Climate scientists at a federally-funded research institute say a massive volcanic eruption during the early 1990s “masked” the acceleration in sea level rise due to man-made global warming.

Scientists with the National Center for Atmospheric Research (NCAR) say the 1991 eruption of Mt. Pinatubo “masked the acceleration that would have otherwise occurred” in sea level rise due to global warming. They also said sea level rise will get even worse as the world warms.

“This study shows that large volcanic eruptions can significantly impact the satellite record of global average sea level change,” Steve Nerem with the University of Colorado Boulder, where NCAR is located, said in a statement.

“So we must be careful to consider these effects when we look for the effects of climate change in the satellite-based sea level record,” Nerem said.

Climate models projected sea levels to rise as greenhouse gases warmed the atmosphere, causing thermal expansion of the oceans and melting the polar ice caps. But that didn’t happen, and sea level rise slowed during the 2000s.

“In stark contrast to this expectation however, current altimeter products show the rate of sea level rise to have decreased from the first to second decades of the altimeter era,” NCAR scientists wrote in their study.

To get their results, Nerem and his colleagues ran 40 theoretical climate model simulations on possible scenarios for sea level rise against models that incorporated aerosols from volcanic eruptions. Scientists say these have a cooling effect on the planet.

Volcanic aerosols slowed human-induced warming, therefore slowing sea level rise that would have accelerated otherwise, according to the NCAR study. The study was funded by the National Science Foundation.

For years, scientists have been trying to figure out why sea level rise has not increased in line with climate model projections. The NCAR study notes sea level rise decreased from 3.5 millimeters per year during the 1990s to 2.7 millimeters per year in the 2000s — though there’s lots of uncertainty in these measurements.

This isn’t the first time, however, that scientists have said volcanic eruptions slowed sea level rise. In 2006, government scientists used climate models to conclude the eruption of Krakatoa in 1883 masked sea level rise acceleration that would have occurred otherwise in the 20th Century.

Though this study was cast into doubt by work done in 2010, and since some scientists have published work arguing sea levels are rising faster.

A 2015 study by Australian scientists claimed sea level rise had accelerated in recent years due to global warming, but critics were quick to point out the Aussies’ results were not “statistically different than zero.”

“You would think that would be the end of the matter as both results are statistically unimportant being not statistically different from zero,” Dr. David Whitehouse of the Global Warming Policy Forum wrote in a critique of the study. “But you would be wrong.”

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