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Wednesday, December 3, 2014

NASA: Antarctic Melt Loss Triples in a Decade

by Peter Sinclair, Climate Crocks, December 3, 2014
Evidence is piling up indicating the planet will be extremely fortunate to stay on the lower end of sea level rise estimates.

And remember, this study covers just one area of Antarctica, where we have good instrument coverage.

A comprehensive, 21-year analysis of the fastest-melting region of Antarctica has found that the melt rate of glaciers there has tripled during the last decade. 
The glaciers in the Amundsen Sea Embayment in West Antarctica are hemorrhaging ice faster than any other part of Antarctica and are the most significant Antarctic contributors to sea level rise. This study by scientists at the University of California, Irvine (UCI), and NASA is the first to evaluate and reconcile observations from four different measurement techniques to produce an authoritative estimate of the amount and the rate of loss over the last two decades.
This work extends the work of Eric Rignot, who stunned the ice sheet community last spring with his “holy shit moment” paper on unstoppable Antarctic melting – discussed here in a very important video, if you have not seen it.

“The mass loss of these glaciers is increasing at an amazing rate,” said scientist Isabella Velicogna, jointly of UCI and NASA’s Jet Propulsion Laboratory, Pasadena, California. Velicogna is a coauthor of a paper on the results, which has been accepted for publication in the journal Geophysical Research Letters. 
Lead author Tyler Sutterley, a doctoral candidate at UCI, and his team did the analysis to verify that the melting in this part of Antarctica is shifting into high gear. “Previous studies had suggested that this region is starting to change very dramatically since the 1990s, and we wanted to see how all the different techniques compared,” Sutterley said. “The remarkable agreement among the techniques gave us confidence that we are getting this right.” 
The researchers reconciled measurements of the mass balance of glaciers flowing into the Amundsen Sea Embayment. Mass balance is a measure of how much ice the glaciers gain and lose over time from accumulating or melting snow, discharges of ice as icebergs, and other causes. Measurements from all four techniques were available from 2003 to 2009. Combined, the four data sets span the years 1992 to 2013.
Worth watching this video from a year ago if you have not yet, backgrounder on the movement of large ice sheets.

The glaciers in the embayment lost mass throughout the entire period. The researchers calculated two separate quantities: the total amount of loss, and the changes in the rate of loss. 
The total amount of loss averaged 83 gigatons per year (91.5 billion U.S. tons). By comparison, Mt. Everest weighs about 161 gigatons, meaning the Antarctic glaciers lost an amount of water weight equivalent to Mt. Everest every two years over the last 21 years. 
The rate of loss accelerated an average of 6.1 gigatons (6.7 billion U.S. tons) per year since 1992.
During the period when the four observational techniques overlapped, the melt rate increased an average of 16.3 gigatons per year — almost three times the rate of increase for the full 21-year period. The total amount of loss was close to the average at 84 gigatons. 
The four sets of observations include NASA’s Gravity Recovery and Climate Experiment satellites, laser altimetry from NASA’s Operation IceBridge airborne campaign and the earlier ICESat satellite, radar altimetry from the European Space Agency’s Envisat satellite, and mass budget analyses using radars and the University of Utrecht’s Regional Atmospheric Climate Model. 
The scientists noted that glacier and ice sheet behavior worldwide is by far the greatest uncertainty in predicting future sea level. “We have an excellent observing network now. It’s critical that we maintain this network to continue monitoring the changes,” Velicogna said, “because the changes are proceeding very fast.”
This is just Antarctica. At the other pole, Greenland is also vulnerable, and estimates of how fast it can shed mass keep increasing. Below, see Dr. Jeff Masters' analysis of that loss, and Dr. James Hansen’s take on what an accelerating Greenland loss might mean to the world.
Dr. Jeff Masters at Weather Underground:
Human-caused global warming has set in motion an unstoppable slow-motion collapse of the glaciers in West Antarctica capable of raising global sea level by 4 feet (1.2 meters) in a few hundred years, said NASA in a May 2014 press release. What’s more, one of the glaciers involved, the Thwaites Glacier, acts as a linchpin on the rest of the ice sheet, which contains enough ice to cause a total of 10 to 13 feet (3 to 4 meters) of global sea level rise over a period of centuries. This unstoppable collapse makes saving Greenland “absolutely essential,” said glaciologist Richard Alley in a May 2014 interview in Mother Jones. 
Greenland’s ice sheet holds enough water to raise global sea levels by 7.36 meters (24.15 feet) were it all to melt, and civilization would be hard-pressed to deal with 10–13 feet of sea level rise from West Antarctica, let alone another 20+ feet from Greenland. “If we’ve committed to 3.3 meters (10.8') from West Antarctica, we haven’t committed to losing Greenland, we haven’t committed to losing most of East Antarctica,” said Alley. “Those are still out there for us. And if anything, this new news just makes our decisions more important, and more powerful.” 
Unfortunately, the Greenland Ice Sheet is much more vulnerable to melting than previously thought, found a May 2014 study by Morlighem et al., "Deeply incised submarine glacial valleys beneath the Greenland ice sheet." The researchers found that widespread ice-covered valleys extend much deeper below sea level and farther inland than previously thought, and would likely melt significantly from steadily warming waters lapping at Greenland’s shores.

Figure 2. Monthly changes in the total mass (in gigatons) of the Greenland ice sheet estimated from GRACE satellite measurements between March 2002–July 2013. The blue and orange asterisks denote April and July values, respectively. Note that the decline in ice mass lost from Greenland is not a straight line – it is exponential, meaning that, in general, more ice loss is lost each year than in the previous year. However, the mass loss during the 2013 summer melt season was probably smaller than during 2012, said the 2013 Arctic Report Card.

Fig. 1 shows that Greenland has been losing mass at a faster and faster rate over the past decade, with the recent rate corresponding to ~1 mm sea level per year (1 mm sea level = 360 Gt ice). The linear fit to the Shepherd et al. data in Fig. 1 yields a Greenland contribution to global sea level of about 30 cm by 2100.
Fig. 1. Annual mass change of Greenland ice sheet based on the input-output method, an analysis of gravity measurements, and a best-estimate composite (Shepherd et al., 2012).
Fig. 1. Annual mass change of Greenland ice sheet based on the inputoutput method, an analysis of gravity measurements, and a best-estimate composite (Shepherd et al., 2012).
The increasing Greenland mass loss in Fig. 1 can be fit just as well by exponentially increasing annual mass loss, a behavior that Hansen (2005, 2007) argues could occur because of multiple amplifying feedbacks as an ice sheet begins to disintegrate. A 10-year doubling time would lead to 1 meter sea level rise by 2067 and 5 meters by 2090. The dates are 2045 and 2057 for 5-year doubling time and 2055 and 2071 for a 7-year doubling time. 

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