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 raising sea levels. It is the story of our lives.
Conversely, when global temperatures plummet, which happens during the ice ages, sea levels drop as the water content is removed from the ocean and frozen once again in huge layers of ice. interiors.
This epic and continuous cycle of reflux and ice flow (the transitions from glacial to interglacial) has occurred since time immemorial. But there is a problem.
For years, scientists following these cycles have suggested that there is a “lack of ice” problem: a mysterious discrepancy between very low sea levels about 20,000 years ago and the volume of ice stored on glaciers at the same time.
Elevation of the ice surface, 20,000 years ago. (Evan Gowan / Alfred Wegener Institute)
Basically, the problem is this. During the peak of the last glacial era on Earth (the last glacial maximum (LGM), which ended about 20,000 years ago), it is believed that sea level was about 130 meters (427 feet) lower than ‘current, based on ancient coral sediment evidence.
But the modeling suggests that the volume of ice on glaciers at that time was not large enough to account for such a low sea level. So how can we explain this “missing” ice?
In a new study led by geophysicist Evan Gowan of the Alfred Wegener Institute in Germany, it appears that researchers have found a solution.
With a new reconstruction called PaleoMIST 1.0, researchers were able to model the evolution of global ice sheets to the past, much further back than even the LGM.
“We seem to have found a new way to reconstruct the past from 80,000 years ago,” Gowan says.
The results of the model suggest that the anomaly in our data is not a case of ice shortage, but erroneous inferences about the low sea level actually during the LGM.
According to the PaleoMIST 1.0 ice physics model, sea level dropped no more and no less than 116 meters below where the waves fall, with an ice volume (fully accounted for) of around 42.2 × 106km3.
“Therefore, we found no basis for the missing ice problem, as our LGM reconstruction is consistent with existing sea-level limitations,” the researchers explain in their study.
According to the team, the misdirection of the argument about the missing ice is due to a couple of factors: first, the over-reliance on distant field indicators (coral sediment tests from other locations places in the world), which may not accurately represent the average sea level as we once thought they did.
Another issue is a long-established but seemingly flawed method used to estimate glacier masses, cycles of oxygen isotope ratio, which appears to produce discrepancies when sea level height and mass are reconciled. the glaciers, at least, to the LGM.
“The isotope model has been widely used for years to determine the volume of ice on glaciers up to many millions of years before our time,” says one of the teams, geophysicist Paolo Stocchi of the Royal Netherlands Institute for Sea Research.
“Our work now raises doubts about the reliability of this method.”
While the missing ice mystery seems to have been solved, researchers don’t expect theirs to be the last word on the subject.
After all, the incompatibility of its own solution with reconstructions based on the cycle of the proportion of oxygen isotopes has, in a way, “created a new missing ice problem,” admits the team. .
Whether and how this new uncertainty can be resolved is a challenge for another day, in future research that may give an even clearer view of the evolution of the ice sheet in the distant past.
The findings are reported in Communications on Nature.