications from nature.