thottleup,
I'll leave it to graywolf to respond personally, but just a short comment; yes, glaciers 'calve'. Regarding Antarctica, the 'mass balance of the ice sheet' is the topic of scientific interest.
Glib is always fun, of course. But when mass loss from 'calving' exceeds mass gain from new ice formation, an ice sheet reduces in mass. Indeed! And science disagrees whether the mass balance is increasing or decreasing, leaving us to wonder what the pictures of glaciers calving was intended to convey.
Indeed glaciers calving is a far more spectacular thing to watch than glaciers gaining ice. In fact usually the view is obscured by snow when they are gaining ice.
icefisher,
You wrote, "And science disagrees whether the mass balance is increasing or decreasing".
The science that I am referring to indicates that Antarctic Mass Balance is decreasing; eg "Recent Antarctic ice mass loss from radar interferometry and regional climate modelling." Nature Geoscience.
www.nature.com/ngeo/journal/v1/n2/abs/ngeo102.htmlWhat science are you referring to?
1.
www.nrcan.gc.ca/earth-sciences/energy-mineral/geology/geodynamics/global-change/8587Global sea level is affected by numerous factors, including human contributions from groundwater pumping and dam building, and the growth and decay of mountain glaciers and polar ice sheets. Research into Antarctic ice mass balance addresses a key unknown in understanding the global sea level budget - is the Antarctic ice sheet growing, stable, or shrinking? Answering this is critical to understanding how sea level is presently changing and how it may change in the future.
An ice sheet that is gaining or losing mass would exert a changing force on the surface of the Earth, a force the Earth would respond to, in the same way that a spring scale responds to a weight. With colleagues from the Jet Propulsion Laboratory, we have been examining how modern geodetic techniques, such as the Global Positioning System (GPS), might be used to measure this response. The figure shows the elastic crustal response (vertical motion) to three differing, but realistic, scenarios of present day Antarctic mass balance (scenario 1, scenario 2, and J92 scenario) based on assessments of the available glaciological and oceanographic data. They give very different predictions of the vertical crustal response due to these changing surface loads, suggesting that GPS observations could help constrain present day mass balance.
Vertical crustal velocities for three scenarios of present Antarctic ice mass change and for a scenario of past ice mass change, the ICE-3G postglacial rebound model of Tushingham and Peltier (1991). Note the differing scale for ICE-3G. Scenario 1 contributes -0.1 mm/yr to sea level rise; scenario 2, -1.1 mm/yr; and J92 scenario, 0.45 mm/yr. This figure appeared in a recent article (Geophysical Research Letters, 22, 973-976, 1995).
However, postglacial rebound is also occurring in Antarctica and is potentially quite large, as shown in the figure for the ICE-3G postglacial rebound model. It will need to be considered when interpreting future GPS-based crustal motion observations in Antarctica. The size and location of postglacial uplift in Antarctica depends on the timing, magnitude, and location of past Antarctic ice mass changes, all of which are rather poorly known, although Antarctica likely contributed 20-30 m to sea level rise since about 20,000 years ago. Determining the past mass balance of Antarctica is critical to interpreting the history of sea level changes worldwide.
2.
rsta.royalsocietypublishing.org/content/364/1844/1627.full.html#ref-list-1pdf here:
www.cpom.org/4A85F6DA-F89E-4718-8276-F8906BB65378/FinalDownload/DownloadId-2416DA0C89E33F49E1917E0AF04CFEC4/4A85F6DA-F89E-4718-8276-F8906BB65378/research/djw-ptrsa364.pdfWe show that 72% of the Antarctic ice sheet is gaining 27±29 Gt yr−1, a sink of ocean mass sufficient to lower global sea levels by 0.08 mm yr−1. The IPCC third assessment (Church & Gregory 2001) partially offset an ongoing sea-level rise due to Antarctic retreat since the last glacial maximum (0.0–0.5 mm yr−1) with a twentieth century fall due to increased snowfall (−0.2–0.0 mm yr−1). But that assessment relied solely on models that neither captured ice streams nor the Peninsula warming, and the data show both have dominated at least the late twentieth century ice sheet. Even allowing a ±30 Gt yr−1 fluctuation in unsurveyed areas, they provide a range of −35–+115 Gt yr−1. This range equates to a sea level contribution of −0.3–+0.1 mm yr−1 and so Antarctica has provided, at most, a negligible component of observed sea-level rise. In consequence, the data places a further burden on accounting (Munk 2003) for the twentieth century rise of 1.5–2 mm yr−1. What is clear, from the data, is that fluctuations in some coastal regions reflect long-term losses of ice mass, whereas fluctuations elsewhere appear to be short-term changes in snowfall. While the latter are bound to fluctuate about the long-term MAR, the former are not, and so the contribution of retreating glaciers will govern the twenty-first century mass balance of the Antarctic ice sheet.
