One of the most significant potential impacts of global climate change is sea level rise. Unless, of course, you are a resident of the Marshall Islands, where there’s nothing “potential” about it. As the New York Times reported on December 2, damage to island communities from rising sea levels is "occurring now."
NOAA satellite data indicate that global average sea level has been rising at a rate of 2.9 mm/year—nearly double that of the past century (1.7mm/year). These numbers may seem small (a penny is about 1 millimeter thick), but they add up over time. For example, in 2014, global average sea level was 2.6 inches (67 mm) above the 1993 average, the highest yearly average in the satellite record (1993 to present). This change is largely explained by the melting glaciers and ice sheets and thermal expansion of seawater, both caused by global climate change.
The Inter-governmental Panel on Climate Change (IPCC) stated in a recent report that global sea levels may increase by half a meter (on average) over the next 50 to 100 years, and maybe more if considerable accelerations occur in the next couple of decades. Such predictions are significant given that nearly 40 percent of the U.S. population lives in coastal areas with relatively high population density. In these locations the specter of rising seas poses a risk to critical infrastructure — roads, bridges, subways, sewage treatment plants and so on — and may exacerbate destructive storm surges, allowing them to push farther inland than they once did.
Of course, as the above image suggests, sea level rise varies around the globe. For example, in the Chesapeake Bay (near Chesapeake City, Maryland), the mean sea level trend ranges from 3.13 to 4.71 millimeters per year. By comparison, in the western Gulf of Mexico (near Sabine Pass, Texas) the mean sea level trend ranges from 4.62 to 6.22 millimeters per year. What accounts for the difference? Scientists point to a variety of factors, including seasonal weather patterns, such as El Niño events, which influence coastal and open ocean circulation.
How Sea Level Is Measured
People are often surprised to learn that the surface of the ocean is not flat. Just like the land, it has topographical variation — hills and depressions — which can make measuring its height a challenge.
One way NOAA’s scientists measure sea-surface height is with satellite-based altimeters, instruments that calculate the height of ocean surfaces by measuring the time it takes a radar pulse to travel from the satellite to the surface of the sea and back. Altimetry, however, is only part of the equation.
For sea height to be measured accurately, the altimeter measurement must be subtracted from the measurement of the satellite’s distance to an earth-based coordinate system. Scientists obtain this second measurement by tracking the orbital position of the satellite using either GPS and/or DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite, a French satellite system used for the determination of satellite orbits).
And that’s not all, says Dr. Laury Miller, Chief of NOAA's Laboratory for Satellite Altimetry in Washington.
“There are also a bunch of “environmental” corrections — literally corrections to account for the fact that radar echoes travel slightly slower through the atmosphere than the speed of light in a vacuum — due to atmospheric water vapor, free electrons in the ionosphere, etc.”
Then the measurements are compared to historical averages to determine if sea levels have risen and by how much.
“We compute an average sea surface height map from all of the satellite measurements, from the beginning of altimetry to the present. Then we look at the spatial and temporal changes with respect to the long term averages,” Miller said.
Ultimately, these measurements can provide an important tool for evaluating climate change models, such as those the IPCC uses in its climate assessment reports.
Jason-3: The Tradition Continues
NOAA began monitoring sea level more than 20 years ago, with the TOPEX/Poseidon satellite in 1992. Then Jason-1 took over from 2001 to 2013. Today, Jason-2, launched in June 2008, produces estimates of global mean sea level every 10 days in fulfillment of its Ocean Surface Topography Mission, a joint effort among NOAA, the National Aeronautics and Space Administration (NASA), France’s Centre National d’Etudes Spatiales (CNES) and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT).
To ensure the continuity of this important record, NOAA launched its newest Jason series satellite, Jason 3, on January 17, 2016. Now operational, Jason-3 works alongside Jason-2 to support the Jason mission’s scientific, commercial and practical applications, including hurricane intensity forecasting, surface wave forecasting for offshore operators, tides and current forecasting for commercial shipping and ship routing, coastal forecasting for response to environmental problems like oil spills and harmful algal blooms, coastal modeling crucial for marine mammal and coral reef research, and El Niño and La Niña forecasting.
For more information on satellite altimetry and sea level rise, visit NOAA’s Laboratory for Satellite Altimetry web page. Additional information on sea-level rise, including an interactive map, is available on the Center for Operational Oceanographic Products and Services website. To learn more about the Jason-3 mission, visit the Joint Polar Satellite Systems’ website.
*Editor's Note* This article has been updated to reflect the launch and operational status of the Jason-3 spacecraft.