Learn how plants respond to higher atmospheric CO2 concentrations

How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic


Ice Sheets (Greenland) -- Summary
Studies of the growth and decay of polar ice sheets are of great importance because of the relationships of these phenomena to global warming and the impacts they can have on sea level. In this summary, therefore, we review a number of such studies that pertain to the Greenland Ice Sheet.

Davis et al. (1998) used Seasat and Geosat radar altimeter data to assess changes in the elevation of the Greenland ice sheet between 1978 and 1988, finding a modest overall thickening of the ice sheet. Over the same time period, McConnell et al. (2000) evaluated ice sheet elevation changes from a model of firn densification and records of annual snow accumulation derived from twelve ice cores obtained at high elevations. Their results agreed well with those of Davis et al. and allowed them to further conclude that "the decadal-scale changes in ice-sheet elevation that occurred during 1978-88 are typical over the last few centuries and well within the natural variability of accumulation-driven elevation change," suggesting little to no long-term change in the overall mass balance of the ice sheet.

Thomas et al. (2000) compared estimates of ice discharge from higher elevations of the Greenland ice sheet (derived from ice motions inferred from Global Positioning System measurements made between 1993 and 1997) with total snow accumulation estimates to calculate ice thickening rates over the past few decades. They too concluded that "the higher elevation parts of the ice sheet have been almost exactly in balance when considered as a whole."

Aircraft laser-altimeter measurements made over southern Greenland in 1993 and 1998 served as the basis for the study of Krabill et al. (1999), who observed a small net thickening of the ice sheet for elevations above 2000 meters, where they considered their data "most reliable." Lower elevations, however, were reported to be thinning; but they said it was "extremely unlikely" the thinning was due to "an increase in summer melting." Overall, the portion of the ice sheet south of 72 N latitude was determined to be "in negative balance."

The following year, Krabill et al. (2000) used aircraft laser-altimeter data obtained over northern Greenland in 1994 and 1999, together with previous data from southern Greenland, to evaluate the mass balance of the entire ice sheet. At elevations above 2000 meters, they found a small net thickening; but after accounting for bedrock uplift, the balance was determined to be essentially "zero." Thinning again predominated along about 70% of the coast; but these results were obtained from estimates of interpolations based on calculations of a hypothetical thinning rate. Perhaps this far removal of their final low-elevation result from any primary data is why the authors say they could find no satisfactory explanation for the thinning, which suggests it may not have been real. Even if it were real, however, the thinning of the coastal portions of the ice sheet is unlikely to have been caused by global or regional warming; for the authors report that "Greenland temperature records from 1900-95 show highest summer temperatures in the 1930s," and they indicate that "the 1980s and early 1990s were about half a degree cooler than the 96-year mean."

Although the Krabill et al. (2000) study was thus rife with many uncertainties, the media sank their teeth into it with great relish (see our Editorial of 26 July 2000). As one news story put it, "a warming climate is melting more than 50 billion tons of water a year from the Greenland ice sheet, adding to a 9-inch global rise in sea level over the last century and increasing the risk of coastal flooding around the world." If its author had done a few simple calculations, however, he or she would have found that to raise global sea level as much as all other natural processes raised it over the last century, the purported thinning of the ice sheet along Greenland's coast (which equates to a sea level rise of 0.005 inch per year) would have to continue for nigh unto two millennia. And as the senior author of the study stated explicitly in a NASA press release, "this amount of sea level rise does not threaten coastal regions."

If the studies reviewed above teach us anything, it is that we have a great need for high-quality, long-term, ice-sheet mass balance data; and in a study of the mass balances of all of earth's glaciers for which such data exist, Braithwaite and Zhang (2000) present even more evidence of this need. Extrapolating what they learned from smaller glaciers to the Greenland ice sheet, for example, they conclude that "the ice sheet can thicken or thin by several meters over 20-30 years without giving statistically significant evidence of non-zero balance under present climate." Hence, they say that the Greenland ice sheet may "have to be monitored over many decades to detect unambiguous evidence of either thinning, due to increased melting, or thickening, due to increased accumulation."

