This has important implications for the coasts: It is much harder to plan for and adapt to accelerating sea-level rise than it is for seas rising at a constant rate.
Before this study was published Monday in the Proceedings of the National Academy of Sciences, scientists had suspected that sea-level rise was accelerating but did not have the data yet to prove it.
Satellite data showed a rise in sea levels over the last 25 years of 2.7 inches (7 centimeters), a rate of roughly 0.12 inches (0.3 cm) per year. Over this short time period, the rate of sea-level rise waxed and waned, and it was difficult to tease out whether its pace was steady or picking up.
But it was only a matter of time before an accelerating trend would become clear, scientists thought. “A detectable acceleration is likely to emerge from the noise of internal climate variability in the coming decade,” a study concluded in 2016.
The study published Monday suggests that enough data is now available to confirm that sea-level rise is in fact speeding up. The authors’ analysis, which took into account cyclical changes in the oceans’ temperatures and measurement uncertainty, found that the chance that sea-level rise is not accelerating is less than 1 percent.
“So far, the difficulty in measuring sea level has made detecting acceleration very hard,” said Joseph Majkut, a climate scientist at the Niskanen Center, a libertarian think tank that favors climate action. “This new paper seems to have done it.”
Majkut, who was not involved in the study, cautioned, however: “It’s only one paper, so we’ll see if it holds up.”
The study authors said the acceleration they calculated is consistent with climate models. If this acceleration is extrapolated into the future, they said, it equates to an increase in average sea level of roughly 26 inches (65 cm) by 2100. This signifies a tripling in the amount of sea-level rise compared to the last century (roughly 8 inches globally). By 2100, the rate of sea-level rise would be double what it is now, the authors reported.
These projections could even be too low. A huge wild card in predicting future sea-level rise is how the Greenland and Antarctic ice sheets will respond to rising temperatures in the coming decades. Their internal dynamics are complex and an area of active research. Processes could kick in that cause the ice to disintegrate faster than models predict.
“If sea level begins changing more rapidly, for example due to rapid changes in ice sheet dynamics, then this simple extrapolation will likely represent a conservative lower bound on future sea-level change,” the study said.
The rather steady sea-level rise observed to date, just starting to accelerate, has already proven disruptive in many U.S. coastal areas.
The frequency of so-called nuisance flooding events, caused by astronomically high tides and/or minor storms, has increased up to several hundred percent. In other words, areas that used to flood only during major storms are now seeing inundation of roads and businesses during much lesser tidal events, sometimes even on sunny days.
During big storms, the implications of rising seas have proven even more severe. During the January “bomb cyclone,” Boston observed its highest tide in recorded history. Superstorm Sandy delivered the highest-recorded water level at the Battery in New York City in 2012.
Another study in the Proceedings of the National Academy of Sciences, published in 2017, found that, due to sea-level rise, flood heights that used to occur every 500 years in New York City now occur every 25 years and are projected to occur every five years over the next three decades.