Slight ocean warming enough to tip the system
Just small increases in ocean temperatures during past geological epochs may have been enough to tip Northern Hemisphere ice sheets toward disintegration, even as air temperatures remained cold. That could spell trouble in the current era of climate warming, according to a new study led by University of Michigan researchers, who said their finding suggest that climate change could cause sea level to rise higher than most models predict.
The study, published in Nature, helps explain the meltdown of the Laurentide Ice Sheet that covered most of North America during the last ice age.
“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 Jeremy Bassis, U-M 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.”
Other recent studies have suggested that process is happening right now in Greenland and parts of Antarctica, according to Bassis, who has developed mathematical models that help show how ice breaks and flows. Last year, other researchers used it to predict that melting Antarctic ice could raise sea levels by more than three feet, as opposed to the previous estimate that Antarctica would only contribute centimeters by 2100.
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.
The ancient periods of rapid ice melt are called Heinrich events, when sea level rose by more than six feet in just a few centuries. The melting and drifting chunks of ice left trails of sediments on the ocean floor that are used to track those sudden climatic shifts. The new study helps explain why the Laurentide ice sheet collapsed during the coldest periods only.
The modeling helps explain why some ocean warming events triggered ice sheet collapses while others did not. According to the study, the Heinrich events were followed by brief periods of rapid warming — as much as 15 degrees Fahrenheit in just a few decades.
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.
“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.”
He added that portions of Antarctica have similar geography to Laurentide: Pine Island, Thwaites glacier, for example.
“We’re seeing ocean warming in those region 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 said. “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.”