Sea level rise has been more rapid than previously understood: Study
“But these simple averages aren’t representative of a true global mean value,” Hay said. “Tide gauges are located along coasts, therefore large areas of the ocean aren’t being included in these estimates. And the records that do exist commonly have large gaps.
“We know the sea level is changing for a variety of reasons,” she added. “There are ongoing effects due to the last ice age, heating and expansion of the ocean due to global warming, changes in ocean circulation, and present-day melting of land ice, all of which result in unique patterns of sea-level change. These processes combine to produce the observed global mean sea-level rise.”
The release notes that the new estimates developed by Hay and Morrow grew out of a separate project aimed at modeling the physics that underpin sea-level “fingerprints.”
“What we were interested in — and remain interested in — was whether we can detect the sea-level fingerprints we predicted in our computer simulations in sea-level records,” Morrow said. “Using a global set of observations, our goal has been to infer how individual ice sheets are contributing to global sea-level rise.”
The challenge, Hay said, is that doing so requires working with “very noisy, sparse records.”
“We have to account for ice age signals, and we have to understand how ocean circulation patterns are changing and how thermal expansion is contributing to both regional patterns and the global mean. We try to correct for all those signals using our simulations and statistical methods, then look at what’s left and see if it fits with the patterns we expect to see from different ice sheets.
“We are looking at all the available sea-level records and trying to say that Greenland has been melting at this rate, the Arctic at this rate, the Antarctic at this rate, etc.,” she continued. “We then sum these contributions and add in the rate that the oceans are changing due to thermal expansion to estimate a rate of global mean sea-level change.”
To the researchers’ surprise, Hay said, it quickly became clear that previous estimates of sea-level rise over most of the twentieth century were too high.
“We expected that we would estimate the individual contributions, and that their sum would get us back to the 1.5 to 1.8 mm per year that other people had predicted,” Hay said. “But the math doesn’t work out that way. Unfortunately, our new lower rate of sea-level rise prior to 1990 means that the sea-level acceleration that resulted in higher rates over the last 20 years is really much larger than anyone thought.”
— Read more in Carling C. Hay et al., “Probabilistic reanalysis of twentieth-century sea-level rise,” Nature (14 January 2015) (doi:10.1038/nature14093)