Reeh (1999) reached much the same conclusion, stating "we do not know" - with respect to the ice covers of both Greenland and Antarctica - "whether the ice sheets are currently in balance; neither do we know if their volume or mass has increased or decreased during the last 100 years." Climate model predictions are of little help either. Working with the two most recent incarnations of the GCM developed by the Max Plank Institute for Meteorology in Hamburg, Germany, for example, Wild and Ohmura (2000) concluded that the sea level change resulting from the combined changes in the ice sheets of both Greenland and Antarctica at the time of a doubled atmospheric CO2 concentration would be either "close to zero" or indicative of a sea level fall rate of 0.6 mm per year.

From a recalibration of oxygen isotope-derived temperatures based on data obtained from central Greenland ice cores, however, Cuffey and Marshall (2000) tentatively determined that the Greenland ice sheet may have been much smaller during the last interglacial than previously thought. If true, this finding implies the potential for further major shrinkage of the Greenland ice sheet ... but only if the planet were to warm substantially more than it did during the past century, i.e., by a several-fold factor (see our review of Petit et al., 1999). Furthermore, this conclusion implies, in Cuffey and Marshall's words, that "high sea levels during the last interglacial should not be interpreted as evidence for extensive melting of the West Antarctic Ice Sheet, and so challenges the hypothesis that the West Antarctic is particularly sensitive to climate change." Therefore, since the Antarctic is by far the most important repository of potential melt-water on the planet, this slight unease about the potential for additional shrinkage of the Greenland ice sheet is more than compensated by the greater confidence it gives us that we do not have to worry about the analogous phenomenon occurring in Antarctica.

Following on the heels of Cuffey and Marshall's study, Fahnestock et al. (2001) used airborne ice-penetrating radar to determine the extent and rate of basal melting for a large portion of the Greenland Ice Sheet, while using other geophysical data to study the underlying topography of the region. In doing so, they discovered a large area of rapid melting in the source region of the rapidly flowing ice stream that drains the north side of the summit dome. This melting was further shown to be occurring above a 1000-m-high topographic disturbance that exhibits a dramatic increase in bed roughness, which suggests, in their words, that "it has undergone less erosion and may be younger than the surrounding bed."

Melt rates of this anomalous area were indicative of geothermal fluxes 15 to 30 times greater than the continental background rate; and Fahnestock et al. say that these fluxes and free-air gravity measurements made over the primary area of basal melting are comparable in magnitude and spatial extent to those of the Yellowstone caldera, and that localized peaks in gravity and rough-surfaced bed topography are suggestive of local extrusive structures. Hence, they concluded that their "limited geophysical evidence suggests the presence of a caldera structure" that leads to "rapid and extensive basal melting in Greenland that has a direct effect on ice flow." Nevertheless, they note that the findings of several studies of thickening and thinning in the high interior of the ice sheet suggest that "the present ice sheet is close to being in balance with the patterns of basal melting."

About the same time, Mosley-Thompson et al. (2001) analyzed a suite of spatially-distributed histories of annual mass accumulation obtained from ice core data collected under the Program for Arctic Regional Climate Assessment (PARCA), including 350-year histories from northwest Greenland and the summit area. Because of the great variability in the results obtained from the different cores, they concluded that "climate reconstructions from a single core must be interpreted cautiously." In addition, the two 350-year histories exhibited large miltidecadal variability. Also, regional composites for northwest and central Greenland were determined to be "strongly in and out of phase for decades at a time," and that since 1940 they have essentially been decoupled. Faced with these realities, the team of nine researchers concluded that much remains to be done before definitive statements can be made about the current mass balance of the Greenland Ice Sheet. Specifically, they note that "longer regional annual net accumulation composites from a suite of carefully sited cores, coupled with longer-term altimeter observations, would contribute substantially to future mass balance evaluations."

In a study of how well numerical models might be able to divine the future of the Greenland Ice Sheet, van der Veen (2002) notes that "it is currently not well known whether or not the ice sheet is growing or shrinking, although most studies agree that the whole of Greenland is not far out of balance in either direction." Consequently, if the ice sheet's past behavior cannot be specified, there is no opportunity to assess model performance. Furthermore, even if a model prediction is demonstrated to be consistent with past observations, van der Veen notes "there is no guarantee that the model will perform equally well when used to predict the future," especially if one of the model parameters extends into a range that is beyond the range within which the model was tested.

