Approximately 3 million years ago, during the Pliocene epoch, atmospheric carbon dioxide levels were similar to today. Global temperatures averaged about 2 to 3 degrees Celsius above pre-industrial levels (a temperature increase we’re on track for in coming decades, unless we significantly reduce global greenhouse gas emissions). And the global mean sea level was upward of 65 feet higher than present, thanks to ice melt. That’s right: 65 feet higher, an increase that would submerge most of the state of Florida, and nearly all of New York City.
Yet most current-day models of sea level rise don’t indicate we’re headed toward anything like 65 feet of sea level rise in the near-future, even at 2 to 3 C of warming. The discrepancy between past and future projection has been an unsettling mystery for climate and ocean scientists for years. Many researchers in the field think the answer comes down to incomplete and inaccurate models. New study findings could be part of solving the puzzle.
The world’s ice sheets are subject to a never-before-identified and worrying climate tipping point, according to the study published June 25 in the journal Nature Geoscience. The discovery, based on a novel mathematical and computer model of ice melt and seawater movement, means we could be in for far more sea level rise in coming years than previously forecasted. “Our projections of sea level rise might be significant underestimates,” said Alex Bradley, lead researcher and a climate scientist with the British Antarctic Survey (BAS), in a press statement.
Here’s how: Coastal ice sheets are made up of floating shelves and grounded sections. This is where ice sits directly on land. In between is the so-called “grounding zone.” Ocean water rushing beneath the grounding zone triggers melt, which can change the structure of the ice mass, subsequently allowing more water to intrude farther and faster in a feedback loop, according to the new analysis. Generally, models assume that all ice sheets have a point of stability where they’ll stop receding, based on the topography of the land that underlies them. Thanks to gravity, it’s hard for water to flow uphill. But the new assessment suggests that, with enough warming and melt progression widening the grounding zone, ocean water will be able to continue intruding deep beneath ice sheets, even upslope.
Climate tipping points occur when a relatively small shift in something like temperature or atmospheric carbon rapidly leads to an outsized impact. In this case, climatologists behind the new research found that subtle increases in ocean warming could lead to very large ice losses and sea level rise. For every ice sheet, including the pivotal expanse of ice that covers Antarctica and contains about 6 million cubic miles of water, there is a point of ocean warming where that feedback loop of melt, structural shifts, and more melt will become a “runaway” process. At a certain ocean temperature, refreezing ice can’t make up for the losses, triggering “dramatic changes” in ice sheet behavior, according to the researchers’ analysis.
Making matters worse, the researchers suggest there will be little warning that Antarctica and other ice sheets are nearing their individual grounding-zone tipping points. Such changes will be “hard to detect…until the tipping point is passed,” they write. The resulting, fated sea level rise would unfold relatively quickly, on the order of decades to centuries.
“This is missing physics, which isn’t in our ice sheet models,” Bradley said, adding that BAS scientists are now working to incorporate it into revised models.
Surprisingly, the Thwaites “Doomsday Glacier” is less at risk of passing this particular tipping point than other ice shelves, per the assessment. (Though seawater intrusion is causing Thwaites to melt faster than expected, according to another recent study.) But other shelves like the Pine Island glacier, which is already Antarctica’s largest contributor to sea level rise, are highly susceptible.
The analysis offers just one initial and simplified model, which doesn’t account for all of the factors relevant for ice sheet melt. For instance, tides aren’t part of the researchers’ equations. And the scientists don’t expect melt triggered by intrusion to proceed indefinitely. Nonetheless, the findings are alarming.
“With every tenth of a degree of ocean warming, we get closer and closer to passing this tipping point, and each tenth of a degree is linked to the amount of climate change that takes place,” Bradley told The Guardian. “We need very dramatic action to restrict the amount of warming that takes place and prevent this tipping point from being passed.” The only way to stop such melt in its tracks is to halt the largest single driver of climate change and rapidly transition away from burning fossil fuels.
