It should be simple. As Earth’s temperatures 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.
Instead, when global temperatures drop, which happens during ice ages, sea levels continue to fall as the water content recedes from the ocean, freezing once again in the huge layers of inland ice.
This epic, continuous cycle of ice flow and ebb – the transitions from glaciers to interglacials – takes place from time immemorial. But there is a problem.
For years, scientists following these cycles have suggested that there is a problem of “lack of ice”: a mysterious discrepancy between the very low sea level, about 20,000 years ago, and the volume of ice stored in glaciers in same time.
The elevation of the ice surface, 20,000 years ago. (Evan Gowan Institute / Alfred Wegener)
In essence, this is the problem. During the peak of the Earth’s last ice age – the last glacial maximum (LGM), which ended about 20,000 years ago – it is believed that sea level was about 130 meters (427 ft) lower than it is today, on the basis of ancient evidence of coral sediments.
But the modeling suggests that the volume of ice in the glaciers at this time was not large enough to explain 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, researchers seem to have found a solution.
With a new reconstruction called PaleoMIST 1.0, researchers have been able to model the evolution of global ice sheets in the past, far beyond even LGM.
“We seem to have found a new way to reconstruct the past 80,000 years ago,” says Gowan.
The results of the model suggest that the anomaly in our data is not a case of missing ice, but wrong inferences about how low the sea level dropped during LGM.
According to the PaleoMIST 1.0 ice physics model, sea level dropped by more than 116 meters below where the waves flow today, with the volume of ice (being fully accounted for) clocking somewhere around 42.2 × 106km3.
“Therefore, we find no basis for the problem of lack of ice, because our LGM reconstruction is compatible with existing constraints at sea,” the researchers explain in their study.
According to the team, the wrong direction of the missing ice argument stems from a number of factors – first and foremost, excessive reliance on distant field indicators (evidence of coral sediments from locations around the world), which may not accurately represent global sea environment as I once thought they did.
Another problem is a long-standing but seemingly flawed method used to estimate glacier masses, oxygen isotope ratio cycles – which appear to produce discrepancies in reconciling sea level and glacier mass at least back to 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,” said team member Geophysicist Paolo Stocchi of the Royal Netherlands Institute for Sea. Research.
“Our work now raises doubts about the reliability of this method.”
While the mystery of the missing ice seems to be solved, researchers do not expect theirs to be the last word on the subject.
After all, the incompatibility of its own solution with the ratio of the oxygen isotope, cycle-based reconstructions, has created, in a way, a new problem of missing ice, the team admits.
Whether and how this new uncertainty can be resolved is a challenge for another day in future research that could take a clearer look at the evolution of the ice sheet in the distant past.
The findings are reported in Communications about nature.