Sea levels rising a few millimeters every year hardly sounds ominous, until NASA program scientist Nadya Vinogradova Shiffer offers a little perspective.
“When my kids asked me if three millimeters is a lot, I told them that is equivalent to adding an extra 300 trillion gallons of water every year,” she says. “If you put this water in milk jars, that would cover the distance from the sun to Pluto and back several times.”
For 30 years, an uninterrupted sequence of international Earth-observing satellites has tracked rising seas—a gauge of human-induced climate change—and other climate-related phenomena, helping researchers understand global weather patterns and forecast how rising tides will impact coastlines.
The Sentinel-6 will see details that previous sea level missions couldn’t. Existing satellites can track large phenomena stretching thousands of miles, such as the Gulf Stream and weather patterns like El Niño and La Niña. But smaller sea-level fluctuations near coastlines exceed their abilities. Sentinel-6’s higher resolution measurements will enable researchers to see finer, more complicated ocean features, especially near shorelines. It will also offer faster data turnarounds—collecting, processing, and releasing data within three hours, down from months and years. The data can be used to more quickly and accurately predict, map, and 3D model coastline changes; weather, such as hurricane intensity; ocean current fluctuations, which dissipate climate energy; and ocean topography and circulation, as movement of heat, salt, pollution, and nutrients impacts marine ecosystems.
“Measuring the world’s vast oceans is a difficult problem,” says JPL project manager Parag Vaze. (Vaze and the others in this story spoke during a virtual October press conference on the mission.) “Having a satellite orbiting 800 miles up, spinning around the Earth at five miles every second, and now being able to do a very accurate measurement of the sea surface height to within just two inches is an incredible feat and one, if you’d asked me years ago [if it was possible], I would have said, ‘People are crazy.'”
The 1.5-ton probes—the first named for pioneering NASA Earth Science Division director Michael Freilich, who passed away in August—will collect sea surface height measurements down to the centimeter for more than 90% of the world’s oceans, as well as fluctuations in atmospheric temperature and humidity. They’ll also illustrate how and how fast climate change is reshaping coastlines by measuring coastal sea-level variations, which affect ship navigation, commercial fishing, and residents of coastal regions.
Melting glaciers and ice sheets, and seawater expansion from greenhouse gas heat absorption cause the rising waters. Global average sea levels have increased 3.3 mm annually from 1993 to 2000, per NASA data from previous satellites. The National Oceanic and Atmospheric Administration, another Sentinel-6 mission partner, noted yearly global sea levels rose from 1.4 millimeters throughout most of the last century to 3.6 mm from 2006 to 2015. From 2018 to 2019, it jumped to 6.1 mm. By the end of this century, the global mean sea level is likely to increase at least one foot above 2000 levels.
A recent study published in an open-access journal called The Cryosphere projected that if greenhouse gas emissions continued at their current pace, melted Greenland and Antarctic ice sheets could contribute more than 15 inches of global sea-level rise by 2100.
However, that increase isn’t uniform worldwide. Water along the Eastern coastline of the United States is rising at a rate of three to four times higher than the global mean average, says Shiffer. “With Sentinel-6, we’d fine-tune those numbers and tap into emerging acceleration.”
The Sentinel-6 uses a suite of instruments that work alone, in tandem, or with other satellites to offer improved measurements of sea-level rise, ocean currents, and wind and wave conditions. Among them is an altimeter that will measure ocean height by bouncing radar pulses off the water’s surface and calculating how long it takes to return to the satellite. Another device will evaluate temperature and humidity levels by detecting microwave emissions from water vapor and liquid water. A new experiment will glean details of the atmospheric layers by calculating how much radio signals from other satellites refract or slow as they pass through them. Other instruments will determine the satellite’s position.
Scientists hope the new climate understanding from Sentinel’s advancements will help policymakers grasp the potential risks and opportunities that inform their decisions.
“The Earth is a global system of intricate and dynamic interactions between ocean, land, ice, the atmosphere, and human communities—and that global system is changing,” says Karen St. Germain, who succeeded Freilich last June as NASA Earth Science Division director. Combining the Sentinel data with that from other Earth science satellites, she says, will “really unleash the power of these observations.”
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