There were problems with the calibration of a sensor on the first of several satellites launched to measure the height of the sea surface using radar. Adjusting the data to remove that error suggests that sea levels are indeed rising at faster rates each year.
Nerem’s team calculated that the rate of sea-level rise increased from around 1.8 millimeters per year in 1993 to roughly 3.9 millimeters per year today as a result of global warming. In addition to the satellite calibration error, his analysis also takes into account other factors that have influenced sea-level rise in the last several decades, such as the eruption of Mount Pinatubo in the Philippines in 1991 and the recent El Niño weather pattern.
His team first compared the satellite data to observations from tide gauges that showed an accelerating rate of sea-level rise. Then the researchers 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 TOPEX/Poseidon’s altimeter to correct any flaws in its data that might be caused by problems with the instrument, such as ageing electronic components. Nerem and his colleagues were not sure that the calibration was necessary — and when they removed it, measurements of sea-level rise in the satellite’s early years aligned more closely 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, Nerem says, the world’s oceans could rise by about 75 centimeters over the next century. That is in line with projections made by the Intergovernmental Panel on Climate Change in 2013.