Underestimating the Rise in Global Sea Level

A recent study, conducted by NASA and the University of Hawaii, has discovered that our historical records of global sea level rise may have been underestimated, impacting any average figures that are quoted for the 20th century.

It is believed that historical measurements were only taken at a minimal number of locations, which may have been subject to, as yet, unexplained local variations not reflective of the global average sea level.

To understand the scope of these variations, the team, led by Dr Stuart Thompson, recreated the historical methods of measurements using 15 tide gauges, and compared these to known global average sea levels. The team found that the gauges underestimated the global levels by up to 28% in some areas. According to Thompson, there is not “something wrong with the instruments or the data, but for a variety of reasons, sea level does not change at the same pace everywhere at the same time. As it turns out, our best historical sea level records tend to be obtained in locations where 20th century sea level rise was most likely less than the true global average.”

Ice melt fingerprints, one of the oceanic processes investigated for this study, played an important role in the team’s discovery. Counter-intuitively, sea levels close to glaciers decrease as the ice melts, while levels further away rise, due to reductions in the glacier’s gravitational attractive force. As several of the tide gauges were located in the Northern Hemisphere near glaciers, the results showed reduced sea level rise compared to the global average.

The researchers concluded that there is less than a 1% chance that the global average sea level rise agrees with previous estimates. Thompson notes the significance of these results because they “suggest that our longest records most likely underestimate past global mean change and allow us to establish the minimum amount of global sea level rise that could have occurred during the last century.” At the very least, the project demonstrates a serious limitation in a formerly well-established sea level measuring technique.

Peter Sherman is studying for a physics degree

Banner Image: Dead tree sea level rise, Arnon Polin

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