Sub-bullet b: Antarctica

Sub-bullet b: "A clear warming trend is evidenced by the Antartctic ice sheets"
Hierarchy: Part II:1:A:iv:b

Back to Intro - Back to Outline - Up to Bullet iv - Back to Sub-bullet a - Forward to Sub-bullet c

Introduction:
Antarctica, like Greenland, has a complex climate and thus the logical structure of Sub-bullet a will be followed. Simply, this means that we will use current understanding of how warming could effect the ice in Antarctica and data showing the trends of the distribution of that ice to conclude whether warming is occurring. The issue of causality will be left at this point unless evidence can be found which attributes the observed behavior to effects other than warming.

The bulk of the ice on Antarctica consists of two main sheets: the East Antarctic Ice Sheet [EAIS] and the West Antarctic Ice Sheet [WAIS]. The EAIS is much higher in elevation and therefore colder and less susceptible to warming than the lower WAIS. Below is a map compiled by NASA's ICESat showing the altitude of the upper surface of the Antarctic ice sheet.
This uneven distribution of ice mass and altitude will be critical for understanding the behavior of the Antarctic ice sheet.


In most respects Antarctica is as different from Greenland as two polar ice sheets can possibly be. In contrast with Greenland, Antarctica is naturally very cold; a large majority of the surface area of the continent averages below freezing year-round. Since Anatarctica is less marginal for ice than Greenland, a small rise in temperature isn't likely to be the difference between ice and liquid water except at the extreme edges. This means that melting is relatively less important in Antarctica as a factor for mass loss. The interior is large and very cold and in this region scientists expect that a warming climate would increase the ice mass since precipitation would increase--perhaps even enough to make the total mass balance increase due to warming. The Antarctic peninsula sticks much farther north and thus is more susceptible to warming trends than the coasts. Antarctica also does not rely on an exterior ocean current to cool it, and contains over 10 times the ice that Greenland does. All of these factors make Antarctica theoretically much less sensitive to small climate effects as Greenland is.


Despite all of these differences, we are looking for the same warming signature for Antarctica as we were for Greenland: positive mass balance in the interior, potentially negative and accelerating mass balance at the edges [since thinning is not just related to melting but to ice dynamics like glacier flow], and glacial dynamics features like accelerating flow. A refinement of the mass balance model predicts that the WAIS will see mass loss and the EAIS will see mass gain; this is the signature we are looking for.
[Refer to bullet iv for further introductory material]

Study 1 : GRACE weighs in on mass balance
Measurements of Time-Variable Gravity Show Mass loss in Antarctica
Isabella Velicogna and John Wahr, Science 311, 1754 (2006)



Antarctic mass rates from GRACE
J. L. Chen et al., GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L11502

Interannual variations of the mass balance of the Antarctica and Greenland ice sheets from GRACE
G. Ramillien et al., Global and Planetary Change 53:3, 198-208 (2006)

Simple: The researchers who conducted two of the three of our GRACE studies for the Greenland mass balance trend also conducted similar studies for Antarctica, and another group worked on the data as well. Their conclusions are the same: interior mass is increasing, peripheral mass is decreasing, with a net decrease in mass that is quickly accelerating. See Sub-bullet a: Greenland for more GRACE discussion.

Details: All three studies found large negative mass balances in the WAIS and mass gain or balance in the EAIS. A figure from Velicogna and Wahr illustrates this [the green line is the EAIS trend and the red line is the WAIS trend]:


Mass balance findings for these three studies are summarized in the table below.

All three studies also corrected the data for post-glacial rebound--the upward swelling of the crust resulting from the loss of glacial period ice--which yields more accurate but less precise data, since the post-glacial rebound is poorly known. Chen et al. describes the limit of knowledge [units, parenthetical statement and emphasis mine]:


"The calculations here show that the estimates of Antarctic snow/ice mass rates from GRACE data are completely dependent on the adopted PGR [post-glacial rebound] model, with uncertainties that might be on the order of 100% Our estimate of -99 or -77 cubic kilometers per year mass loss in West Antarctica is consistent with that of Velicogna and Wahr of -148 cubic kilometers per year, given the large PGR uncertainty and that here we only compute the mass loss of the Amundsen Sea Embayment in West Antarctica."

The take-home message is that we shouldn't have much faith in the exact numerical values of the mass balance, but that the trend of mass loss in the WAIS and mass gain in the EAIS is clear.


Discussion: The two studies with the most comprehensive geographical scope [Velicogna and Wahr and Remillien et al. ] agree within quoted error limits. The total mass balance isn't the first clue to a warming climate that Antarctica offers anyway [although it's the driving global factor in sea-level change, which has the potential for enormous effects]. What is important is also what is clear from the data: Antarctica is undeniably showing the signs of warming--decreasing ice mass in the WAIS and increasing or stable ice mass in the EAIS.

Conclusion: The GRACE data constitute strong evidence of a south polar warming trend.

Study 2: Sattelite radar altimetry measures mass balance
Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992–2002
H. Jay Zwally et al., Journal of Glaciology, Vol. 51, No. 175, 2005

Not-yet-published sattelite radar altimetry Antarctic mass balance study
Eric Rignot

Simple: The European remote sensing sattelites ERS-1 and -2 used radar altimetry to measure the altitude of ice in the Antarctic ice sheets. The first study is comprised of data from 1992-2002 which shows a mass loss in the WAIS and a mass gain in the EAIS which contributed to an overall negative mass balance. Additionally, the as-yet-unpublished second study--another by Eric Rignot--apparently fuses newer ERS-1 and -2 data with similar data from Japanese and Canadian sattelites and finds that ice loss in the WAIS increased extremely sharply during the decade 1996-2006.