Admittedly, these observations appear to suggest that it is essentially impossible for a model to ever be "proven" to be a valid tool for assessing the likelihood of future events; and that perspective is correct. At best, says van der Veen, models can only be confirmed "by matching observational data that were not used to calibrate model parameters." But even then, considering the observations of the preceding paragraph, it really becomes a matter of faith as to how well one believes a model that has successfully replicated the past will predict the future.

Laying these considerations aside - but remembering they imply that whatever follows may be even less well defined than what is suggested by the numbers - van der Veen calculates that within the context of greenhouse-warming-induced sea level change, uncertainties in model parameters are sufficiently great to yield a 95% confidence range of projected contributions from Greenland and Antarctica that encompass global sea-level lowering as well as rise by AD 2100 for low, middle and high warming scenarios based on surface mass balance calculations. Hence, even for the worst of the global warming projections - which could well be way off base itself, as we personally believe it is - there could be little to no change in mean global sea level due to the ongoing rise in the air's CO2 content.

In view of these findings, van der Veen concludes that the confidence level that can be placed in current ice sheet mass balance models "is quite low." Paraphrasing an earlier assessment of the subject, in fact, he says that today's best models "currently reside on the lower rungs of the ladder of excellence." Hence, it is not surprising that he states that "considerable improvements are needed before accurate assessments of future sea-level change can be made."

Moving forward a few years, the situation does not become much clearer, as evidenced by Oppenheimer and Alley (2005), who discuss what they call a key issue, i.e., "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 both the West Antarctic Ice Sheet (WAIS) and the Greenland Ice Sheet (GIS). After a brief overview of the topic, they note 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 answering these questions, they say 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 key questions, Oppenheimer and Alley say their review "leads to a multitude of questions [our italics] with respect to the basic science [our italics] 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.

(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.

In concluding their essay, Oppenheimer and Alley describe this list of deficiencies in our knowledge of things related to ice-sheet dynamics as "gaping holes in our understanding" that "will not be closed unless governments provide adequate resources for research." More importantly - and incredulously! - they state that "if emissions of the greenhouse gases are not reduced while uncertainties are being resolved, there is a risk of making ice-sheet disintegration nearly inevitable [our italics]."

Clearly, there is a risk - be it ever so small - that almost anything could occur. But how probable are such high-risk phenomena? To claim there is a risk of making ice-sheet disintegration nearly inevitable if emissions of greenhouse gases are not reduced while uncertainties are being resolved, is totally illogical, especially in light of what they say are "gaping holes in our understanding" of the subject, as enumerated in the list above. 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 that we would ever say that outcome is "nearly inevitable."

Continuing, in our Editorial of 2 Nov 2005 we discuss the 21 October 2005 issue of Science that contains a major review of recent ice-sheet and sea-level changes (Alley et al., 2005), wherein it is claimed that "the Greenland Ice Sheet may melt entirely from future global warming," which worn-out climate-alarmist claim is buttressed with the statement that "recently detected rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered."

The assessment sounded pretty ominous. But are the data on which it was based really that solid?

Alley et al. say that "for Greenland, updated estimates based on repeat altimetry, and the incorporation of atmospheric and runoff modeling, indicate increased net mass loss." Between 1993-94 and 1998-99, for example, they say "the ice sheet was losing 54 14 gigatons per year (Gt/year) of ice, equivalent to a sea-level rise of ~0.15 mm/year." What is more, they report that "despite highly anomalous excess snowfall in the southeast in 2002 to 2003, net mass loss over the 1997-to-2003 interval was higher than the loss between 1993 and 1999, averaging 74 11 Gt/year or ~0.21 mm/year sea-level rise."

Attempting to give these observations even more weight, Alley et al. went on to say they "are broadly similar to those from a mesoscale atmospheric model used to simulate the surface mass balance of the Greenland Ice Sheet from 1991 to 2000," where "accounting for additional mass loss from iceberg discharge and basal melting yielded an estimated net mass loss of 78 Gt/year."

