Warmer Oceans, Not Air, Causing Disintegration of Ice Sheets

(CN) – New findings that may explain why the North American ice sheet melted during one of the coldest periods of the last Ice Age also add to mounting evidence that climate change could lead to greater sea level rise than most existing models predict.

In a study published Thursday in the journal Nature, researchers from the University of Michigan show how small spikes in ocean temperature – rather than air temperature – likely fueled the eventual disintegration of the Laurentide Ice Sheet, which used to cover much of North America.

What caused the ice sheet to melt has puzzled scientists for decades, as the period in which it melted and splintered into the sea aligned with some of the coldest times in the last Ice Age, which ended about 10,000 years ago.

“We’ve shown that we don’t really need atmospheric warming to trigger large-scale disintegration events if the ocean warms up and starts tickling the edges of the ice sheets,” said lead author Jeremy Bassis.

The same mechanism is likely at work today on the Greenland ice sheet – and possibly Antarctica, according to the team.

“It is possible that modern-day glaciers, not just the parts that are floating but the parts that are just touching the ocean, are more sensitive to ocean warming than we previously thought.”

Scientists identified this potential mechanism in part due to a new, more accurate method to mathematically describe how ice breaks and flows, which Bassis developed several years ago. His model has enabled a deeper understanding of how ice could react to spikes in ocean or air temperature, and how such changes may affect sea level rise.

Other researchers used the model to predict in 2016 that melting Antarctic ice could increase sea levels by more than three feet. The previous estimate projected Antarctica would only contribute a few centimeters to sea level rise by 2100.

For the new study, Bassis and his team applied a version of this model to the climate of the last Ice Age, using ocean-floor sediment and ice-core records to estimate variations in water temperature. Their goal was to see if what’s happening today in Greenland could explain the behavior of Laurentide.

The collapse of the ancient ice sheet is an example of periods of rapid ice disintegration known as Heinrich events, which can be seen in sediment cores across the North Atlantic basin. The unusual sediment layers are what allowed scientists to initially identify Heinrich events.

“Decades of work looking at ocean sediment records has shown that these ice sheet collapse events happened periodically during the last Ice Age, but it has taken a lot longer to come up with a mechanism that can explain why the Laurentide Ice Sheet collapsed during the coldest periods only,” co-author Sierra Petersen said.

“This study has done that.”

The team set out to understand the size and timing of Heinrich events from the last Ice Age, creating simulations that were able to predict both. They even identified an additional Heinrich event that had previously been missed.

The model also factors in how the Earth’s surface reacts to the weight of the ice on top of it. Heavy ice depresses the planet’s surface, even pushing it below sea level – the point at which ice sheets are most vulnerable to warm ocean temperatures.

“There is currently large uncertainty about how much sea level will rise and much of this uncertainty is related to whether models incorporate the fact that ice sheets break,” Bassis said. “What we are showing is that the models we have of this process seem to work for Greenland as well as in the past, so we should be able to more confidently predict sea level rise.”

Bassis added that portions of Antarctica have similar geography to Laurentide.

“We’re seeing ocean warming in those regions and we’re seeing these regions start to change,” he said. “What we saw in our simulations is that just a small amount of ocean warming can destabilize a region if it’s in the right configuration, and even in the absence of atmospheric warming.”