A Houston neighborhood inundated by Hurricane Harvey in August 2017. Photo: AP

When one thinks about sea level rise, as one does in 2017, the tendency is to picture the oceans creeping steadily upward toward some catastrophic spillover. But that’s not really how it works. Sea level is rising in fits and starts, at different rates all over the world. Even on a global scale, some scientists think that instead of a gradual rise, our coastlines may be in for sudden pulses of extra sea level as climate change progresses.

A study published Thursday in Nature Communications affirms that view. Examining fossil reef beds that died more than 12,000 years ago, researchers at Rice University and Texas A&M concluded that as the Earth was warming toward the end of the Pleistocene, sea levels—at least in the Gulf of Mexico, and perhaps globally—went up in pulses, surging as much as a meter or more in a matter of decades.

It’s a finding that may have implications for coastal communities all over the world. “An important takeaway is that these pulse events exist,” Pankaj Khanna, a graduate student at Rice University and lead author of the new study, told Earther. “People are going to be affected if that finding is not incorporated into future [model] predictions.”

Indeed, while global climate models treat sea level rise as gradual, the geologic past paints a different picture. Between 14,600 and 13,800 years ago, for instance, sea levels rose at least ten meters, an event aptly-termed meltwater pulse 1A. In a later, more controversial meltwater pulse event, some scientists think sea levels rose more than 20 meters in just 500 years. But as important as these events are, they don’t tell us much about how sea level rise plays out over the shorter time intervals—decades to a single century—that matter to us mortals.

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The new study attempted to fill this gap, with a detailed analysis of ten drowned fossil reefs offshore of Corpus Christie, Texas. Based on previous analyses of this coastline, the researchers know that the reefs began forming roughly 20,000 years ago, as ice sheets melted and sea levels rose.

Using state of the art sonar equipment to produce high-resolution maps of the seafloor, Khanna and his colleagues identified a series of six stair-like “terraces” that grew on the reefs between 14,500 and 12,500 years ago. Based on their shape and spacing, the researchers interpret the features to represent periods when sea level rose rapidly, causing the reef to “back-step” and develop a terrance.

A 3D map of a fossil coral reef off the coast of Texas that records rapid pulses of sea level rise. Image Courtesy P. Khanna/Rice University

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Comparing the estimated ages of the terraces to a Greenland ice core that records past climate change, the researchers were able to link most of the terraces to periods of warming, when ice sheets suddenly collapsed.

“These events are finer scale [than the meltwater pulses],” Khanna said. “And what caused them is punctuated sea level rise—sea level rise of a meter or two in decades to a century causing stress on the reef.”

Andrea Dutton, a sea level rise expert at the University of Florida Gainsville who was not involved with the study, praised the researchers for their dataset and agreed that the terrace formation seems to represent short-term pulses of sea level rise.

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“We have known for quite a long time ice sheets don’t decay gradually necessarily—that they can retreat in a stepwise fashion,” she told Earther. “But the idea that you could get these small pulses, very quickly spaced, is kind of new.”

Dutton cautions that we can’t be sure exactly how fast sea levels rose in the absence of age data for the fossil corals. Khanna agrees. He’s hoping that in the future, his team can drill into the reefs to collect samples for isotopic dating.

More studies in other parts of the world are also needed to verify that the sea level pulse events recorded in the reef were truly global.

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“We need these kinds of studies, replicated in other parts of the world, so we have a stronger grip on these pulses,” Khanna said. “Right now, there is no component of punctuated sea level rise used in future predictions.”

Which is to say, whether the world is in for a pulse of rapid sea level rise in the future is a multi-trillion dollar question without a good answer. But as the new study shows, it’s an idea we need to be considering, if not actively planning for.

“If the Greenland and Antarctic ice sheet behave in a similar fashion,” to melting ice sheets at the end of the last glacial period, “we might expect more of this staircase-type sea level rise in the future,” Dutton said. “Which is really important in terms of coastal planning.”

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“If the rise comes very rapidly, that’s a game changer,” she added.