Rising sea levels, which menace coastal populations and infrastructure, are one of the most visible threats of climate change. And according to a new report by researchers at the University of Colorado at Boulder, not only are the oceans continuing to rise, but the pace of that change is increasing, meaning that the effect could be more grave than we thought.

Climate scientists have long known that sea levels were rising at a rate of at least three millimeters per year. But according to a new analysis of satellite data collected over the past 25 years and published this week in the Proceedings of the National Academy of Sciences, that rate is increasing by nearly a tenth of a millimeter per year — meaning that by the turn of the next century, the ocean could be continuing to rise at more than three times its current rate.

"This acceleration, driven mainly by accelerated melting in Greenland and Antarctica, has the potential to double the total sea level rise by 2100 as compared to projections that assume a constant rate," said lead researcher Steve Nerem in a press release. “And this is almost certainly a conservative estimate.”

Climate change can cause sea levels to rise in two ways: by melting ice sheets, and because warmer water expands in volume.

The danger of water overtaking familiar coastal cities will happen gradually, starting with the lowest-lying parts of city infrastructure, and is already a concern to city planners. A recent New York Times feature revealed that the New York City officials have already started to shore up the city’s subway system by installing trap doors and vent covers over the system’s least-elevated openings.