The smoking caldera of Mount Pinatubo on June 22, 1991. Image: USGS

As our planet heats up, the pace of sea level rise is expected to quicken, making it harder for cities like Miami to stay above water. But since 1992, scientists have studied Earth’s mean sea level via satellites, and they’ve watched it rise at a steady 3 millimeters per year—no evidence for acceleration.

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Now, after more than twenty years of head-scratching, we finally have an explanation: the Mt. Pinatubo volcanic eruption of 1991. The largest eruption of the late 20th century, Mt. Pinatubo blew its top less than two years before modern sea level record-keeping began. According to research published today in Scientific Reports, the eruption cooled the oceans enough to briefly depress global sea level, masking the expected acceleration in the record so far.

“We got a very biased view of sea level rise, based on the happenstance timing of the launch of [the first] altimeter satellites,” lead study author John Fasullo of the National Center for Atmospheric Research told Gizmodo.

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Accounting for Mt. Pinatubo, Fasullo and his co-authors conclude that sea level rise is already escalating today, and will continue to do so in the future.

Estimated rate of global sea level rise based on the satellite altimeter record, which began in late 1992. Image: Fasullo et al. 2016

There’s a finite amount of water on our planet, but it rearranges itself in all sorts of ways when the climate shifts. In warmer periods of Earth’s history, sea level gets higher, due to the combined effect of thermodynamics (hot liquids expand to occupy more space than cold liquids) and an increase in the total amount of water in the ocean as ice sheets melt. From the end of the last ice age to today, global mean sea level has risen approximately 410 feet (125 meters).

Perhaps the most important thing we’ve learned about sea level by studying Earth’s past is that it doesn’t rise linearly—it goes in fits and starts, accelerating dramatically as the ice sheets disintegrate. Whether the pace of modern sea level rise will start to escalate, in keeping with the pattern of the past, has enormous implications for the hundreds of millions of people living along vulnerable coastlines. Acceleration could be the difference between two and twenty feet of sea level rise by the century’s end.

A) Changes in the reflectivity of the atmosphere over the tropical oceans following the Mt. Pinatubo eruption (black) caused a sudden drop in global mean sea level (blue). B) Corresponding changes in ocean heat content (red) atmospheric water vapor (blue) and water storage on the global land surface (green). Image: Fasullo et al. 2016

Now, we have our first firm evidence that the rate of sea level rise is already quickening. Analyzing numerous model simulations with and without natural factors that can affect global sea level, Fasullo and his colleagues were able to pick out the signal of the Mt. Pintaubu eruption, which occurred on June 15th, 1991. They found that aerosols from the eruption blocked enough sunlight to temporarily cool the oceans, causing sea level to fall by about six millimeters.

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By some rotten luck, the eruption took place right before the first altimeter, the TOPEX/Poseidon satellite, was launched into orbit in late 1992. In the early days of monitoring sea level from space, Earth’s oceans were rebounding from the temporary effect of the volcano, causing the rate of sea level rise to be artificially high. “That skewed our impression of acceleration,” Fasullo said.

Accounting for the depression and sudden rebound of sea levels due to the eruption, Fasullo and his colleagues determined that the expected warming-induced acceleration is already under way. “In the next five to 10 years, we should see a clear acceleration emerge from the record,” he said.

Fasullo was reluctant to estimate to how quickly sea level rise will accelerate, or what the total damage will be the end of the century. The Intergovernmental Panel on Climate Change estimates about three feet of sea level rise by the century’s end, while the National Oceanic and Atmospheric Administration predicts four to six and a half feet. Other scientists have considered the possibility of rapid ice sheet disintegration and come to much higher estimates.

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“The science just isn’t there yet,” Fasullo said. “There’s a wealth of research going on, trying to quantify the contribution of ice sheet [melting] to acceleration. That’s where the main uncertainty lies.”

[Nature Scientific Reports]