First, a question in the comments
a little more than a little while ago regarding an alleged contradiction between recent studies on increasing Antarctic ice sheet loss and a paper (Tedesco 2009) on recent reduction in Antarctic snowmelt (i.e. presumably, “if there’s less melting, surely there can’t be more ice loss”). This is a non sequitur- snowmelt extent and ice sheet mass balance being two distinct phenomena- although it’s easy to see why it sounds plausible at first blush. Of course it doesn’t help that the denialosphere confused the issue by falsely claiming a reduction in “ice melt” rather than snowmelt index. (Whether they do this stuff out of a deliberate desire to mislead people or sheer incompetence, the end result is the same and both are indefensible.)
As I wrote then, the snowmelt index is defined as the number of days multiplied by the extent of surface melt (duration times area) rather than an actual “amount” (either volume or mass) of melting ice (Zwally 1994). In other words, there is no inherent contradiction between greater ice sheet decay and reduced surface snowmelt. Furthermore, rather than exculpate anthropogenic influence on Antarctica as the denialists suggest, the decreased snowmelt might actually be being driven at least in part by human activities. Tedesco and Monaghan finger couplings between positive summertime phases of the SAM with positive ENSOs. And of course anthropogenic ozone depletion and greenhouse gas emissions are suspected to at least partially contribute to the increasingly positive SAM (Arblaster 2006). And of course, we know that that the Antarctic ice sheet loss is accelerating, because we can measure it happening.
But wait! Didn’t a study (Bevis 2009) just show that ice sheet decay as measured by GRACE was exaggerated, contradicting GRACE findings of accelerating ice sheet decay? “Yes!” cried the denialosphere. “Not so fast,” warn the literate.
The Bevis, et al. study was concerned with long term bias in GRACE measurements (due to underestimates of post-glacial rebound), which aren’t that significant on the timescales discussed in papers like Velicogna 2009. Bevis et al. note:
any sudden increase in the rate of ice loss will be resolved unambiguously by GRACE since the mass rates associated with PGR [post-glacial rebound] do not change significantly over several years.
In other words, even if the GRACE data were systematically biased, such a bias would be relatively constant on short timescales and not meaningfully contribute to/contaminate measurements of large changes happening over interannual periods, especially changes in the rate of loss. So while the total amount of ice lost may be revised downward, the acceleration in recent/current decay is very much real.
And it’s not as though the GRACE data are the only method of establishing accelerating ice loss. In a recent paper in Science, van den Broeke 2009 calculate GrIS loss using a “mass budget method, which quantifies the individual components of ice sheet mass balance [surface mass balance (SMB) and ice discharge (D)":
For SMB, we used the monthly output of a 51-year climate simulation (1958–2008) with the Regional Atmospheric Climate Model (RACMO2/GR) at high horizontal resolution (~11 km)... The modeled SMB from RACMO2/GR agrees very well with in situ observations [N = 265, correlation coefficient (r) = 0.95], without need for post-calibration . For D, we used ice flux data from 38 glacier drainage basins, covering 90% of the ice sheet… corrected for SMB between flux gate and grounding line and updated to include 2008
Van den Broeke, et al. compare their modeled/in situ data with GRACE, and found good agreement between the two:
The temporal evolution of the cumulative SMB-D anomaly was evaluated using monthly GRACE mass changes. The spatial distribution of GrIS mass changes was compared to a regionally distributed GRACE solution, updated to include 2008… The high correlation (r = 0.99) between the two fully independent time series and the similarity in trends support the consistency of the mass balance reconstruction. A linear regression on the SMB-D time series yields a 2003– 2008 GrIS mass loss rate of –237 ± 20 Gt year^−1.
Recently, Rignot 2011 combined van den Broeke’s SMB-D method (which they call “mass budget method” or MBM) with GRACE data to reconstruct changes in GrIS and Antarctic mass balance over two decades, from 1992-2009.
Initial concerns over accelerating Greenland ice loss arose when dramatic loss was recorded in the early to mid-2000s. However, this regional melting appeared to taper off around 2005, which lead to predictable crowing from climate contrarians who sought to portray this change as a reason to stop worrying about Greenland ice sheet decay.
Pritchard 2009 noted, however, that the extent of dynamic melt was more widespread than has been previously assessed- especially in the northwest- and has penetrated in some areas more than 100km inland and to altitudes as high as 2000m. 81 out of 111 Greenland glaciers surveyed showed melt rates more than twice as fast as nearby flowing ice.
Additional research- using data from GPS in addition to GRACE- further supported this overall behavior, noting melt had again accelerated in some areas around 2005, especially in the northwest (Kahn 2010).
Rignot, et al. have confirmed that the long term trend for the GrIS is not just one of melt, but one of accelerating mass balance loss:
Rignot, et al.:
The mass losses estimated from MBM and GRACE are within ± 20 Gt/yr, or within their respective errors of ± 51 Gt/yr and ± 33 Gt/yr. The acceleration in mass loss is 19.3 ± 4 Gt/yr^2 for MBM [ed: 21.9 ± 1 Gt/yr^2 over 1992-2009] and 17.0 ± 8 Gt/yr^2 for GRACE. The GRACE-derived acceleration is independent of the GIA reconstruction, a constant signal during the observational period.
