NASA-led study reveals causes of sea-level rise since 1900

A new study led by the National Aeronautics and Space Agency's Jet Propulsion Laboratory (NASA JPL) uses new techniques to "balance the budget" between factors that have caused sea levels to rise in the last century and those that have cause them to fall.

The study, titled "The Causes of Sea Level Rise Since 1900," was published August 19 in Nature. In addition to scientists from JPL, the project involved researchers from Old Dominion University, Caltech, Université Catholique de Louvain in Belgium, University of Siegen in Germany, the National Oceanography Centre in the UK, Courant Institute in New York, Chinese Academy of Sciences, and Academia Sinica in Taiwan.

Scientists know the factors that play a role in sea level rise: Melting glaciers and ice sheets add water to the seas, and warmer temperatures cause water to expand. Other factors are known to slow the rise, such as dams impounding water on the land, stymying its flow into the sea. The researchers, reexamining each of the known contributors to sea level rise from 1900 to 2018, used improved estimates and applied satellite data to better understand historic measurements.

The researchers found that estimates of global sea level variations based on tide-gauge observations had slightly overestimated global sea levels before the 1970s. (Located at coastal stations scattered around the globe, tide gauges are used to measure sea level height.) They also found that mountain glacier meltwater was adding more water to the oceans than previously realized but that the relative contribution of glaciers to sea level rise is slowly decreasing. And they discovered that glacier and Greenland ice sheet mass loss explain the increased rate of sea level rise before 1940.

In addition, the new study found that during the 1970s, when dam construction was at its peak, sea level rise slowed to a crawl. Dams create reservoirs that can impound freshwater that would normally flow straight into the sea.

"That was one of the biggest surprises for me," said lead researcher Thomas Frederikse, a postdoctoral fellow at JPL, referring to the peak in global dam projects at that time. "We impounded so much freshwater, humanity nearly brought sea level rise to a halt."

Since the 1990s, however, Greenland and Antarctic ice sheet mass loss and thermal expansion have accelerated sea level rise, while freshwater impoundment has decreased. As our climate continues to warm, the majority of this thermal energy is absorbed by the oceans, causing the volume of the water to expand. In fact, ice sheet melt and thermal expansion now account for about two-thirds of observed global mean sea level rise. Mountain glacier meltwater currently contributes another 20%, while declining freshwater water storage on land adds the remaining 10%.

All told, sea levels have risen on average 1.6 millimeters (0.063 inches) per year between 1900 and 2018. In fact, sea levels are rising at a faster rate than at any time in the 20th century. But previous estimates of the mass of melting ice and thermal expansion of the ocean fell short of explaining this rate, particularly before the era of precise satellite observations of the world's oceans, creating a deficit in the historic sea level budget.

Finding a Balance

In simple terms, the sea level budget should balance if the known factors are accurately estimated and added together. It's a bit like balancing the transactions in your bank account: Added together, all the transactions in your statement should match the total. If they don't, you may have overlooked a transaction or two.

The same logic can be applied to the sea level budget: When each factor that affects sea level is added together, this estimate should match the sea level that scientists observe. Until now, however, the sea level budget has fallen short of the observed sea level rise.

"That was a problem," Frederikse said. "How could we trust projections of future sea level change without fully understanding what factors are driving the changes that we have seen in the past?"

Frederikse led an international team of scientists to develop a state-of-the-art framework that pulls together the advances in each area of study - from sea level models to satellite observations - to improve our understanding of the factors affecting sea level rise for the past 120 years.

The latest satellite observations came from the pair of NASA - German Aerospace Center (DLR) Gravity Recovery and Climate Experiment (GRACE) satellites that operated from 2002-2017, and their successor pair, the NASA - German Research Centre for Geosciences (GFZ) GRACE Follow-On (launched in 2018). Additional data from the series of TOPEX/Jason satellites - a joint effort of NASA and the French space agency Centre National d'Etudes Spatiales -that have operated continuously since 1992 were included in the analysis to enhance tide-gauge data.

"Tide-gauge data was the primary way to measure sea level before 1992, but sea level change isn't uniform around the globe, so there were uncertainties in the historic estimates," said Sönke Dangendorf, an assistant professor of oceanography at Old Dominion University in Norfolk, Virginia, and a coauthor of the study. "Also, measuring each of the factors that contribute to global mean sea levels was very difficult, so it was hard to gain an accurate picture."

But over the past two decades, scientists have been "flooded" with satellite data, added Dangendorf, which has helped them precisely track the physical processes that affect sea levels.

For example, GRACE and GRACE-FO measurements have accurately tracked global water mass changes, melting glaciers, ice sheets, and how much water is stored on land. Other satellite observations have tracked how regional ocean salinity changes and thermal expansion affect some parts of the world more than others. Up-and-down movements of Earth's crust influence the regional and global levels of the oceans as well, so these aspects were included in the team's analysis.

"With the GRACE and GRACE-FO data we can effectively back-extrapolate the relationship between these observations and how much sea level rises at a particular place," said Felix Landerer, project scientist at JPL for GRACE-FO and a coauthor of the study. "All observations together give us a pretty accurate idea of what contributed to sea level change since 1900, and by how much."

JPL managed the GRACE mission and manages the GRACE-FO mission for NASA's Earth Science Division of the Science Mission Directorate at NASA Headquarters in Washington. Based in Pasadena, California, Caltech manages JPL for NASA.

Read more: https://www.jpl.nasa.gov/news/news.php?feature=7731

Read the study: https://www.nature.com/articles/s41586-020-2591-3