Scientists have gained new insights into the processes that have driven ocean level variations for over a century, helping us prepare for the rising seas of the future.
To make better predictions about the future impacts of sea
level rise, new techniques are being developed to fill gaps in the historic
record of sea level measurements. We 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.
When each factor 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, leading scientists to question why
the budget wouldn’t balance.
A new study published on Aug.19 seeks to balance this budget.
By gaining new insights to historic measurements, scientists can better
forecast how each of these factors will affect sea level rise in the future.
For example, in its recent flooding
report, the National Oceanic and Atmospheric Administration (NOAA) noted a
rapid increase in sea level rise-related flooding events along U.S. coasts over
the last 20 years, and they are expected to grow in extent, frequency, and
depth as sea levels continue to rise.
Factors Driving Our Rising
Seas
On reexamining each of the known contributors to sea level
rise from 1900 to 2018, the research, led by NASA’s Jet Propulsion Laboratory
in Southern California, uses improved estimates and applies 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,” said
Frederikse. “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.”
The study, titled “The
Causes of Sea Level Rise Since 1900,” was published Aug. 19 in Nature. In
addition to scientists from JPL and Old Dominion University, the project
involved researchers from Caltech,
Université Catholique de Louvain in Belgium, University of Siegen in Germany, the
National Oceanography Centre in the United Kingdom, Courant Institute in New
York, Chinese Academy of Sciences, and Academia Sinica in Taiwan.
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 on
Pasadena, California, Caltech manages JPL for NASA.
News Media Contact
Ian J. O’Neill / Jane J. Lee
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-2649 / 818-354-0307
ian.j.oneill@jpl.nasa.gov / jane.j.lee@jpl.nasa.gov
2020-163
Source: Jet Propulsion Laboratory