Details: Covering a time period prior to most of the studies we've seen so far, the first ERS data is crucial in establishing a longer-term trend. This analysis is the most sophisticated to come out of the ERS program; the authors attempt to correct for some small but important factors which most other studies ignore, having to do with the dynamics of ice compaction. The dH/dt [rate of change of ice sheet altitude] map below represents the corrected data from Zwally et al.:


Rignot's technique is different than Zwally's; Rignot measures the amount of ice leaving the continent indirectly, by monitoring the flow of ice off the continent in glaciers. The benefit of this method is that it allows the researchers to ignore the problems that Zwally had to design sophisticated corrections for, such as the sensitivity of compactibility of new ice to temperature. Additionally, Rignot combined many different large datasets and has data up to 2006, greatly enhancing the completeness and relevancy of the data.

Quote from article on Rignot's upcoming paper: "The team found that the net loss of ice mass from Antarctica increased from 112 (plus or minus 91) gigatonnes a year in 1996 to 196 (plus or minus 92) gigatonnes a year in 2006."

Zwally et al. Quote: "The ice sheet inWest Antarctica (WA) is losing mass (–47 +/- 4Gt per year) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 +/- 11 Gt per year) for a combined net change of –31
+/- 12 Gt per year (+0.08mma–1 SLE). "

Discussion: The data from this study solidly confirms the existence of the bimodial east/west mass gain/loss signature in Antarctica; the signature of warming. Furthermore, Rignot's study--which used data from the same sources and identical analysis--found solid evidence that the ice mass loss in Antarctica is accelerating at a phenomenal pace; a solid sign of warming. Two more sattelite radar altimetry studies, Wingham et al. and Davis and Li found a slight mass increase. However, both of these studies found it difficult to analyze the peripheral regions and thus left them out of the analysis altogether. Since a majority of the loss we see occurs at the periphery, these studies do nothing but confirm the other studies' findings of a mass increase in the interior.
[Zwally et al. study also analyzed the Greenland ice sheet and found a small positive mass balance, which is not inconsistent with the studies reported in the Greenland section for this time period]

Conclusion: The ERS data constitutes evidence of a south polar warming trend.

Study 3: Laser altimetry analyzes glacial dynamics
Accelerated sea-level rise from West Antarctica
Thomas, Rignot, et al., Science 8 October 2004:Vol. 306. no. 5694, pp. 255 - 258

Simple: Using laser altimetry data from sattelites and aircraft, Thomas et al. have measured the discharge from the glaciers in a section of the WAIS. They discovered a steeply accelerating flow from the 1990's until 2003; a glacier dynamics marker indicative of warming.

Details: Thomas et al. found that 60% more ice was flowing out of the catchment basin feeding the Amundsen sea in west Antarctica than was accumulating due to precipitation. They collected precise ice thickness data from aircraft flown from Chile and compared it with various sattelite data sets to create a map of ice thinning rates for the catchment area. A quote: "The catchment regions of Amundsen Sea glaciers contain enough ice to raise sea level by 1.3 m. Our measurements show them collectively to be 60% out of balance, sufficient to raise sea level by 0.24 mm/year. Although these glaciers are the fastest in Antarctica, they are likely to flow considerably faster once the ice shelves are removed and glacier retreat proceeds into the deeper part of glacier basins."

Discussion: Not only does this large imbalance of outflow/inflow point to conditions now being different than they were previously, but the study reports an acceleration of the imbalance, which signifies compounding factors in recent years. Even more impressive is the potential for yet higher rates of mass loss when/if the ice shelves melt, effectively pulling the rug out from under the glaciers.

Conclusion: This glacial dynamics study constitutes strong evidence of south polar warming.

Other studies
Recent sea-level contributions of the Antarctic and Greenland ice sheets
Andrew Shepherd and Duncan Wingham, Science 315, 1529 (2007)

Simple: This is a review of mass-balance data. It does a much better job than I can, and it reviews most of the same data I have.

Mass balance of polar ice sheets
Eric Rignot and Robert H. Thomas

Simple: Another review but older. Same conclusions.

[Further studies requested]

Discussion
The evidence is accumulating and the doubt is thinning; the Antarctic ice sheets are shrinking at fantastic rates, and the rate of increase is itself increasing at a fantastic rate. Two years ago there might have been argument, but at this point it is obvious that something big is cooking at the south pole. The issue of causality is less complicated than Greenland, but still the strongest statement we can make is that warming is the only candidate known for the cause of the changes observed. Given the scale of the changes--and the recently discovered fact that ice outflow increases dramatically after ice shelves melt--I feel comfortable agreeing with the large majority of these researchers in this point: the loss of ice in Antarctica will likely reach astonishing proportions in the near future if current conditions persist. Two ice shelves collapsed in the last few years, triggering massive acceleration of glacial flow, and similar events should be expected in the near future.


A summary of the mass balance studies cited in this post:


Conclusion
This data constitutes strong evidence of north polar warming.

Sub-bullet value: TRUE