Infrastructure protectionExplaining uneven rise in sea levels

Published 21 May 2012

If there is a global warming trend, one of its consequences would a rise in sea levels, which will require massive mitigation efforts to protect coastal infrastructure; rather than a uniform rise in sea level, however, the records show sea levels rising in some areas and dropping in others; Harvard researchers offer an explanation for this phenomenon

It was used to help Apollo astronauts navigate in space, and has since been applied to problems as diverse as economics and weather forecasting, but Harvard scientists are now using a powerful statistical tool not only to track sea level rise over time, but to determine where the water causing the rise is coming from.

As described in an 27 April paper in the Proceedings of the National Academy of Sciences (PNAS), graduate students Eric Morrow and Carling Hay demonstrate the use of a statistical tool called a Kalman smoother to identify “sea level fingerprints” — tell-tale variations in sea level rise — in a synthetic data set. Using those fingerprints, scientists can determine where glacial melting is occurring.

The goal was to establish a rigorous and precise method for extracting those fingerprints from this very noisy signal,” Professor of Geophysics Jerry Mitrovica, who oversaw the research, said. “What Carling and Eric have come up with is very elegant and it provides a powerful method for detecting the fingerprints. In my view everyone is soon going to be using this method.”

A Harvard University release reports that at the heart of the new technique is the idea, first proposed by Mitrovica and others more than a decade ago, that variability in sea level changes amount to a “fingerprint” researchers can use to identify the source of water pouring into the oceans.

With the public unconvinced about the effects of climate change, Mitrovica proposed the fingerprint idea as a way to refute the argument that melting ice sheets would cause a uniform sea level rise, he said, “like what you see when you turn on the tap in the bathtub — it all goes up uniformly.”

Rather than a uniform rise in sea level, skeptics pointed to records that showed levels rising in some areas and dropping in others as evidence that man-made climate change was a myth.

This variability, however, is exactly what researchers expect to see, Mitrovica said. “As ice sheets around the world melt, there is an extremely variable effect on sea level,” he explained. “That variation is actually very beautiful — it has information embedded in it. By looking at the differences in sea level changes around the globe, we should, in principle, be able to determine if the changes are the result of melting in Alaska, Greenland, Antarctica, or elsewhere.”

That variation in sea level change is the result, in part,