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Study by Researchers, Including ODU's Hamlington, Suggests Sea Level Rise May be Underestimated in Some Areas

By Brendan O'Hallarn

A study by researchers from Old Dominion University, the University of Hawaii at Mānoa and the NASA Jet Propulsion Laboratory demonstrate that the longest and highest-quality records of historical ocean levels may underestimate the amount of global average sea level rise that occurred during the 20th century.

In new research published in the journal Geophysical Research Letters, Ben Hamlington, assistant professor of ocean, earth and atmospheric sciences at ODU; and Philip Thompson associate director of the University of Hawai'i Sea Level Center, show how various processes that cause sea levels to change differently in different places may have affected past measurements.

The study, which uses NASA satellite data, finds that tide gauges may have underestimated the amount of global average sea level rise that occurred during the 20th century. The study concludes it is highly unlikely that global average sea level rose less than 5.5 inches during the 20th century, and the figure was likely closer to 6.7 inches.

"It's not that there's something wrong with the instruments or the data," said Thompson, lead author on the study. "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 located where past sea level rise was most likely less than the true global average."

Hamlington, who has collaborated with Thompson on many sea level rise studies utilizing this methodology, said the findings have important implications for ongoing sea level rise research.

"When taken in the context of the measurements made by satellites over the past two decades, an improved estimate of 20th century global mean sea level rise gives us a better indication of the magnitude of acceleration that might be occurring," Hamlington said.

One of the key processes the researchers looked at is the effect of "ice melt fingerprints," which are global patterns of sea level change caused by deviations in Earth's rotation and local gravity that occur when a large ice mass melts. To determine the unique melt fingerprint for glaciers, ice caps and ice sheets, the team used data from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites on Earth's changing gravitational field, and a novel modeling tool recently developed at NASA's Jet Propulsion Laboratory.

During the 20th century, the dominant sources of global ice melt were in the Northern Hemisphere. The results of this study showed that many of the highest-quality historical water level records are taken from places where the melt fingerprints of Northern Hemisphere sources result in reduced local sea level change compared to the global average.

Furthermore, the scientists found that factors capable of enhancing sea level rise at these locations, such as wind or Southern Hemisphere melt, were not likely to have counteracted the impact of fingerprints from Northern Hemisphere ice melt.

These melt fingerprints and the influence of wind might have caused sea level rise to be overestimated in the past. Hamlington said this research demonstrates that is not likely, and allows for a minimum amount of global sea level rise to have occurred during the past century.

The full paper can be found HERE.

During the past seven years, Hamlington has been researching ways to better use satellite sensor data about sea levels. The data point to an overall rise in global sea level, but the rise is uneven in various regions of the world. He came to Old Dominion's Department of Ocean, Earth and Atmospheric Science in 2014, joining a multidisciplinary team of researchers studying the issue of sea level rise and recurrent flooding.

In 2014, the University was tasked by the White House to convene an Intergovernmental Pilot Project, which consists of a multi-stakeholder effort that seeks a whole of government, whole of community approach to sea level rise mitigation and adaptation.

In April 2016, Gov. Terry McAuliffe signed a bill creating the Commonwealth Center for Recurrent Flooding Resiliency, a joint venture of Old Dominion University, the College of William & Mary and the Virginia Institute of Marine Science which is envisioned as a one-stop shop for scientific, socioeconomic, legal and policy analyses to build Virginia's flooding resiliency.

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