That last part is consistent with the earlier discussion of Bevis 2009- an adjustment to total mass balance loss numbers in GRACE due to glacial isostatic adjustment does not contradict the reality that Greenland is melting, and it’s melting at an accelerating pace.
Once upon a time, discussion of significant melting of Antarctic ice was restricted to the West Antarctic Ice Sheet (WAIS). The East Antarctic Ice Sheet (EAIS) was believed to be much more stable, and in much less danger of melting. A sense of complacency regarding the state of EAIS melt might be furthered in the public’s perception due to conflicting estimates of surface warming in that region. Ice sheet decay in Antarctica is not driven primarily by surface warming, however, and the Southern Ocean is warming significantly. And indeed, recent analysis of GRACE data has shown that the EAIS- long thought to be the more “safe” (e.g. less affected by warming) of the two Antarctic ice sheets- has been shown to be melting as well. (Chen 2009).
Additionally, concerns over Antarctic contribution to sea level rise have increasingly focused on the Pine Island Glacier- e.g., according to Wiki, “The Pine Island and Thwaites Glaciers are two of Antarctica’s five largest ice streams. Scientists have found that the flow of these ice streams has accelerated in recent years, and suggested that if they were to melt, global sea levels would rise by 0.9–1.9 m (1–2 yards), destabilizing the entire West Antarctic Ice Sheet and perhaps sections of the East Antarctic Ice Sheet.”
Recent modeling results have suggested that Pine Island Glacier might already have crossed a threshold of stability (Katz 2010):
[O]ur results suggest that, in contrast to earlier assessments, the scenario of unstable grounding-line recession on retrograde beds in West Antarctica is likely. Indeed, in the case of the Pine Island glacier, it may be presently occurring.
Some had hoped that an increase in precipitation over Antarctica could result in an off-setting increase in surface mass balance. Unfortunately, Rignot, et al. note:
In Antarctica, Pine Island Glacier accelerated exponentially over the last 30 years: 0.8% in the 1980s, 2.4% in the 1990s, 6% in 2006 and 16% in 2007-2008 (Rignot, 2008), and quadrupled its thinning rate in 1992-2008 (Wingham et al., 2009). Simple model projections predict a tripling in glacier speed once the grounding line retreats to a deeper and smoother bed (Thomas et al., 2003). Dynamic losses are therefore likely to persist and spread farther inland in this critical sector. A small positive increase in Antarctic SMB could offset these coastal losses, but this effect has not yet been observed.
For the Antarctic continent as a whole, Rignot, et al. find:
an acceleration in mass loss from the GRACE data of 13.2 ± 10 Gt/yr^2… [and for] the same time period, the acceleration in mass loss from the MBM data is 15.1 ± 12 Gt/yr^2 [ed: and 14.5 ± 2 Gt/yr^2 over 1992-2009].
The take home message from Rignot, et al. is stark.
When we use the extended time period 1992-2009, the significance of the trend improves considerably. The MBM record indicates an acceleration in mass loss of 21.9 ± 1 Gt/yr^2 for Greenland and 14.5 ± 2 Gt/yr^2 for Antarctica… When the mass changes from both ice sheets are combined together…, the data reveal an increase in ice sheet mass loss of 36.3 ± 2 Gt/yr^2.
Greenland and Antarctica are melting. Moreover, they’re melting at an accelerating rate. This is not an artifact of instrumental bias in GRACE, or due to an insufficiently short time period.
Rignot, et al. conclude:
This study reconciles two totally independent methods for estimating ice sheet mass balance, in Greenland and Antarctica, for the first time: the MBM method comparing influx and outflux of ice, and the GRACE method based on time-variable gravity data. The two records agree in terms of mass, M(t), mass change, dM(t)/dt, and acceleration in mass change, d2M/dt2. The results illustrate the major impact of monthly-to-annual variations in SMB on ice sheet mass balance. Using the two-decade long MBM observation record, we determine that ice sheet loss is accelerating by 36.3 2 Gt/yr2, or 3 times larger than from mountain glaciers and ice caps (GIC). The magnitude of the acceleration suggests that ice sheets will be the dominant contributors to sea level rise in forthcoming decades, and will likely exceed the IPCC projections for the contribution of ice sheets to sea level rise in the 21st century.
This last finding will come as no surprise to those who know that the IPCC AR4 sea level rise estimates didn’t account for nonlinear ice sheet decay, but rather assumed a contribution from Greenland and Antarctica at the observed linear rate from 1993-2003. Using semi-empirical methods (that still don’t explicitly account for catastrophic ice sheet collapse) provides an additional line of evidence for accelerating ice sheet contribution to SLR (e.g. Vermeer 2009).
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