Yes, the data and theory match wonderfully, but the actual observations were very spotty; and a new analysis of a much more comprehensive data set shows the central conclusion of Alley et al. - and that of the atmospheric model - to be 180 degrees out of phase with reality. And in an incredible irony, the new observations of Johannessen et al. (2005) were reported in a Sciencexpress paper posted online just one day before the Alley et al. paper appeared in print in Science.

In introducing their new observational study, Johannessen et al. note that previous mass balance work on the Greenland Ice Sheet was "based on some tracks of aerial laser altimetry, unevenly sampled in space and time," and that "the surface-elevation data sets analyzed previously have been discontinuous and relatively short." Overcoming these problems, they derived and analyzed, for practically all of Greenland, a continuous satellite-altimeter height record of ice sheet elevations for the 11-year period 1992-2003.

So what did Johannessen et al. find? Below 1500 meters, the mean change of ice sheet height with time was a decline of 2.0 0.9 cm/year, qualitatively in harmony with the statements of Alley et al.; but above 1500 meters, there was a positive growth rate of fully 6.4 0.2 cm/year. Averaged over the entire ice sheet, the mean result was also positive, at a value of 5.4 0.2 cm/year, which when adjusted for an isostatic uplift of about 0.5 cm/year yielded a mean growth rate of approximately 5 cm/year, for a total increase in the mean thickness of the Greenland Ice Sheet of about 55 cm over the 11-year period, which was primarily driven by accumulation of increased snowfall over the ice sheet. These results turn the central conclusion of Alley et al. (that the Greenland Ice Sheet is shrinking) on its head; and they signal the existence of serious problems with the climate model they cited as agreeing with their faulty view of reality.

In concluding this chapter of the story, the data of Johannessen et al. indicate that over the past decade or so, at the apex of a global warming that has been characterized by climate alarmists as having been the greatest of the past two millennia, the Greenland Ice Sheet has not been wasting away, as climate alarmists claim, and as even reputable scientists have been led to believe. In fact, it appears to have been growing, and growing at a very respectable pace.

Shortly thereafter, and continuing in much the same vein, Zwally et al. (2005) determined changes in ice mass "from elevation changes derived from 10.5 years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2." This work revealed, in their words, that "the Greenland ice sheet is thinning at the margins (-42 2 Gt a-1 below the equilibrium-line altitude (ELA)) and growing inland (+53 2 Gt a-1 above the ELA) with a small overall mass gain (+11 3 Gt a-1; -0.03 mm a-1 SLE (sea-level equivalent))." Likewise, they say that "the ice sheet in West Antarctica (WA) is losing mass (-47 4 Gt a-1) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 11 Gt a-1) for a combined net change of -31 12 Gt a-1 (+0.08 mm a-1 SLE)." Hence, they report that "the contribution of the three ice sheets to sea level is +0.05 0.03 mm a-1." Furthermore, although not impacting sea level, they note that "the Antarctic ice shelves show corresponding mass changes of -95 11 GT a-1 in WA and +142 10 Gt a-1 in EA."

To put these findings in perspective, we often hear horror stories about the potential for Greenland and Antarctica to add many meters to the level of the seas in response to global warming. However, Zwally et al. say the real-world data they processed indicate that the ongoing contribution of the Greenland and Antarctic ice sheets to sea-level "is small relative to the potential contribution from ice sheets." How small? At the current sea-level-equivalent ice-loss rate of 0.05 millimeters per year, it would take an entire millennium to raise global sea level by just 5 cm, and it would take fully 20,000 years to raise it a single meter.

However, and in spite of these many reassuring findings, there is an unending flow of papers that attempt to paint a more ominous picture of the Greenland Ice Sheet. As we reported in our Editorial of 8 Mar 2006, for example, considerable fanfare was accorded the publication of the study of Rignot and Kanagaratnam (2005), who used satellite radar interferometry observations of Greenland to detect "widespread glacier acceleration." Calculating that this phenomenon had led to a doubling of the ice sheet mass deficit in the last decade and, therefore, a comparable increase in Greenland's contribution to rising sea levels, they went on to claim that "as more glaciers accelerate ... the contribution of Greenland to sea-level rise will continue to increase."

