Global warming already accelerating sea level rise, study finds – ‘Accelerated sea level rise is real, and it’s ongoing’Posted by Jim at Thursday, August 11, 2016
By Laura Snider
10 August 2016
BOULDER, Colorado (NCAR) – Greenhouse gases are already having an accelerating effect on sea level rise, but the impact has so far been masked by the cataclysmic 1991 eruption of Mount Pinatubo in the Philippines, according to a new study led by the National Center for Atmospheric Research (NCAR).
Satellite observations, which began in 1993, indicate that the rate of sea level rise has held fairly steady at about 3 millimeters per year. But the expected acceleration due to climate change is likely hidden in the satellite record because of a happenstance of timing: The record began soon after the Pinatubo 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.
The study lends support to climate model projections, which show the rate of sea level rise escalating over time as the climate warms. The findings were published today in the open-access Nature journal, Scientific Reports.
"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."
Study co-author Steve Nerem, from the University of Colorado Boulder, added: “This study shows that 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."
The findings have implications for the extent of sea level rise this century and may be useful to coastal communities planning for the future. In recent years, decision makers have debated whether these 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 study was funded by NASA, the U.S. Department of Energy, and the National Science Foundation, which is NCAR's sponsor.
Reconstructing a pre-Pinatubo world
Climate change triggers sea level rise in a couple 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, the pace of warming and melting has accelerated, and scientists have expected to see a corresponding increase in the rate of sea level rise. But analysis of the relatively short satellite record has not borne that out.
To investigate, Fasullo, Nerem, and Benjamin Hamlington of Old Dominion University worked to pin down how quickly sea levels were rising in the decades before the satellite record began.
Prior to the launch of the international TOPEX/Poseidon satellite mission in late 1992, sea level was mainly measured using tide gauges. While records from some gauges stretch back to the 18th century, variations in measurement technique and location mean that the pre-satellite record is best used to get a ballpark estimate of global mean sea level.
To complement the historic record, the research team used a dataset produced by running the NCAR-based Community Earth System Model 40 times with slightly different—but historically plausible—starting conditions. The resulting simulations characterize the range of natural variability in the factors that affect sea levels. The model was run on the Yellowstone system at the NCAR-Wyoming Supercomputing Center.
A separate set of model runs that omitted volcanic aerosols — particles spewed into the atmosphere by an eruption — was also assessed. By comparing the two sets of runs, the scientists were able to pick out a signal (in this case, the impact of Mount Pinatubo's eruption) from the noise (natural variations 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."
Using models to understand observations
Analyzing the simulations, the research team found that Pinatubo's eruption caused the oceans to cool and sea levels to drop by about 6 millimeters immediately before TOPEX/Poseidon began recording observations.
As the sunlight-blocking aerosols from Mount Pinatubo dissipated in the simulations, sea levels began to slowly 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 would otherwise have been.
This higher-than-normal rate of sea level rise in the early part of the satellite record makes it appear that the rate of sea level rise has not accelerated over time and may actually have decreased somewhat. In fact, according to the study, if the Pinatubo eruption had not occurred—leaving sea level at a higher starting point in the early 1990s—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."
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.
“These scientists have disentangled the major role played by the 1991 volcanic eruption of Mt. Pinatubo on trends in global mean sea level,” said Anjuli Bamzai, program director in the National Science Foundation’s Division of Atmospheric and Geospace Sciences, which funded the research. “This research is vital as society prepares for the potential effects of climate change."
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.
Since the original TOPEX/Poseidon mission, other satellites have been launched—Jason-1 in 2001 and Jason-2 in 2008—to continue tracking sea levels. The most recent satellite, Jason-3, launched on 17 January 2016.
"Sea level rise is potentially one of the most damaging impacts of climate change, so it's critical that we understand how quickly it will rise in the future," Fasullo said. "Measurements from Jason-3 will help us evaluate what we've learned in this study and help us better plan for the future."
By Chris Mooney
10 August 2016
(Washington Post) – On a warming Earth, seas inevitably rise, as ice on land melts and makes its way to the ocean. And not only that — the ocean itself swells, because warm water expands. We already know this is happening — according to NASA, seas are currently rising at a rate of 3.5 millimeters per year, which converts to about 1.4 inches per decade.
However, scientists have long expected that the story should be even worse than this. Predictions suggest that seas should not only rise, but that the rise should accelerate, meaning that the annual rate of rise should itself increase over time. That’s because the great ice sheets, Greenland and Antarctica, should lose more and more mass, and the heat in the ocean should also increase.
The problem, or even mystery, is that scientists haven’t seen an unambiguous acceleration of sea level rise in a data record that’s considered the best for observing the problem — the one produced by the TOPEX/Poseidon satellite, which launched in late 1992 and carries an instrument, called a radar altimeter, that gives a very precise measurement of sea level around the globe.
This record actually shows a decrease in the rate of sea level rise from the first decade measured by the satellite (1993 to 2002) to the second one (2003 to 2012). “We’ve been looking at the altimeter records and scratching our heads, and saying, ‘why aren’t we seeing an acceleration in the satellite record?’ We should be,” said John Fasullo, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado. […]
“What we’ve shown is that sea level acceleration is real, and it continues to be going on, it’s ongoing, and we understand why you don’t see it in the short satellite record,” said Fasullo, who conducted the research along with scientists from the University of Colorado in Bolder and Old Dominion University. […]
“Our initial impression of sea level rise was not only influenced by climate change and the rate of change, but the response and the recovery from the eruption itself,” says Fasullo. “Those effects largely have ebbed by now, and once we get a few more years into the altimeter record, we should see a clear acceleration. That’s really the punchline of the article.” […]
The key question then becomes just how fast the annual rate of sea level rise can actually increase. In one thought experiment recently, former NASA climate scientist James Hansen calculated the consequences if the “doubling time” for ice loss is as fast as 10 years — finding dramatic sea level increases as a result.
“Doubling times of 10, 20 or 40 years yield sea level rise of several meters in 50, 100 or 200 years,” Hansen’s study concluded. However, it is far from clear at this point that ice loss is actually increasing this rapidly. […]
Fasullo says that debate — over precisely how fast acceleration happens, or where that leaves us in 2100 — remains unresolved. For now, he says, at least it’s pretty clear that the acceleration is actually happening as expected.
“Accelerated sea level rise is real, and it’s ongoing, and it’s not something we should doubt based on the altimeter record,” said Fasullo. [more]
ABSTRACT: Global mean sea level rise estimated from satellite altimetry provides a strong constraint on climate variability and change and is expected to accelerate as the rates of both ocean warming and cryospheric mass loss increase over time. 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. Here, a combined analysis of altimeter data and specially designed climate model simulations shows the 1991 eruption of Mt Pinatubo to likely have masked the acceleration that would have otherwise occurred. This masking arose largely from a recovery in ocean heat content through the mid to late 1990 s subsequent to major heat content reductions in the years following the eruption. A consequence of this finding is that barring another major volcanic eruption, a detectable acceleration is likely to emerge from the noise of internal climate variability in the coming decade.