Make no mistake, global sea level rise is accelerating faster than ever. Previous estimates based on satellite data may have suggested that sea level rise was actually slowing down over the last couple decades, but according to a new Nature Climate Change study, measurement errors may have concealed the ongoing acceleration.
Beginning in 1993, a groundbreaking satellite-era time series described one of strongest and most robust signs of climate change: global mean sea level rise. It used radar observations of sea surface height from three consecutive satellite altimeter missions: TOPEX/Poseidon, Jason-1, and OSTM/Jason-2. According to this seminal climate record, sea level rise was lower in the past decade than during the decade before -- which would totally be good news, if only it was true.
This recent slowdown has puzzled researchers for a while, especially when melting ice in West Antarctica and Greenland is increasingly contributing to sea level rise, according to Christopher Watson from the University of Tasmania. And that may be because the satellite-based studies didn’t take into account vertical land movement. The sites where gauges sit can shift during earthquakes, or subside from groundwater withdrawal or sediment settling, Science explains. These natural changes to Earth’s surface may end up producing what appears to be sea level changes, even though they have nothing to do with the ocean.
So Watson and colleagues tried to identify inaccuracies in the satellite data by combining GPS measurements of vertical land movement with hourly data from a worldwide network of tide gauges. TOPEX/Poseidon may have slightly overstated sea level rise, Science reports, and the inflated numbers gave the appearance that sea level rise was decelerating. When in fact, it was masking the ongoing acceleration, New Scientist explains. Furthermore, natural climate variation contributed to the slowdown as well. Around 2011, "there was a major dip in sea level associated with major flooding events in Australia and elsewhere," says study co-author John Church of CSIRO.
Rather than a rise of 3.2 millimeters per year based on previous satellite-based estimates, the team found that the overall rate of global sea level rise was between 2.6 to 2.9 millimeters per year from 1993 to 2014. The first six years of the satellite-based record were affected the most by these new corrections: estimates for 1993 through 1999 had to be scaled down by 0.9 to 1.5 millimeter per year.
This new recalculation means that, compared with the 20th century, the rate of sea level rise has actually accelerated in recent years -- by an additional 0.04 millimeters per year. This revised acceleration fits with the melting ice sheets during this period, and "it is consistent with all the projections," Watson adds.