I came across a very interesting article by following a google image back to the source.
Antarctic Ice Mass Controversies | Al-Ayham Saleh Aggregator
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Like many others, I was interested in the recent controversy arising from findings of Zwally et al 2015 that there had been ice mass gain gain of ~112±61 Gt/year over 1992-2001 and ~82±25 Gt/year over 2003-2008. Zwally’s findings obviously contradict a widely held contrary belief, expressed, for example, in IPCC AR5’s assertion there was “high confidence” that the Antarctic Ice Sheet had been losing mass for the prior two decades and that the rate of loss had “likely increased” to ~147±75 GT/year over 2002-2011 or in NASA’s widely cited statement that “the continent of Antarctica has been losing about 134 billion metric tons of ice per year since 2002”.
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IPCC AR4
AR5 took a far more aggressive line on Antarctic ice mass loss than AR4, which, in retrospect, was rather cautious. AR4 reported that there had been attempts to measure Antarctic ice mass balance using a variety of techniques: radar and laser altimetry, the GRACE gravity surveys and input-output balances (using climate models to estimate accumulations and satellite data to measure glacier flow over their grounding lines. In its summary, it was unable to reach a conclusion as to whether the mass balance was positive or negative:
Assessment of the data and techniques suggests overall Antarctic Ice Sheet mass balance ranging from growth of 50 Gt yr-1 to shrinkage of 200 Gt yr-1 from 1993 to 2003.
AR4 was considerably more candid than AR5 about the continuing impact of the end of the last Ice Age. In a section entitled “4.6.3.2 Ongoing Dynamic Ice Sheet Response to Past Forcing”, AR4 contained an interesting comment that “retreat of the West Antarctic grounding line in response to the end of the last ice age” was estimated to contribute “about 90 Gt yr-1” to ice mass loss:
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AR4 to 2011
Subsequent to AR4, interest in Antarctica mass loss increased tremendously, not least because of the availability of new satellite data measuring gravity, altimetry and grounding lines. The GRACE gravity surveys had become available in mid-2002 and have continued to the present. The high resolution IceSat laser altimetry satellite became available in 2003 and operated until 2009, but, prior to Zwally et al 2015, does not appear to have resulted in a technically published estimate of Antarctic mass loss (though estimates were noted in the Supplementary Information of Shepherd et al 2012, as discussed below).
In contrast, between 2006 and 2011, there were numerous articles estimating Antarctic mass loss using GRACE data. Articles in this period used “early” models for glacial isostatic adjustments (Peltier 2004; Ivins and James 2005), estimates which were dramatically reduced in 2011-12. Although not evident in most GRACE articles, the size of the glacial isostatic adjustment was the same order of magnitude as the ice mass loss itself (and even larger). Velicogna and Wahr 2006 (somewhat anomalously) reported both items in their calculation, showing that the “uncorrected” GRACE trend was actually positive and that the estimated mass loss of 152 km3/year (~139 Gt/year) arose from the glacial isostatic adjustment of 192 km3/year (~176 Gt/year):
We subtracted this PGR [post-glacial rebound: the viscoelastic response of the solid Earth to glacial unloading over the past several thousand years] contribution from the GRACE-minus-leakage ice mass estimates … The PGR contribution (192 ± 79 km3/year) is much larger than the uncorrected GRACE trend (39 ± 14 km3/year)… The best-fitting linear trend, and our final estimate of the decrease in total Antarctic mass between the summers of 2002 and 2005, is 152 ± 80 km3/year
"
"
Although the IceSat satellite had operated from 2003-2009, at the time of the IMBIE intercomparison in 2012, there (remarkably) does not appear to have been any formal publication of IceSat laser altimetry results. Zwally et al 2015 appears to be long overdue in this respect. For the IMBIE intercomparison, four centers (Louise Sørensen and René Forsberg (SF) at the Technical University of Denmark, Hamish Pritchard (HP) at the British Antarctic Survey, Donghui Yi and Jay Zwally (YZ) at NASA Goddard Space Flight Center, and Benjamin Smith (BS) at the University of Washington) were commissioned to estimate mass change from the IceSat data. Their estimates of volume change were reported in Shepherd et al Table S7 and mass change in Table S8, both in the SI. All four centers estimated volume gain in East Antarctica exceeding the volume loss in West Antarctica plus Antarctica Peninsula. The Zwally (YZ) estimates of volume increase was greater than, but similar to, volume changes from other groups, with Sorensen and Forsberg (SF) estimates being particularly similar. Both Zwally (YZ) and Sorensen-Forsberg got similar mass gains for the AIS as a whole, with subsector results by the other two groups being similar as shown in their Table S8, excerpted below.
