Posted with permission from Newsweek

Sea levels aren’t just steadily rising—they’re accelerating, according to a new assessment based on 25 years of satellite data. The findings, published Monday in Proceedings of the National Academy of Sciences, confirm what climate projections have already told us. The results reveal that sea level rise has been accelerating for the past 25 years, rather than steadily rising the same amount each year.

Assuming the acceleration rate stays the same, which the lead author said is unlikely, sea levels will surge 26 inches by 2100 from climate change alone. 

Scientists used satellite data to pinpoint acceleration of sea level rise over 25 years, beginning in 1993. A second part of the study used satellite data tracking tiny fluctuations in gravity due to ice mass loss to trace the acceleration back to melting ice in Greenland and Antarctica. “This study would not have been possible without satellite measurements,” Steve Nerem, lead author and fellow of the Cooperative Institute for Research in Environmental Sciences at University of Colorado Boulder, told Newsweek. “They really are our kind of eye on the Earth.”

These so-called eyes on the Earth revealed that sea level rise is not steadily rising at .1 inch every year, but rather, that increase is increasing, itself—by .003 inches each year. Though these numbers sound small, tiny increases across the globe over several decades can result in devastating consequences from sea level rise on coastal cities alone. Storm surges and salt water intrusion into aquifers where some communities get their drinking water are just two examples.

Assuming the acceleration rate that this study found stays consistent throughout the century, sea levels will rise by 26 inches by 2100, the authors concluded. But, that number, Nerem said, “is almost certainly a conservative estimate of future sea level change,” he said. “The acceleration will probably go up as ice sheets start to respond more to the warming.”

Another caveat: this assessment did not account for sea level changes from other factors, including the climate phenomena known as El Niño—Southern Oscillation (ENSO), which affects ocean temperatures and precipitation, and the 1991 eruption of Mount Pinatubo in the Philippines. (Volcanic eruptions have a mild cooling effect that can, in turn, affect sea levels). Disentangling the various causes of sea level changes allowed researchers to pinpoint the acceleration rate due to climate change alone, which is critical to illuminate what sea level changes will happen over a time period of several decades.

"We have known from observations that the ice sheets and mountain glaciers have been losing mass at an accelerated rate over the past decade or so," Fernando Paolo, postdoctoral scholar at NASA’s Jet Propulsion Laboratory, who was not involved with the study, told  Newsweek. This study shows "a clear acceleration in the 25-year sea level rise, which can be linked to the accelerated ice loss." He added that the long record of 25 years allowed researchers to separate out natural processes, including ENSO and the 1991 volcanic eruption of Mount Pinatubo. 

The 26-inch sea level rise increase by 2100 is similar to model projections based on the Intergovernmental Panel on Climate Change 5th Assessment Report. That assessment, which Nerem said some describe as a conservative estimate, has been organized by the United Nations since 1988. Model projections include various factors of Earth and ice physics, accounting for nearly everything that may affect climate or sea levels, according to Nerem.

“[Our study] is much more simplistic, but it’s kind of providing a check on those model projections,” he explained. The acceleration rate uncovered in this study would more than double sea level rise by 2100 in comparison to steady sea level rise without acceleration.

Melting ice sheets in Greenland and Antarctica are the main cause of sea level rise and this acceleration, according to the study. Nerem said: “The big question in sea level science today is how are the ice sheets going to respond to the warming, and how quickly.”