With respect to these contentions, we have no problem with what the two researchers observed with respect to Greenland's glaciers; but we feel compelled to note that what they calculated with respect to the mass balance of Greenland's ice sheet and what they say it implies about sea level are diametrically opposed to the story told by other more inclusive real-world data. Where Rignot and Kanagaratnam went wrong was in estimating Greenland's mass gain via snowfall over the vast interior of the ice sheet during the time that coastal glaciers were accelerating. Instead of relying on real-world measurements for this evaluation, they relied on the calculations of Hanna et al. (2005), who used meteorological models "to retrieve annual accumulation, runoff, and surface mass balance."

When actual measurements of the ice sheet via satellite radar altimetry are employed, quite a different perspective is obtained, as demonstrated by the study of Zwally et al. (2005), who found that although "the Greenland ice sheet is thinning at the margins," it is "growing inland with a small overall mass gain," as previously described, and as has also been demonstrated by Johannessen et al. (2005), who found that "below 1500 meters, the elevation-change rate is -2.0 0.9 cm/year, in qualitative agreement with reported thinning in the ice-sheet margins," but that "an increase of 6.4 0.2 cm/year is found in the vast interior areas above 1500 meters." Spatially averaged over the bulk of the ice sheet, therefore, the net result, according to Johannessen et al., is a mean increase of 5.4 0.2 cm/year, "or ~60 cm over 11 years, or ~54 cm when corrected for isostatic uplift," as we have also previously described.

Consequently, and in direct contradiction of the claim of Rignot and Kanagaratnam, Greenland has experienced no "ice sheet mass deficit in the last decade." Quite to the contrary, it appears to have been host to a net accumulation of ice, which Zwally et al. find to be "contributing -0.03 0.01 mm a-1 to sea-level change." As a result, the net accretion of ice on Greenland over the past decade has actually been ever so slightly lowering global sea level.

In light of these several observations, it is a sad commentary on the politicization of science that the American Association for the Advancement of Science's press release about the Rignot and Kanagaratnam paper was entitled "Greenland glaciers dumping ice into Atlantic at faster pace." Although technically correct, it failed to convey the far more important knowledge that earth's hydrologic cycle was sucking water out of the ocean and depositing it on Greenland in the form of snow at an even faster pace.

But the negative spin never ceases. In an attempt to downplay the significance of these inconvenient findings, Kerr quotes Zwally as saying he believes that "right now" the Greenland ice sheet is experiencing a net loss of mass. Why? Kerr says Zwally's belief is "based on his gut feeling about the most recent radar and laser observations." Fair enough. But gut feelings are a poor substitute for comprehensive real-world measurements; and even if the things that Zwally's intestines are telling him are ultimately proven to be correct, their confirmation would only demonstrate just how rapidly the Greenland environment can change. Also, we would have to wait and see how long the mass losses prevailed in order to assess their significance within the context of the CO2-induced global warming debate. For the present and immediate future, therefore, we have no choice but to stick with what the existent data and analyses suggest, i.e., that cumulatively since the early 1990s, and conservatively (since the balance is likely still positive), there has been no net loss of mass from the Greenland ice sheet.

Nevertheless, the onslaught continues. In the 24 March 2006 issue of Science, as we describe in our Editorial of 29 Mar 2006, a number of commentaries heralded accelerating discharges of glacial ice from both Greenland and Antarctica, while dispensing dire warnings of an imminent large, rapid and accelerating sea-level rise (Bindschadler, 2006; Joughin, 2006; Kerr, 2006; Kennedy and Hanson, 2006). This distressing news was based primarily on three reports published in the same issue of Science (Ekstrom et al., 2006; Otto-Bliesner et al., 2006; Overpeck et al., 2006), wherein the impending sea level rise was blamed on anthropogenic-driven global warming, which is widely claimed to be due primarily to increases in the air's CO2 content that are believed to be driven by the burning of ever increasing quantities of coal, gas and oil. But does all of this make any sense?

