Keeping a Steady Eye on Sea Level Change From Space

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The Sentinel-6/Jason-CS satellite mission will add to a long-term sea level dataset that’s become the gold standard for climate studies from orbit.


Over the course
of nearly three decades, an uninterrupted series of satellites has circled our
planet, diligently measuring sea levels. The continuous record of ocean height
that they’ve built has helped researchers reveal the inner workings of weather phenomena
like El Niño and to forecast how much the ocean could encroach on coastlines
around the world. Now, engineers and scientists are preparing two identical
satellites to add to this legacy, extending the dataset another decade.

Both spacecraft
are a part of the Sentinel-6/Jason-CS (Continuity of Service) mission, a
U.S.-European collaboration that aims to make some of the most accurate
measurements of sea levels around the world. The first satellite to launch, Sentinel-6
Michael Freilich, will lift off in November. Its twin, Sentinel-6B, will launch
in 2025. Both will assess sea levels by sending electromagnetic signals down to
the ocean and measuring how long it takes for them to return to the spacecraft.

“This
mission will continue the invaluable work of accurately measuring sea surface
height,” said Karen St. Germain, director of NASA’s Earth Science
Division. “These measurements enable us to understand and predict sea
level changes that will affect people living in coastal regions around the
world.”

The satellite
will build on efforts that began in 1992 with the launch of the TOPEX/Poseidon
mission and that continued with three more missions over the years: Jason-1,
OSTM/Jason-2, and Jason-3. Sentinel-6/Jason-CS aims to extend the nearly 30-year
sea level dataset that these previous missions built by another 10 years.

Measuring the
height of the ocean gives scientists a real-time indication of how Earth’s
climate is changing, said Josh Willis, the mission’s project scientist at
NASA’s Jet Propulsion Laboratory in Southern California. The oceans absorb
about 90% of the excess heat from the planet’s warming climate. Seawater
expands as it heats up, resulting in about a third of the modern-day global average
sea level rise. Melting ice from land-based sources like glaciers and ice
sheets accounts for the rest.

To understand
how rising seas will affect humanity, researchers need to know how fast this is
happening, said Willis. “Satellites are the most important tool to tell us
this rate,” he explained. “They’re kind of a bellwether for this
creeping global warming impact that’s going to inundate coastlines around the
world and affect hundreds of millions of people.”

Currently, sea
levels rise an average of 0.13 inches (3.3 millimeters) per year, more than
twice the rate at the start of the 20th century. “By 2050, we’ll have a
different coastline than we do today,” said Willis.

“As more
and more people move to coastal regions, and coastal megacities continue to
develop, the impact of sea level change will be more profound on those
societies,” said Craig Donlon, mission project scientist at the European
Space Agency.

Setting the
Standard

The information
that Sentinel-6 Michael Freilich gathers will join a dataset that’s become the
gold standard for climate studies from space. This is because the chain of overlapping
satellites that started with TOPEX/Poseidon has continuously measured ocean
heights since the early 1990s. That continuity is key to this dataset’s success.

Some of the
long-term datasets climate scientists rely on, like ocean temperature or the
height of tides, have gaps or major changes in how data was collected (like
before and after satellite records began) that make understanding the long-term
climate signal challenging. Researchers must account for these variations to
ensure that their results are truly representative of the phenomena they’re looking
at.

The satellites
that followed TOPEX/Poseidon – Jason-1, OSTM/Jason-2, and Jason-3 – flew in the
same orbit as one another, each launching before the older one was
decommissioned. When Sentinel-6 Michael Freilich lifts off later this year, it
will orbit Earth 30 seconds behind the Jason-3 satellite, which launched in
2016. Scientists will then spend a year cross-calibrating the data collected by
the two satellites to ensure the continuity of measurements from one mission to
the next. Engineers and scientists will do the same cross-calibration with
Sentinel-6 Michael Freilich’s twin in five years as its predecessor’s mission winds
down.

Without these
satellites and the data they’ve collected, researchers would have a much rougher
understanding of the rate of sea level rise, as well as of phenomena like El
Niño. This is a weather pattern triggered by a huge shift in the winds that normally
blow from east to west across the equatorial Pacific Ocean. An El Niño can shift
ocean currents and global weather patterns, bringing torrential rain to the Southwestern
U.S. and triggering droughts in Asia and Australia. Its counterpart, La Niña,
can have the opposite effect.

One of the
discoveries to come out of this sea level dataset is the far-reaching effects that
El Niño and La Niña can have on the world. “In 2010, there was a massive
La Niña and it essentially flooded huge parts of Australia and Southeast Asia.
It rained so much on land, it dropped global sea levels by one centimeter [0.4
inches],” said Willis. “We had no idea it could have such a massive
impact on global sea level.”

The global view that the Sentinel-6 Michael
Freilich satellite will provide, together with sea level data from models and observing
stations, will provide invaluable information for governments and local
authorities tasked with planning for things like sea level rise and storms,
said Donlon.

More About
the Mission

Copernicus Sentinel-6/Jason-CS is being
jointly developed by the European Space Agency (ESA), the European Organisation
for the Exploitation of Meteorological Satellites (EUMETSAT), NASA, and the
National Oceanic and Atmospheric Administration (NOAA), with funding support
from the European Commission and support from France’s National Centre for
Space Studies (CNES).

The first Sentinel-6/Jason-CS satellite
that will launch was named after the former director of NASA’s Earth Science
Division, Michael Freilich. It will follow the most recent U.S.-European sea
level observation satellite, Jason-3, which launched in 2016 and is currently
providing data.

NASA’s contributions to the Sentinel-6/Jason-CS
mission are three science instruments for each of the two Sentinel-6 satellites:
the Advanced Microwave Radiometer, the Global Navigation Satellite System -
Radio Occultation, and the Laser Reflector Array. NASA is also contributing
launch services for those satellites, ground systems supporting operation of
the JPL-developed science instruments, the science data processors for two of
these instruments, and support for the international Ocean Surface Topography
Science Team.

To learn more about NASA’s study of sea
level rise, visit:

https://sealevel.nasa.gov

News Media Contact

Jane J. Lee / Ian J. O’Neill
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0307 / 818-354-2649
jane.j.lee@jpl.nasa.gov / ian.j.oneill@jpl.nasa.gov

2020-126



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