Coastal resilienceSea level rise accelerated over the past two decades, research finds
Sea level rise sped up over the last two decades rather than slowing down as previously thought, according to new research. The research corrects other studies which relied on records from tide gauges and satellites, records which have shown sea level rise to be slowing slightly over the past twenty years. This slowing down surprised scientists: As the ice sheets of West Antarctica and Greenland melt and send huge amounts of water into the ocean, climate models predicted that sea level rise would accelerate, not slow down. The new research, in which researchers used data sets generated by both tidal gauges and altimetric satellites, found, however, that the record of sea level rise during the early 1990s was too high. When adjustments are made for the initial error, the rate of sea level rise is not slowing down but accelerating, and the IPCC climate modelling proves right.
Sea level rise sped up over the last two decades rather than slowing down as previously thought, according to new research.
Records from tide gauges and satellites have shown sea level rise to be slowing slightly over the past twenty years – something which surprised scientists: As the ice sheets of West Antarctica and Greenland melt and send huge amounts of water into the ocean, climate models predicted that sea level rise would accelerate, not slow down.
“The thing that was really puzzling us was that the last decade of sea level rise was marginally slower, ever so subtly slower, than the decade before it,” Dr. Christopher Watson from the University of Tasmania who led the new study, told the Guardian.
Watson’s team found that the record of sea level rise during the early 1990s was too high. The Guardian reports that the error gave the illusion of the rate of sea level rise decreasing by 0.058 mm/year2 between 1993 and 2014, when in reality sea level rise accelerated by between 0.041 and 0.058 mm/year2 during that period. This means that the records is in line with the modelling of the UN’s climate science body, the Intergovernmental Panel on Climate Change (IPCC).
“We see acceleration, and what I find striking about that is the fact that it’s consistent with the projections of sea level rise published by the IPCC,” said Watson. “Sea level rise is getting faster. We know it’s been getting faster over the last two decades than it’s been over the twentieth century and its getting faster again.”
Professor Jonathan Gregory from the University of Reading and a lead author of the IPCC’s most recent climate report, told the Guardian that the study was “interesting and useful” and shored up the predictions of the models.
“The better agreement of the altimeter record after the correction … is a reason for greater confidence in the projections,” he said.
The rise of sea level is measured by using tide gauges on shorelines around the world and, since 1993, by also using altimetric satellites. The sets of data generated by both methods are imperfect.
The tide gauges sit on land, and this land is constantly shifting. For example, said Watson, the melting of vast amounts of ice in Alaska by global warming causes the measurements to be thrown out by the continent rebounding upward because it is now subject to lighter downward pressure by a heavy ice sheet: the more the ice sheet melts, the lighter the ice sheet becomes, allowing the continent to move upward. In Perth, Australia, on the other hand, the continental plate is subsiding.
The satellites circle the Earth at four miles per second at on orbit of 746 miles above the Earth. The satellites sends radar beams at the surface of the sea, recording the time it takes to bounce back. Watson said the accuracy of satellite measurements was “staggering.” He noted, though, that the level of precision required to measure the significant – if slight — changes in sea level driven by climate change is very high. During the 1990s the satellite instrumentation degraded, losing some of its accuracy.
Watson says his team was able to compare the two data sets and identify where each went wrong. The results revise downward the average rate of sea level rise since the 1990s. The 2013 IPCC report found the sea had risen on average by 3.2 mm per year since 1993. Waston notes that his study found the rate was slightly slower, between 2.6 and 2.9 mm per year.
“I have no doubt there are members of the community who may wish to reevaluate [the predictions for sea level rise]. But as a scientist I come back to the data,” said Watson, preempting claims that the study was a scaling down of the threat of climate change to coastal communities.
“A single number implies that that rate is constant over time. And I think what is emerging here is that that’s not the case. That rate of change is actually increasing. For everyone that lives around the coastal margin, that’s a really concerning fact.”
In 2013 Gregory’s report to the IPCC predicted that sea level could rise between 28cm and 98cm by 2100 depending on how much carbon human industry emits this century.
“There is no reason to change the projections,” said Gregory.
— Read more in Christopher S. Watson et al., “Unabated global mean sea-level rise over the satellite altimeter era,” Nature Climate Change (11 May 2015) (doi:10.1038/nclimate2635)
