New research on the behavior of the Laurentide ice sheet that once covered a great deal of North America — long something of a puzzle for researchers, owing to the fact that it often melted and splintered into the ocean at very cold points during the last ice age — suggests that sea levels may rise more than current models predict as a result of anthropogenic climate change.
To put the findings of the study in simple language: ocean temperature has apparently been a much larger driver of earlier ice sheet disintegration events than supposed, apparently being much more important in many regards than air temperature.
The second gif here makes the point pretty clearly.
“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,” explained Jeremy Bassis, University of Michigan associate professor of climate and space sciences and engineering. “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.”
The researchers involved in the work note that this mechanism is likely part of what’s now happening in Greenland, and possibly also what’s happening in Antarctica as well.
The press release provides more:
In the new study, Bassis and his colleagues applied a version of this model to the climate of the last Ice Age, which ended about 10,000 years ago. They used ice core and ocean-floor sediment records to estimate water temperature and how it varied. Their aim was to see if what’s happening in Greenland today could describe the behavior of the Laurentide Ice Sheet.
Scientists refer to these bygone periods of rapid ice disintegration as Heinrich events: Icebergs broke off the edges of Northern Hemisphere ice sheets and flowed into the ocean, raising sea level by more than 6 feet over the course of hundreds of years. As the icebergs drifted and melted, dirt they carried settled onto the ocean floor, forming thick layers that can be seen in sediment cores across the North Atlantic basin. These unusual sediment layers are what allowed researchers to first identify Heinrich events. …
Bassis and his colleagues set out to understand the timing and size of the Heinrich events. Through their simulations, they were able to predict both, and also to explain why some ocean warming events triggered Heinrich events and some did not. They even identified an additional Heinrich event that had previously been missed.
Heinrich events were followed by brief periods of rapid warming. The Northern Hemisphere warmed repeatedly by as many as 15 degrees Fahrenheit in just a few decades. The area would stabilize, but then the ice would slowly grow to its breaking point over the next thousand years. Their model was able to simulate these events as well.
Bassis’ model takes into account how the Earth’s surface reacts to the weight of the ice on top of it. Heavy ice depresses the planet’s surface, at times pushing it below sea level. That’s when the ice sheets are most vulnerable to warmer seas. But as a glacier retreats, the solid Earth rebounds out of the water again, stabilizing the system. From that point the ice sheet can begin to expand again.
It’s very noteworthy here that there are parts of Antarctica that are similar geographically to those examined in the new study.
“We’re seeing ocean warming in those region(s) and we’re seeing these regions start to change. In that area, they’re seeing ocean temperature changes of about 2.7 degrees Fahrenheit,” Bassis noted. “That’s pretty similar magnitude as we believe occurred in the Laurentide events, and 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.”
So, the takeaway is that — as some other recent research has also shown — Antarctica’s ice sheets are probably much more prone to rapid disintegration than has been previously supposed (publicly anyway).
The work is detailed in a paper published in the journal Nature.
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