Antarctic ice - growing or shrinking? NASA vs Princeton and Leeds etc Sou | 11:26 PM
There's a new paper out about Antarctic ice, from H. Jay Zwally and colleagues at NASA. They report that over the period from 2003 to 2008, there was a net increase in ice over all Antarctica of 82±25 Gt/year. This paper looks to be based on a conference paper at a SCAR workshop back in July 2012 (though that doesn't explain why there wasn't data from the past six years in the final published paper). (edit. ICESat's primary instrument failed in February 2010, with data collected over periods from Feb 2003 to October 2009. Appropriately enough the remnants of the decommissioned satellite crashed to Earth on August 30, 2010 over the Barents Sea.)
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These findings are different to the results of work reported earlier this year from two scientists at Princeton, Christopher Harig and Frederik J. Simons. The Princeton team found that over the period January 2003 to 2014, there was a loss of ice overall. the overall mass loss from Antarctica since January 2003 at 92 ±10 Gt/yr.
It's also different from the results reported in a paper by Malcolm McMillan and colleagues last year. They estimated the current mass loss over all Antarctica at 159 ± 48 Gt/year. ".
So one group of scientists find that ice has been on balance increasing, while others find that ice has been on balance decreasing.
Note that the periods differ. The new paper stops in 2008. Harig15 is for 2003-2014 inclusive, and McMillan14 doesn't to an average number, but reports "the contemperaneous loss" - based on from 2010 to 2013.
In 2012, a paper was published in Science mag by a whole bunch of people (h/t ATTP). That paper combined data sets of different types: satellite altimetry, interferometry, and gravimetry. They found there was good agreement between the different satellite methods, and concluded:
Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year-1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year-1 to the rate of global sea-level rise.
I'm obviously not in a position to "pick sides" and say who's right and who's not. The scientists involved are all experts, and experts don't always agree. What is agreed is that Western Antarctica is losing ice. And it's losing ice very quickly.
There's a simplified diagram in Zwally15, which shows the different processes that affect mass balance in Antarctica:
 | | Fig. 1 | The principal processes affecting the mass balance and dynamics of the ice sheets are ice mass input from snowfall with losses from sublimation and drifting. Surface melting on the grounded ice of Antarctica is very small, and subject to refreezing in the firn. Interaction with the ocean occurs at the undersides of the floating ice shelves and glacier tongues, and consequent changes in thickness affect the rate of ice flow from the grounded ice. Source: Zwally15 |
Antarctica is huge at around 14 million sq kilometres it's almost twice as big as Australia; and almost twice as big as the contiguous USA (not including Alaska). Most of it has never been visited by people, so satellites are relied on a lot to figure out what is happening there.
In Zwally15, the authors divided up Antarctica into 27 different regions. They separated dynamic-driven mass changes and accumulation-driven mass changes. The accumulation-driven changes are from snowfall. Unlike Greenland, there's not much surface melting in Antarctica. It's too cold. There are losses from warm water getting under the ice and melting it - particularly in Western Antarctica, the Peninsula and some parts east. These processes are shown in the diagram above.
In Zwally15 the authors discussed changes for the periods 1992 to 2001, based on ERS radar altimetry, and from 2003 to 2008 using ICESat data. They explained how they calibrated the two so they could be compared.
As Jay Zwally explained in the press release, scientists are pretty much in agreement about the increasing ice losses in parts of Western Antarctica and on the Peninsula:
"We're essentially in agreement with other studies that show an increase in ice discharge in the Antarctic Peninsula and the Thwaites and Pine Island region of West Antarctica," said Jay Zwally, a glaciologist with NASA Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study, which was published on Oct. 30 in the Journal of Glaciology. "Our main disagreement is for East Antarctica and the interior of West Antarctica -- there, we see an ice gain that exceeds the losses in the other areas." Zwally added that his team "measured small height changes over large areas, as well as the large changes observed over smaller areas."
And they may not be in a heap of disagreement up to 2008 either. For example, here is a chart from Harig and Simons. I've animated it, blocking out the period after 2008, and then showing the entire period:
 | | Fig.2. Ice mass changes (mass corrected using the GIA model by Ivins et al., 2013) in gigatons (Gt). The black lines are monthly GRACE observations with 2s error bars determined from our analysis. The solid blue lines are the best-fit quadratic curves. Source: Harig and Simons 2015 |
So you can see that most of the loss since 2000 occurred after 2008. It would have been more obvious without the blue line - just focus on the monthly data. The figure in the paper has more panels, and shows that West Antarctica is losing a lot of ice, while Droning Maud Land region was reported to be gaining ice.
I expect there will be a lot more research to come, because Antarctica is the biggest concern in regard to rising seas, together with Greenland.
Where is the water coming from?
Sea levels are rising from both the expansion of water as it warms, and from melting ice. So if Antarctica is reducing sea level by 0.23 mm a year, and sea level is rising by at least 3.3 mm a year, then where is the extra water coming from?
Other papers have reported variously that Antarctica is adding to sea level rise, for example:
I was going to write a lot more, and there's probably much more that could be said. One is that the melting of Antarctica and Greenland will cause big problems sooner or later this century.
This article might not be all that satisfactory, in that it leaves unresolved questions. Is Antarctica really gaining mass? If it was gaining mass from 2003 to 2008 is it still gaining mass or is it now losing mass? However, it does cover more ground than is covered at WUWT (archived here), and shows where there are differences and where the findings are not so different.
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