A new study says that global sea level rise is accelerating incrementally over time, rather than increasing at a steady rate, as previously thought. The study, published February 12, 2018, in the journal Proceedings of the National Academy of Sciences, is based on 25 years of NASA and European satellite data.

The researchers say that this 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.

If the rate of ocean rise continues to change at this pace, the researchers suggest, sea level will rise 26 inches (65 centimeters) by 2100. That’s enough to cause significant problems for coastal cities.

Steve Nerem, the study’s lead author, 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. Nerem said in a statement:

This is almost certainly a conservative estimate. 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 (7 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 measurements since 1992 from multiple satellite, managed by multiple agencies in both the U.S. and Europe. According to the data, the rate of sea level rise in the satellite era has risen from about 0.1 inch (2.5 millimeters) per year in the 1990s to about 0.13 inches (3.4 millimeters) per year today.

Even with a 25-year data record, detecting acceleration is challenging. Episodes like volcanic eruptions can create variability: the eruption of Mount Pinatubo in 1991 decreased global mean sea level, for example. Global sea level can also fluctuate due to climate patterns such as El Niños and La Niñas, which influence ocean temperature and global precipitation patterns. For the study, the researchers used climate models to account for the volcanic effects and other datasets to determine the El Niño/La Niña effects.

Bottom line: According to a new study based on 25 years of NASA and European satellite data, global sea level rise has been accelerating in recent decades, rather than increasing steadily.

Read more about the study from NASA