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GIA specialists (Ivins, Peltier, Whitehouse and others) have themselves taken the position that their earlier GIA estimates were incorrect in light of improved knowledge and additional data. Zwally et al 2015 used up-to-date GIA estimates from Ivins et al 2013 – estimates that have also been widely used in more recent GRACE articles (e.g. Velicogna et al 2014).
Further, to the extent that Bamber is correct that there is “poor agreement among different estimates of GIA” over East Antarctica, this is a much larger problem for the GRACE gravity estimates, which are approximately six times more sensitive (as Gt/year relative to mm/year) than altimetry estimates. If this is an issue for Bamber, he ought not to have been silent on the GRACE studies.
"
"
Conclusions
While it is obviously up to specialists to try to ultimately figure out whether Antarctic ice mass was increasing in the periods 1992-2001 and 2003-2008 (per Zwally) or whether it was decreasing (as IPCC and others had previously asserted), there does not appear to be any objective basis by which, for example, Gavin Schmidt could reasonably “pin more weight” to highly negative estimates from GRACE gravity data than to Zwally’s positive estimates from laser altimetry.
The size of the GIA adjustment for GRACE gravity estimates is the same order of magnitude as the estimate of ice mass loss and, in many cases, is larger. These GIA adjustments have been dramatically reduced by specialists over the past decade and have concurrently reduced estimates of ice mass loss.
"
of course I love this kind of stuff. lots of detail, links to past papers, history of how things are evolving. especially when it supports and quantifies what I have been saying for years .the GRACE estimates would have to be severely clawed back as more data comes in.
Antarctic Ice Mass Controversies | Al-Ayham Saleh Aggregator
"
Like many others, I was interested in the recent controversy arising from findings of Zwally et al 2015 that there had been ice mass gain gain of ~112±61 Gt/year over 1992-2001 and ~82±25 Gt/year over 2003-2008. Zwally’s findings obviously contradict a widely held contrary belief, expressed, for example, in IPCC AR5’s assertion there was “high confidence” that the Antarctic Ice Sheet had been losing mass for the prior two decades and that the rate of loss had “likely increased” to ~147±75 GT/year over 2002-2011 or in NASA’s widely cited statement that “the continent of Antarctica has been losing about 134 billion metric tons of ice per year since 2002”.
"
"
IPCC AR4
AR5 took a far more aggressive line on Antarctic ice mass loss than AR4, which, in retrospect, was rather cautious. AR4 reported that there had been attempts to measure Antarctic ice mass balance using a variety of techniques: radar and laser altimetry, the GRACE gravity surveys and input-output balances (using climate models to estimate accumulations and satellite data to measure glacier flow over their grounding lines. In its summary, it was unable to reach a conclusion as to whether the mass balance was positive or negative:
Assessment of the data and techniques suggests overall Antarctic Ice Sheet mass balance ranging from growth of 50 Gt yr-1 to shrinkage of 200 Gt yr-1 from 1993 to 2003.
AR4 was considerably more candid than AR5 about the continuing impact of the end of the last Ice Age. In a section entitled “4.6.3.2 Ongoing Dynamic Ice Sheet Response to Past Forcing”, AR4 contained an interesting comment that “retreat of the West Antarctic grounding line in response to the end of the last ice age” was estimated to contribute “about 90 Gt yr-1” to ice mass loss:
"
"
AR4 to 2011
Subsequent to AR4, interest in Antarctica mass loss increased tremendously, not least because of the availability of new satellite data measuring gravity, altimetry and grounding lines. The GRACE gravity surveys had become available in mid-2002 and have continued to the present. The high resolution IceSat laser altimetry satellite became available in 2003 and operated until 2009, but, prior to Zwally et al 2015, does not appear to have resulted in a technically published estimate of Antarctic mass loss (though estimates were noted in the Supplementary Information of Shepherd et al 2012, as discussed below).
