From the NATIONAL CENTER FOR ATMOSPHERIC RESEARCH/UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH, and the department of “let’s not show a graph of sea level rise in the press release” comes this real PR spin job.
Climate change already accelerating sea level rise, study finds
Pinatubo eruption masked acceleration in satellite record
BOULDER, Colo. — 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 Jan. 17 of this year.
“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.”
Compare the title of the press release to the title of the paper:
Is the detection of accelerated sea level rise imminent?
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.
Access to open source article: http://www.nature.com/articles/srep31245
We have had some very good essays about the hoped for acceleration in sea level rise. Most recently this one in March of 2016:
An answer to: Is the rise in sea levels accelerating? by Jan Kjetil Andersen where he determines:
Above: IPCC AR5 page 289 their Figure 3.14
According to the figure, there has been no accelerating since 1920. We see that both Church & White and Ray & Douglas observe approximately the same annual rise of around 2.5 mm/year between 1920 and 1940, and then there is a fall 1 mm/year before the rate rebounds to 2.5 mm in the late 1980-ies.
The Jevrejeva et. Al observes a maximum rate of 4 mm/year in the 1940-es but the series stop before the increase in recent decades. Satellite altimeters shows a steady rise with a relative small variation between 2.9 and 3.9 mm/year.
The satellite measurements has a very short series in the figure because the series only goes from 1992 to 2012. That means that only two years can be calculated with an 18-years trend.
I think a very important piece of information comes out of this figure. We see here that in reality IPCC find no evidence for accelerating sea level rise after 1920. The rise before 1920 was real, but can hardly have been caused by the small amounts of greenhouse gases emitted at that time. The CO2 level in 1920 was according to Nasa 303 ppm, or just 10% above the pre-industrial level and the warming that could have caused the sea level rise has to come before that. (http://data.giss.nasa.gov/modelforce/ghgases/Fig1A.ext.txt)
And of course, Willis did an analysis back in 2011, Putting the Brakes on Acceleration, that is still germane today. This is the graph that NCAR won’t show you in their press release:
Figure 1. Satellite-measured sea level rise. Errors shown are 95% confidence intervals.Data Source.