Sea Level Rise Picking Up Speed

The tides they are a'changin, for global sea level rise is picking up speed, according to new research that found its acceleration is much larger than scientists previously thought.

Previous estimates had placed sea level rise at between 1.5 and 1.8 millimeters (mm) annually over the 20th century (1900-1990). However, according to co-lead researchers Carling Hay and Eric Morrow, these numbers over-estimated global sea level rise by as much as 30 percent. The figure was actually closer to 1.2 mm per year.

While that may appear as good news, currently the general consensus is that global sea level has risen by about 3 mm since then. And therefore, due to the previous miscalculation, sea level rise is accelerating at a rate significantly faster than scientists realized.

"What this paper shows is that sea-level acceleration over the past century has been greater than had been estimated by others," Morrow, a recent PhD graduate at Harvard University's Department of Earth and Planetary Sciences (EPS), said in a news release. "It's a larger problem than we initially thought."

"Another concern with this is that many efforts to project sea-level change into the future use estimates of sea level over the time period from 1900 to 1990," he added. "If we've been over-estimating the sea-level change during that period, it means that these models are not calibrated appropriately, and that calls into question the accuracy of projections out to the end of the 21st century."

So the research team decided to take a different approach. Usually to calculate global sea level rise, scientists rely on data from tide gauges that measure ocean currents in each of the world's oceanic sub-regions. The results are then compiled and averaged together to create a global estimate. However, tide gauges only measure along the coastline, leading to inaccurate data and large gaps in the record.

This time around Hay and Morrow decided to take into account a variety of factors that play a role in sea level rise. Aside from obvious ones like global warming and melting of sea ice, they also considered effects from the last ice age, ocean heating and expanding, and changes in ocean circulation.

"We are looking at all the available sea-level records and trying to say that Greenland has been melting at this rate, the Arctic at this rate, the Antarctic at this rate, etc," Hay explained. "We then sum these contributions and add in the rate that the oceans are changing due to thermal expansion to estimate a rate of global mean sea-level change."

To their surprise, simulations and statistical methods showed that over the last 20 years sea level acceleration "is really much larger than anyone thought," she said.

The results were published in the journal Nature.

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