In contrast, between 2006 and 2011, there were numerous articles estimating Antarctic mass loss using GRACE data. Articles in this period used “early” models for glacial isostatic adjustments (Peltier 2004; Ivins and James 2005), estimates which were dramatically reduced in 2011-12. Although not evident in most GRACE articles, the size of the glacial isostatic adjustment was the same order of magnitude as the ice mass loss itself (and even larger). Velicogna and Wahr 2006 (somewhat anomalously) reported both items in their calculation, showing that the “uncorrected” GRACE trend was actually positive and that the estimated mass loss of 152 km3/year (~139 Gt/year) arose from the glacial isostatic adjustment of 192 km3/year (~176 Gt/year):
We subtracted this PGR [post-glacial rebound: the viscoelastic response of the solid Earth to glacial unloading over the past several thousand years] contribution from the GRACE-minus-leakage ice mass estimates … The PGR contribution (192 ± 79 km3/year) is much larger than the uncorrected GRACE trend (39 ± 14 km3/year)… The best-fitting linear trend, and our final estimate of the decrease in total Antarctic mass between the summers of 2002 and 2005, is 152 ± 80 km3/year
"
"
Although the IceSat satellite had operated from 2003-2009, at the time of the IMBIE intercomparison in 2012, there (remarkably) does not appear to have been any formal publication of IceSat laser altimetry results. Zwally et al 2015 appears to be long overdue in this respect. For the IMBIE intercomparison, four centers (Louise Sørensen and René Forsberg (SF) at the Technical University of Denmark, Hamish Pritchard (HP) at the British Antarctic Survey, Donghui Yi and Jay Zwally (YZ) at NASA Goddard Space Flight Center, and Benjamin Smith (BS) at the University of Washington) were commissioned to estimate mass change from the IceSat data. Their estimates of volume change were reported in Shepherd et al Table S7 and mass change in Table S8, both in the SI. All four centers estimated volume gain in East Antarctica exceeding the volume loss in West Antarctica plus Antarctica Peninsula. The Zwally (YZ) estimates of volume increase was greater than, but similar to, volume changes from other groups, with Sorensen and Forsberg (SF) estimates being particularly similar. Both Zwally (YZ) and Sorensen-Forsberg got similar mass gains for the AIS as a whole, with subsector results by the other two groups being similar as shown in their Table S8, excerpted below.
"
"
GIA specialists (Ivins, Peltier, Whitehouse and others) have themselves taken the position that their earlier GIA estimates were incorrect in light of improved knowledge and additional data. Zwally et al 2015 used up-to-date GIA estimates from Ivins et al 2013 – estimates that have also been widely used in more recent GRACE articles (e.g. Velicogna et al 2014).
Further, to the extent that Bamber is correct that there is “poor agreement among different estimates of GIA” over East Antarctica, this is a much larger problem for the GRACE gravity estimates, which are approximately six times more sensitive (as Gt/year relative to mm/year) than altimetry estimates. If this is an issue for Bamber, he ought not to have been silent on the GRACE studies.
"
"
Conclusions
While it is obviously up to specialists to try to ultimately figure out whether Antarctic ice mass was increasing in the periods 1992-2001 and 2003-2008 (per Zwally) or whether it was decreasing (as IPCC and others had previously asserted), there does not appear to be any objective basis by which, for example, Gavin Schmidt could reasonably “pin more weight” to highly negative estimates from GRACE gravity data than to Zwally’s positive estimates from laser altimetry.
The size of the GIA adjustment for GRACE gravity estimates is the same order of magnitude as the estimate of ice mass loss and, in many cases, is larger. These GIA adjustments have been dramatically reduced by specialists over the past decade and have concurrently reduced estimates of ice mass loss.
"
of course I love this kind of stuff. lots of detail, links to past papers, history of how things are evolving. especially when it supports and quantifies what I have been saying for years .the GRACE estimates would have to be severely clawed back as more data comes in.
