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The 1991 eruption of Mount Pinatubo may have skewed rising sea level readings...

Recent reports that suggest sea levels aren't rising as fast as expected – and may even be dropping – could be inaccurate, according to new research.

Experts from the National Center for Atmospheric Research (NCAR) have discovered that the 1991 eruption of Mount Pinatubo in the Philippines could have masked the true reading, and this could have dire consequences for the future.

Satellite observations, which began in 1993, show that the rate of sea level rise has held fairly steady at about 3 millimeters per year. However, these records began soon after the eruption, which temporarily cooled the planet, causing sea levels to drop.

The new study finds that the lower starting point effectively distorts the calculation of sea level rise acceleration for the last couple of decades. It also lends support to projections that show the rate of sea level rise escalating over time as the climate warms.

"When we used climate model runs designed to remove the effect of the Pinatubo eruption, we saw the rate of sea level rise accelerating in our simulations," said NCAR scientist John Fasullo, who led the study. "Now that the impacts of Pinatubo have faded, this acceleration should become evident in the satellite measurements in the coming decade, barring another major volcanic eruption."

Climate change is said to trigger a rise in sea level rise in a number of ways – by warming the ocean, which causes the water to expand, and by melting glaciers and ice sheets, which drain into the ocean and increase its volume.

In recent decades, warming and melting has increased and scientists had expected to see a corresponding acceleration in the rate of sea level rise. However, this has not been seen in the analysis of the satellite record.

The caldera of Mount Pinatubo on June 22, 1991 USGS

To investigate, Fasullo, Nerem, and Benjamin Hamlington from Old Dominion University studied how quickly sea levels were rising in the decades before the satellite record began.

Prior to the launch of the international Topex/Poseidon satellite in 1992, sea level was predominantly measured using tide gauges. While records from some gauges date back to the 18th century, variations in measurement technique and location mean the pre-satellite record is best used to get an estimate of global mean sea level.

To complement the historic record, the research team used data from the Community Earth System Model. It ran a series of simulations, including one that omitted volcanic particles released into the atmosphere by an eruption. By comparing the different simulations and climate models, the scientists were able to pick out a signal – in this case, the impact of Mount Pinatubo's eruption – from the natural changes in ocean temperature and other factors that affect sea level.

"You can't do it with one or two model runs – or even three or four," Fasullo said. "There's just too much accompanying climate noise to understand precisely what the effect of Pinatubo was. We could not have done it without large numbers of runs."

"This study shows volcanic eruptions can impact the satellite record. We must be careful to consider this when we look for the effects of climate change"

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Analysing the simulations, the research team found that Pinatubo's eruption caused the oceans to cool and sea levels to drop by about 6 millimetres immediately before Topex/Poseidon began recording observations.

As the volcanic aerosols blocking the sun and cooling the air were removed from the simulations, sea levels began to rebound to pre-eruption levels. This rebound "swamped the acceleration caused by the warming climate" and made the rate of sea level rise higher in the mid- to late 1990s than it may otherwise have been.

This higher-than-normal level makes it seem like the rate of sea level rise has not accelerated over time and may actually have decreased. In fact, according to the study, if the Pinatubo eruption had not occurred, the satellite record would have shown a clear acceleration.

"The satellite record is unable to account for everything that happened before the first satellite was launched, " Fasullo said. "This study is a great example of how computer models can give us the historical context that's needed to understand some of what we're seeing in the satellite record."

Nasa/JPL

"This study shows large volcanic eruptions can significantly impact the satellite record of global average sea level change. 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," study co-author Steve Nerem, from the University of Colorado Boulder added.

Understanding whether the rate of sea level rise is accelerating or remaining constant is important because it drastically changes what sea levels might look like in 20, 50, or 100 years. Also, because the study's findings suggest that acceleration due to climate change is already under way, the acceleration should become evident in the satellite record in the coming decade, Fasullo said.

In recent years, decision makers have debated whether communities should make plans based on the steady rate of sea level rise measured in recent decades or based on the accelerated rate expected in the future by climate scientists.

The findings were published in the Nature journal Scientific Reports and the study was funded by Nasa, the US Department of Energy, and the National Science Foundation.

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