A team of scientists has studied 25 years' worth of satellite data, and calculated that the sea level isn't rising at a steady rate, it's accelerating. If the trend continues, the total sea level rise could be twice as high as previous projections by 2100.
It's been observed for decades that the sea level is rising, with the waves lapping about 2.6 in (6.6 cm) higher on average in 2014 than they were in 1993. To study just how fast it's rising, the researchers looked at altimeter measurements gathered over the last 25 years by satellites such as TOPEX/Poseidon and the three Jason satellites, as well as ground-based tide gauge data and climate simulations.
Altogether, the researchers calculated that the rate of sea level rise is accelerating by about 0.08 mm per year. If left unchecked, that trend could mean the seas would rise by at least 10 mm per year by the end of the century, which would wreak plenty of havoc on the world's coastal cities.
"This acceleration, driven mainly by accelerated melting in Greenland and Antarctica, has the potential to double the total sea level rise by 2100 as compared to projections that assume a constant rate — to more than 60 cm (23.6 in) instead of about 30 (11.8 in)," says Steve Nerem, lead researcher on the project. "And this is almost certainly a conservative estimate. Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that's not likely."
Satellite altimeter data alone doesn't paint a clear enough picture, due to fluctuations caused by El Niño and La Niña weather patterns, and events like volcanic eruptions. To see through these outliers to the underlying trend, the scientists used climate models that determine how much of an effect these events would have. Tide gauge data, gathered by ground-based stations all over the world, can also chip in to counter any potential errors in the altimeter readings.
"The tide gauge measurements are essential for determining the uncertainty in the GMSL (global mean sea level) acceleration estimate," says Gary Mitchum, co-author of the study. "They provide the only assessments of the satellite instruments from the ground."
The researchers say that their findings are just the beginning. The 25-year period studied so far is long enough for the acceleration to be detected, but further data will be gathered by the ongoing Jason-3 project and other altimetry satellites, as well as more advanced ground stations.
The research team involved scientists from the Cooperative Institute for Research in Environmental Sciences (CIRES), the University of Colorado Boulder, the University of South Florida, NASA Goddard Space Flight Center, Old Dominion University, and the National Center for Atmospheric Research.
The study was published in the journal Proceedings of the National Academy of Sciences.
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