As the Earth warmed and glaciers and ice sheets thawed, decades of satellite data seemed to show that the rate of sea level rise was stable or even declining. How is that possible? For years, scientists’ estimates ran counter to data provided by a joint US-French mission that began collecting data in late 1992, TOPEX / Poseidon . Everything was the result of an error in the calibration.
Well, adjusting the data to eliminate that error suggests that sea levels are rising at higher rates each year , according to University of Colorado Boulder remote sensing expert Steven Nerem, who led the reanalysis.
Nerem’s team calculated that the rate of sea level rise increased 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 satellite calibration error, their analysis also takes into account other factors that have influenced sea level rise in recent decades, such as the 1991 eruption of Mount Pinatubo in the Philippines and the recent El Niño weather pattern.
The results align with three recent studies that have raised questions 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 team 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 TOPEX / Poseidon backup sensor. 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 a discrepancy between sea level data collected by satellites and those from tide gauges scattered around the world.
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. The researchers then began looking for factors that could explain the difference between the two data sets.
The team eventually identified a minor calibration that had been built into the TOPEX / Poseidon altimeter to correct any gaps in its data that could be caused by problems with the instrument, such as aging electronics. 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 about 75 centimeters over the next century.” The message is quite clear. Humanity needs to reduce its production of greenhouse gas emissions, and quickly.
References:
Satellite snafu masked true sea-level rise for decades. Nature 547, 265-266 ( 20 July 2017 ) doi : 10.1038 / nature.2017.22312
