It should be simple. As temperatures on Earth rise, massive 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 life.
In contrast, when Earth’s temperatures plummet, which happens during ice ages, sea levels plummet as water content retreats from the ocean and freezes again in huge inland ice caps.
This epic, perpetual cycle of ebb and flow of ice – the transitions from ice ages to interglacials – has been going on from time immemorial. But there is a problem.
Scientists tracking these cycles have suggested for years that there is a “missing ice” problem: a mysterious discrepancy between very low sea levels, about 20,000 years ago, and the amount of ice stored in glaciers at the same time.
Height of the ice surface, 20,000 years ago. (Evan Gowan / Alfred Wegener Institute)
Essentially this is the problem. During the peak of Earth’s last Ice Age – the Last Glacial Maximum (LGM), which ended about 20,000 years ago – sea level would have been about 130 meters (427 ft) lower than it is today, based on ancient coral deposition evidence.
But modeling suggests that the ice volume in glaciers was not large enough at this time 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, researchers seem 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 far into the past, much further back than even the LGM.
“It seems we 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 rather erroneous conclusions about how low the sea level actually fell during the LGM.
According to the PaleoMIST 1.0 ice physics model, sea level fell no more than 116 meters below the point where the waves are scurrying around today, with the ice volume (fully charged) clocking somewhere around 42.2 x 10.6km3
“We therefore cannot find a basis for the missing ice problem, as our LGM reconstruction is compatible with existing sea-level restrictions,” 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, over-reliance on far field indicators (evidence of coral deposition from locations elsewhere in the world), which may not accurately reflect global mean sea levels. as we once thought they did.
Another problem is a long-standing but seemingly flawed method used to estimate glacial masses, oxygen isotope ratio cycles – which appears to produce discrepancies in reconciling sea level height and glacial masses as far back as the LGM.
“The isotope model has been widely used for years to determine the ice volume in glaciers up to many millions of years before our time”, says one of the team members, 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 appears to have been resolved, the researchers don’t expect to be the final word on the topic.
The incompatibility of their own solution with cycle-based reconstructions of the oxygen isotope ratio has, in a sense, “created a new missing ice problem,” the team admits.
Whether and how that new uncertainty can be resolved is a challenge for another day, in future research that may provide even clearer glimpses of the evolution of ice sheets in the distant past.
The findings are reported in Nature Communications