3.
scienceandpublicpolicy.org/commentaries_essays/west_antarctic_ice_sheet.htmlAnderson and Andrews (1999) analyzed grain size and foraminiferal contents of radiometrically-dated sediment cores collected from the eastern Weddell Sea continental shelf and the western Weddell Sea deep-sea floor in an attempt to better understand the behavior of both the East and West Antarctic ice sheets. In doing so, their data led them to conclude that "significant deglaciation of the Weddell Sea continental shelf took place prior to the last glacial maximum," and that the ice masses that border the Weddell Sea today "are more extensive than they were during the previous glacial minimum." Hence, they concluded "that the current interglacial setting is characterized by a more extensive ice margin and larger ice shelves than existed during the last glacial minimum, and that the modern West and East Antarctic ice sheets have not yet shrunk to their minimum." It is thus to be expected -- independent of what global air temperature may currently be doing, because of the great inertial forces at work over much longer time scales -- that the modern East and West Antarctic Ice Sheets may well continue to shrink and release more icebergs to the Southern Ocean over the coming years, decades and centuries, thereby slowly raising global sea level. Nothing man has done is responsible for these phenomena, however; and nothing man can do will impact them in any way.
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In an Editorial Essay published in the journal Climatic Change, Oppenheimer and Alley (2005) discussed "the degree to which warming can affect the rate of ice loss by altering the mass balance between precipitation rates on the one hand, and melting and ice discharge to the ocean through ice streams on the other," with respect to the WAIS and Greenland Ice Sheet (GIS). After a brief overview of the topic, they noted that "the key questions with respect to both WAIS and GIS are: What processes limit ice velocity, and how much can warming affect those processes?" In answer to these questions, they said that "no consensus has emerged about these issues nor, consequently, about the fate of either ice sheet, a state of affairs reflecting the weakness of current models and uncertainty in paleoclimatic reconstructions."
After a cursory review of the science related to these two key questions, Oppenheimer and Alley say their review "leads to a multitude of questions with respect to the basic science of the ice sheets," which we list below. However, instead of listing them in their original question form, we post them in the form of statements that address what we do not know about the various sub-topics mentioned, which is obviously what prompts the questions in the first place and validates the content of the statements.
(1) We do not know if the apparent response of glaciers and ice streams to surface melting and melting at their termini (e.g., ice shelves) could occur more generally over the ice sheets.
(2) We do not know if dynamical responses are likely to continue for centuries and propagate further inland or if it is more likely that they will be damped over time.
(3) We do not know if surface melting could cause rapid collapse of the Ross or Filchner-Ronne ice shelves, as occurred for the smaller Larsen ice shelf.
(4) We do not know if ice sheets made a significant net contribution to sea level rise over the past several decades.
(5) We do not know what might be useful paleoclimate analogs for sea level and ice sheet behavior in a warmer world.
(6) We do not know the reliability of Antarctic and Southern Ocean temperatures (and polar amplification) that are projected by current GCMs, nor do we know why they differ so widely among models, nor how these differences might be resolved.
(7) We do not know the prospects for expanding measurements and improving models of ice sheets nor the timescales involved.
(8) We do not know if current uncertainties in future ice sheet behavior can be expressed quantitatively.
(9) We do not know what would be useful early warning signs of impending ice sheet disintegration nor when these might be detectable.
(10) We do not know, given current uncertainties, if our present understanding of the vulnerability of either the WAIS or GIS is potentially useful in defining "dangerous anthropogenic interference" with earth's climate system.
