By Laura Marcos
As the Earth warmed and glaciers and ice sheets thawed, decades of data from satellites seemed to show thatthe rate of sea level riseit remained stable or even declining. How is that possible? For years, scientists' estimates rangedagainst the dataprovided by a joint US-French mission that began collecting data in late 1992, theTOPEX / Poseidon. It was all the fruit of acalibration error.
Well, adjusting the data to eliminate that error suggests thatsea levels are rising at higher rates every yearaccording to University of Colorado Boulder remote sensing expert Steven Nerem, who is leading the reanalysis.
Nerem's team calculated that the rate of sea level riseincreased from about 1.8 millimeters per year in 1993 to about 3.9 millimeters per year today as a result of global warming. In addition to the satellite calibration error, their analysis also takes into account other factors that have influenced sea level rise in recent decades, such asthe 1991 eruption of Mount Pinatubo in the Philippines and the recent El Niño weather pattern.
The results are aligned with three recent studies that have raisedquestions about the first observations of the height of the sea surface. "Whatever the methodology, we all come to the same conclusions," says Anny Cazenave, a geophysicist at the Laboratory for the Study of Space Geophysics and Oceanography (LEGOS) in Toulouse, France.
In an analysis published in Geophysical Research Letters in April, Cazenave's teamhe noted the various contributions to sea level rise, including the expansion resulting from warming ocean waters and melting ice in places like Greenland. Their results suggest that satellite altimetry measurements were too high during the first six years they were collected. After this point, scientists began using the backup sensor of TOPEX / Poseidon. The error in those early measurements distorted the long-term trend,masking a long-term increase in the rate of sea level rise.
The problem was first identified in 2015 by a group that included John Church, an oceanographer at the University of New South Wales in Sydney, Australia. Church and his colleagues identified adiscrepancy between sea level data collected by satellites and from tide gauges scattered around the world.
If sea level rise continues to accelerate at the current rate, the world's oceans could rise by about 75 centimeters over the next century.
To identify what had gone wrong with the satellite measurements, Nerem and his team first compared the satellite data with observations from tide gauges that showed an acceleration of sea level rise. Then the researchers started lookingfactors that could explain the difference between the two data sets.
The team finallyidentified a minor calibration that had been built into the altimeter ofTOPEX / Poseidon to correct any flaws in your data that could be caused by problems with the instrument, such as aging electronic components. Nerem and his colleagues weren't sure that calibration was necessary and when they pulled it out, measurements of sea level rise in the satellite's early years were more closely aligned with the tide gauge data. The adjusted satellite data showed an increasing rate of sea level rise over time.
If sea level rise continues to accelerate at the current rate, according to Nerem, "the world's oceans could rise by 75 centimeters over the next century." The message is quite clear.Humanity needs to reduce its production of greenhouse gas emissions, and quickly.