Sea level rise accelerating faster than thought (Science)

High tides swamp a playground in coastal Wales.

DIMITRIS LEGAKIS/SPLASH NEWS/NEWSCOM. High tides swamp a playground in coastal Wales.

If you’re still thinking about buying that beach house, think again. A new study suggests that sea levels aren’t just rising; they’re gaining ground faster than ever. That’s contrary to earlier work that suggested rising seas had slowed in recent years.

The result won’t come as a shock to most climate scientists. Long-term records from coastal tide gauges have shown that sea level rise accelerated throughout the 20th century. Models predict the trend will continue. However, previous studies based on satellite measurements—which began in 1993 and provide the most robust estimates of sea level—revealed that the rate of rise had slowed in the past decade compared with the one before.

That recent slowdown puzzled researchers, because sea level contributions from melting ice in Antarctica and Greenland are actually increasing, says Christopher Watson, a geodesist at the University of Tasmania in Australia. So he and colleagues took a closer look at the available satellite and tide gauge data, and tried to correct for other factors that might skew sea level measurements, like small changes in coastal elevation.

The results, published today in Nature Climate Change, show that global mean sea level rose slightly slower than previously thought between 1993 and 2014, but that sea level rise is indeed accelerating. The new findings agree more closely with other records of changing sea levels, like those produced by tide gauges and bottom-up accounting of the contributions from ocean warming and melting ice.

In the past, researchers have used tide gauges to keep tabs on the performance of satellite altimeters, which use radar to measure the height of the sea surface. The comparison allowed them to sniff out and cope with any issues that cropped up with the satellite sensors. Tide gauges themselves are not immune to problems, however; the land on which they rest can shift during earthquakes, or subside because of groundwater withdrawal or sediment settling. These processes can produce apparent changes in sea level that have nothing to do with the oceans.

So Watson’s team tried to correct for the rise and fall of tide gauge sites by using nearby GPS stations, which measure land motions. If no GPS stations were present, they used computer models to estimate known changes, such as how some regions continue to rebound from the last glaciation, when heavy ice sheets caused land to sink.

The newly recalibrated numbers show that the earliest part of the satellite record, collected between 1993 and 1999 by the first altimetry mission, known as TOPEX/Poseidon, appears to have overstated sea level rise. That’s probably because a sensor deteriorated, ultimately forcing engineers to turn on a backup instrument. When combined with data from subsequent satellite missions, those inflated TOPEX/Poseidon numbers gave the appearance that sea level rise was decelerating, even as the global climate warmed.

Also contributing to the apparent slowdown was a hiccup caused by natural climate variation, says John Church, a climate scientist at the Commonwealth Scientific and Industrial Research Organisation in Hobart, Australia, and a co-author of the new study. Around 2011, “there was a major dip in sea level associated with major flooding events in Australia and elsewhere,” he says. Intense rainfall transferred water from the oceans to the continents, temporarily overriding the long-term sea level trend.

The corrected record now shows that sea level rose 2.6 millimeters to 2.9 millimeters per year since 1993, compared with prior estimates of 3.2 millimeters per year. Despite the slower rates, the study found that sea level rise accelerated by an additional 0.04 millimeters per year, although the acceleration is not statistically significant. Watson says he expects that trend to grow stronger as researchers collect more data.

The acceleration falls in line with predictions from the Intergovernmental Panel on Climate Change (IPCC), Watson notes. “We’re tracking at that upper bound” of the IPCC’s business-as-usual scenario for greenhouse gas emissions, he says, which could bring up to one meter of sea level rise by 2100.

Others say it’s too early to tell. “The IPCC is looking way out in time,” says geodesist Steve Nerem of the University of Colorado, Boulder, who was not involved in the study. “This is only 20 years of data.”

In the meantime, Nerem says, the altimetry community needs to focus on continuing to improve the satellite data. He thinks Watson’s team “addressed it in the best way we can right now,” but it would be even better “to have a GPS receiver at every tide gauge, and right now that’s not the case.”

Regardless, the underlying message is clear, Church says: Sea levels are rising at ever increasing rates, and society needs to take notice.

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