Consider the report of Ekstrom et al., who studied "glacial earthquakes" caused by sudden sliding motions of glaciers on Greenland. Over the period Jan 1993 to Oct 2005, they determined that (1) all of the best-recorded quakes were associated with major outlet glaciers on the east and west coasts of Greenland between approximately 65 and 76N latitude, (2) "a clear increase in the number of events is seen starting in 2002," and (3) "to date in 2005, twice as many events have been detected as in any year before 2002." With respect to the reason for the recent increase in glacial activity on Greenland, Clayton Sandell of ABC News (23 March 2006) quotes Ekstrom as saying "I think it is very hard not to associate this with global warming," which sentiment appears to be shared by almost all of the authors of the seven Science articles. Unwilling to join in that conclusion, however, was Joughin, who in the very same issue presented histories of summer temperature at four Greenland coastal stations located within the same latitude range as the sites of the glacial earthquakes, which histories suggest that it was warmer in this region back in the 1930s than it was over the period of Ekstrom et al.'s analysis. On the basis of these data, therefore, Joughin concluded that "the recent warming is too short to determine whether it is an anthropogenic effect or natural variability," a position that is supported -- and in some cases even more rigorously -- by numerous scientists who have researched the issue in depth and whose findings we describe in reviews of the following studies: Hanna and Cappelen (2003), Przybylak (2000), Comiso et al. (2001) and Chylek et al. (2004).

In light of the many real-world observations described in the above-referenced studies, it should be clear to all that the recent upswing in glacial activity on Greenland likely has had nothing to do with anthropogenic-induced global warming, as temperatures there have yet to rise either as fast or as high as they did during the great warming of the 1920s, which was clearly a natural phenomenon.

The set of Science papers and associated news reports also make much of recent ice discharges from Antarctica, particularly along the Antarctic Peninsula, which has warmed more than any other place on earth over the past fifty years. Little to nothing, however, is said about the fact that the great bulk of the continent has actually cooled over this period, which as in the case of Greenland has also been demonstrated by numerous researchers, including Comiso (2000), Doran et al. (2002) and Turner et al. (2005). The observations of these researchers reveal that over the latter part of the 20th century, when according to climate alarmists the earth experienced the most dramatic global warming of the entire past two millennia, fully 80% of the Antarctic coastal stations with sufficiently long temperature records experienced either an intensification of cooling or a reduced rate of warming; while four coastal sites and one interior site actually shifted from warming to cooling.

In summing up the bottom-line take-home message of all of these many studies, perhaps the fairest thing that could be said is that we really do not know if there is any long-term positive or negative mass balance change occurring on either the Greenland or Antarctic Ice Sheets. Hence, it is important that we continue collecting data in these two polar regions, so that someday we will be able to unambiguously discern whatever trends or non-trends are representative of reality. In the mean time, don't believe anything about these ice sheets that sounds either too good or too bad. Neither is likely to be correct.

References
Alley, R.B., Clark, P.U., Huybrechts, P. and Joughin, I. 2005. Ice-sheet and sea-level changes. Science 310: 456-460.

Bindschadler, R. 2006. Hitting the ice sheets where it hurts. Science 311: 1720-1721.

Braithwaite, R.J. and Zhang, Y. 2000. Relationships between interannual variability of glacier mass balance and climate. Journal of Glaciology 45: 456-462.

Chylek, P., Box, J.E. and Lesins, G. 2004. Global warming and the Greenland ice sheet. Climatic Change 63: 201-221.

Comiso, J.C. 2000. Variability and trends in Antarctic surface temperatures from in situ and satellite infrared measurements. Journal of Climate 13: 1674-1696.

Comiso, J.C., Wadhams, P., Pedersen, L.T. and Gersten, R.A. 2001. Seasonal and interannual variability of the Odden ice tongue and a study of environmental effects. Journal of Geophysical Research 106: 9093-9116.

Cuffey, K.M. and Marshall, S.J. 2000. Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet. Nature 404: 591-594.

Davis, C.H., Kluever, C.A. and Haines, B.J. 1998. Elevation change of the southern Greenland ice sheet. Science 279: 2086-2088.

Doran, P.T., Priscu, J.C., Lyons, W.B., Walsh, J.E., Fountain, A.G., McKnight, D.M., Moorhead, D.L., Virginia, R.A., Wall, D.H., Clow, G.D., Fritsen, C.H., McKay, C.P. and Parsons, A.N. 2002. Antarctic climate cooling and terrestrial ecosystem response. Nature advance online publication, 13 January 2002 (DOI 10.1038/nature710).

