In a finding that challenges climate skeptics, a team of scientists reported this week that a seeming slowdown in global sea-level rise in the last decade was not a slowdown at all.

Their work—released in Nature Climate Change—found that a natural weather cycle known as La Niña provides most of the explanation for the pause in sea-level rise observed after 2003. The phenomenon lowered precipitation over the oceans in the last decade, dampening the warming signal, they said.

The research also supports a theory that a parallel pause in air temperature rise in recent years may result from storage of heat in the deep ocean.

"What we show is that for the last decade, the rate of global mean sea-level rise is greater than previously believed," said Anny Cazenave, a French scientist at Laboratoire d'Etudes en Géophysique et Océanographie Spatiales and lead author of the study.

Global sea levels rose at a rate of about 3.5 millimeters annually in the 1990s, a dynamic largely attributed to rising temperatures. However, the rate of rise dropped about 30 percent between 2003 and 2011 to approximately 2.4 millimeters a year, driving speculation—and skeptic criticism—about the cause of the pause.

The French scientific team benefited from data from NASA's twin Gravity Recovery and Climate Experiment satellites, launched in 2002, that measure ocean mass and water storage variations on land. Combined with earlier water resource monitoring and hydrological modeling, the information let them estimate fluctuations in ocean mass.

Unlike much of the 1990s, the 2003-11 period favored La Niña episodes, or natural cycles characterized by unusually low ocean temperatures in the eastern equatorial Pacific Ocean. La Niña's cousin, El Niño, is characterized by the opposite, or unusually high temperatures. The El Niño-La Niña cycle, or El Niño-Southern Oscillation, has been blamed for various climate catastrophes from wildfires to floods.

A slowdown disappears
La Niñas also mean that more rain falls over land and less falls over the ocean, particularly in the Pacific, Cazenave explained. That in turn can suppress sea levels in a way they would not be suppressed under average conditions or with an El Niño, she said. "Less water is filling the ocean basins" with La Niñas.

When the scientists corrected for this La Niña effect on rain patterns in the past decade, they found that the slowdown in sea-level rise disappeared and fell in line with the 1994-2002 pace, coming to about 3.3 millimeters a year. The results call for re-examination of long-term sea-level records to detect the true warming signal, the paper says. The results also may help projections of the future, according to Cazenave.

"Climate models need to be validated against observations," she said. However, it is not known whether climate change will lead to more La Niñas, she added.

Additionally, the paper supports the theory that heat storage in the deep ocean may be partly responsible for the parallel pause in Earth's surface temperatures over the past 13 years. Scientists have been grappling with why there was a recent slowdown in the warming pace from the early 20th century.

One theory is that the Pacific Ocean is absorbing much of the "missing heat" with help from strengthened trade winds (ClimateWire, Feb. 10).

The warming pause has been "exploited by climate skeptics to refute global warming," the paper states.

Because existing phenomena—such as thermal expansion of water from warming—do not fully explain the corrected sea-level-rise number of 3.3 millimeters, stored heat in the deep ocean may be making a significant contribution, Cazenave said. There is no way to know that for sure, but the revised sea-level numbers are consistent with the idea that the oceans are absorbing much of the lost heat in the past decade.

'Big elephants' make future sea-level rise hard to predict
Some previous papers mirror the new findings. Last year, for instance, a team led by federal scientists found that a mysterious drop in ocean level beginning in 2010 likely resulted from a combination of three temporary climate influences, including La Niña, that spawned heavy rain over Australia.

The new paper is unique for showing that the climate change signal is constant across decades, said John Fasullo, a climate scientist at the National Center for Atmospheric Research and author of the earlier Australia study.

"Every time some unexpected change in any climate parameter happens (e.g., sea level going down), the question will be raised on whether this will be a continuing trend. ... One could say that the authors are putting pieces of the puzzle together to make the information more accessible to the public," Carmen Boening, a NASA scientist who did not participate in the research, added in an email.

Felix Landerer, research scientist at NASA's Jet Propulsion Laboratory, said he agreed with the overall conclusion about no slowdown in sea-level rise but said he was reluctant to attribute such a big role to La Niña, as other processes also can cause sea-level undulation in the short term. It's also important to remember that the "big elephants" with future sea-level rise are the pace and dynamic of melting from Antarctica and Greenland, he said.

"This makes future projections extremely challenging—anything from 10 centimeters to over a meter is currently on the table for sea-level rise due to melting ice sheets," Landerer said in an email.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500