It should be simple. As the temperature on Earth warms, large amounts of ice trapped in giant glaciers begin to thaw, releasing water into the oceans and raising sea levels. This is the story of our lives.
In contrast, as global temperatures drop, which occurs during ice ages, sea levels drop as the water content recedes from the ocean and freezes again in large inland ice sheets.
This epic, continuous cycle of ice ebb and flow – the transitions from glaciers to glaciers – has been going on since time immemorial. But there is a problem.
For years, scientists watching these cycles have suggested that there is a ‘missing ice’ problem: a mysterious difference between very low sea levels about 20,000 years ago and the amount of ice stored in glaciers at the same time.
Ice surface height, 20,000 years ago. (Evan Gowan / Alfred Wegener Institute)
The core 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 – the sea level is thought to be about 130 meters lower than today, based on evidence of ancient coral sediment.
But modeling suggests that ice volume in glaciers at this time was not large enough to explain such a low sea level. How then can we explain this ‘missing’ ice cream?
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, far beyond even the LGM.
“We seem to have found a new way to reconstruct the past up to 80,000 years,” Gowan says.
The results of the model suggest that the anomaly in our data is not the case of ice, but incorrect inferences about how low the sea level actually dropped during the LGM.
According to the ice physics model of PaleoMIST 1.0, the sea level did not drop more than 116 meters where the waves circulate today, and the ice volume (which is fully taken into account) clocked somewhere around 42.2 × 10.6km3.
“We therefore find no basis for the missing ice problem, as our LGM reconstruction is compatible with existing sea level constraints,” the researchers explain in their study.
According to the team, the incorrect designation of the missing ice argument stems from a number of factors – firstly, an excessive reliance on far-field indicators (evidence of coral descent from places elsewhere in the world), which may not accurately reflect the world average sea level. do not represent as we once thought they do.
Another problem is a widely established but seemingly flawed method used to calculate glacier masses, oxygen isotope ratios – which apparently causes inconsistencies when sea level rise and glacier masses are at least united.
“The isotope model has been widely used for many years to determine the amount of ice in glaciers up to many millions of years before our time,” says one of the team, geologist Paolo Stocchi of the Royal Netherlands Institute of Marine Research.
“Our work now raises doubts about the reliability of this method.”
Although the missing ice mystery seems to have been solved, the researchers do not expect it to be the last word on this topic.
The incompatibility of their own solution with oxygen-isotope ratio-cycle-based reconstructions has in a way “created a new problem for the missing ice,” the team admits.
Whether and how the new uncertainty can be resolved is a challenge for another day, in future research that may yield even clearer glimpses of the evolution of ice sheets in the distant past.
The findings are presented in Nature communication.