Ekstrom, G., Nettles, M. and Tsai, V.C. 2006. Seasonality and increasing frequency of Greenland glacial earthquakes. Science 311: 1756-1758.

Fahnestock, M., Abdalati, W., Joughin, I., Brozena, J. and Gogineni, P. 2001. High geothermal heat flow, basal melt, and origin of rapid ice flow in central Greenland. Science 294: 2338-2342.

Hanna, E. and Cappelen, J. 2003. Recent cooling in coastal southern Greenland and relation with the North Atlantic Oscillation. Geophysical Research Letters 30: 10.1029/2002GL015797.

Hanna, E., Huybrechts, P., Janssens, I., Cappelin, J., Steffen, K. and Stephens, A. 2005. Journal of Geophysical Research 110: 10.1029/2004JD005641.

Johannessen, O.M., Khvorostovsky, K., Miles, M.W. and Bobylev, L.P. 2005. Recent ice-sheet growth in the interior of Greenland. Sciencexpress / www.sciencexpress.org / 20 October 2005.

Joughin, I. 2006. Greenland rumbles louder as glaciers accelerate. Science 311: 1719-1720.

Kerr, R.A. 2006. A worrying trend of less ice, higher seas. Science 311: 1698-1701.

Krabill, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., Wright, W. and Yungel, J. 1999. Rapid thinning of parts of the southern Greenland ice sheet. Science 283: 1522-1524.

Krabill, W., Abdalati, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., Wright, W. and Yungel, J. 2000. Greenland ice sheet: High-elevation balance and peripheral thinning. Science 289: 428-430.

McConnell, J.R., Arthern, R.J., Mosley-Thompson, E., Davis, C.H., Bales, R.C., Thomas, R., Burkhart, J.F. and Kyne, J.D. 2000. Changes in Greenland ice sheet elevation attributed primarily to snow accumulation variability. Nature 406: 877-879.

Mosley-Thompson, E., McConnell, J.R., Bales, R.C., Li, Z., Lin, P.-N., Steffen, K., Thompson, L.G., Edwards, R. and Bathke, D. 2001. Local to regional-scale variability of annual net accumulation on the Greenland ice sheet from PARCA cores. Journal of Geophysical Research 106: 33,839-33,851.

Oppenheimer, M. and Alley, R.B. 2005. Ice sheets, global warming, and article 2 of the UNFCCC. Climatic Change 68: 257-267.

Otto-Bliesner, B.L., Marshall, S.J., Overpeck, J.T., Miller, G.H., Hu, A., and CAPE Last Interglacial Project members. 2006. Simulating Arctic climate warmth and icefield retreat in the last interglaciation. Science 311: 1751-1753.

Overpeck, J.T., Otto-Bliesner, B.L., Miller, G.H., Muhs, D.R., Alley, R.B. and Kiehl, J.T. 2006. Paleoclimatic evidence for future ice-sheet instability and rapid sea-level rise. Science 311: 1747-1750.

Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E., and Stievenard, M. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429-436.

Przybylak, R. 2000. Temporal and spatial variation of surface air temperature over the period of instrumental observations in the Arctic. International Journal of Climatology 20: 587-614.

Reeh, N. 1999. Mass balance of the Greenland ice sheet: Can modern observation methods reduce the uncertainty? Geografiska Annaler 81A: 735-742.

Rignot, E. and Kanagaratnam, P. 2005. Changes in the velocity structure of the Greenland Ice Sheet. Science 311: 986-990.

Thomas, R., Akins, T., Csatho, B., Fahnestock, M., Gogineni, P., Kim, C. and Sonntag, J. 2000. Mass balance of the Greenland ice sheet at high elevations. Science 289: 426-428.

van der Veen, C.J. 2002. Polar ice sheets and global sea level: how well can we predict the future? Global and Planetary Change 32: 165-194.

Wild, M. and Ohmura, A. 2000. Change in mass balance of polar ice sheets and sea level from high-resolution GCM simulations of greenhouse warming. Annals of Glaciology 30: 197-203.

Zwally, H.J., Giovinetto, M.B., Li, J., Cornejo, H.G., Beckley, M.A., Brenner, A.C., Saba, J.L. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002. Journal of Glaciology 51: 509-527.

Last updated 23 August 2006