(11) We do not know if the concept of a threshold temperature is useful.
(12) We do not know if either ice sheet seems more vulnerable and thus may provide a more immediate measure of climate "danger" and a more pressing target for research.
(13) We do not know if any of the various temperatures proposed in the literature as demarking danger of disintegration for one or the other ice sheet are useful in contributing to a better understanding of "dangerous anthropogenic interference."
(14) We do not know on what timescale future learning might affect the answers to these questions.
Clearly, there is a chance -- be it ever so small -- that almost anything could occur. But how probable are such high-risk phenomena? To claim, as Oppenheimer and Alley do, that ice-sheet disintegration is nearly inevitable if emissions of greenhouse gases are not reduced, is incredibly illogical, especially in light of the existence of what they say are "gaping holes in our understanding," as enumerated in the above list. In fact, given the degree of deficiency in our knowledge of the matter, it is perhaps as likely as not that a continuation of the planet's recovery from the relative cold of the Little Ice Age could actually lead to a buildup of polar ice; but there is no way we would ever say that that outcome is "nearly inevitable."
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Velicogna and Wahr (2006) used measurements of time-variable gravity from the Gravity Recovery and Climate Experiment (GRACE) satellites to determine mass variations of the Antarctic ice sheet for the 34 months between April 2002 and August 2005.
... the two researchers note that the GRACE mass solutions "do not reveal whether a gravity variation over Antarctica is caused by a change in snow and ice on the surface, a change in atmospheric mass above Antarctica, or post-glacial rebound (PGR: the viscoelastic response of the solid Earth to glacial unloading over the last several thousand years)."
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In yet another contemporary study, Remy and Frezzotti (2006) reviewed "the results given by three different ways of estimating mass balance, first by measuring the difference between mass input and output, second by monitoring the changing geometry of the continent, and third by modeling both the dynamic and climatic evolution of the continent." In describing their findings, the two researchers state that "the East Antarctica ice sheet is nowadays more or less in balance, while the West Antarctica ice sheet exhibits some changes likely to be related to climate change and is in negative balance." In addition, they report that "the current response of the Antarctica ice sheet is dominated by the background trend due to the retreat of the grounding line, leading to a sea-level rise of 0.4 mm/yr over the short-time scale," which they describe in terms of centuries. However, they note that "later, the precipitation increase will counterbalance this residual signal, leading to a thickening of the ice sheet and thus a decrease in sea level."
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At long last, we finally move from 2006 to 2007, as we conclude our Summary with a brief review of the paper of Krinner et al. (2007), who used the LMDZ4 atmospheric general circulation model (Hourdin et al., 2006) to simulate Antarctic climate for the periods 1981-2000 (to test the model's ability to adequately simulate present conditions) and 2081-2100 (to see what the future might hold for the mass balance of the Antarctic Ice Sheet and its impact on global sea level). This work revealed, first of all, that "the simulated present-day surface mass balance is skilful on continental scales," which gave them confidence that their results for the end of the 21st century would be reasonably skilful as well. Of that latter period a full century from now, they determined that "the simulated Antarctic surface mass balance increases by 32 mm water equivalent per year," which corresponds "to a sea level decrease of 1.2 mm per year by the end of the twenty-first century," which would in turn "lead to a cumulated sea level decrease of about 6 cm." This result, in their words, occurs because the simulated temperature increase "leads to an increased moisture transport towards the interior of the continent because of the higher moisture holding capacity of warmer air," where the extra moisture falls as precipitation, causing the continent's ice sheet to grow.
The results of this study -- based on sea surface boundary conditions taken from IPCC Fourth Assessment Report simulations (Dufresne et al., 2005) that were carried out with the IPSL-CM4 coupled atmosphere-ocean general circulation model (Marti et al., 2005), of which the LMDZ4 model is the atmospheric component -- argue strongly against climate-alarmist predictions of future catastrophic sea level rise due to mass wastage of the Antarctic Ice Sheet caused by CO2-induced global warming. In fact, they suggest just the opposite, i.e., that CO2-induced global warming would tend to buffer the world against such an outcome.
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Sorry for the long post. I just thought it might be helpful.