Global sea level rise is accelerating incrementally over time rather than increasing at a steady rate, as previously thought, according to a new study based on 25 years of NASA and European satellite data.

The acceleration, driven mainly by increased melting in Greenland and Antarctica, has the potential to double the total sea level rise projected by 2100 when compared to projections that assume a constant rate of sea level rise, according to lead author Steve Nerem. Nerem is a professor of Aerospace Engineering Sciences at the University of Colorado Boulder, a fellow at Colorado’s Cooperative Institute for Research in Environmental Sciences (CIRES), and a member of NASA’s Sea Level Change team.

If the rate of ocean rise continues to change at this pace, sea level will rise 26 inches by 2100—enough to cause significant problems for coastal cities, according to the new assessment by Nerem and colleagues from NASA’s Goddard Space Flight Center in Greenbelt, Maryland; CU Boulder; the University of South Florida in Tampa; and Old Dominion University in Norfolk, Virginia. The team, driven to understand and better predict Earth’s response to a warming world, published their work in the journal Proceedings of the National Academy of Sciences.

“This is almost certainly a conservative estimate,” Nerem said. “Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that’s not likely.”

Rising concentrations of greenhouse gases in Earth’s atmosphere increase the temperature of air and water, which causes sea level to rise in two ways. First, warmer water expands, and this thermal expansion of the ocean has contributed about half of the 2.8 inches (seven centimeters) of global mean sea level rise we’ve seen over the last 25 years, Nerem said. Second, melting land ice flows into the ocean, also increasing sea level across the globe.

These increases were measured using satellite altimeter measurements since 1992. The rate of sea level rise in the satellite era has risen from about 0.1 inch per year in the 1990s to about 0.13 inches per year today.

In 2018, NASA will launch two new satellite missions that will be critical to improving future sea level projections: the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, a partnership with GeoForschungsZentrum (GFZ) in Germany, will continue measurements of the mass of the Greenland and Antarctic ice sheets; while the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) will make highly accurate observations of the elevation of ice sheets and glaciers.