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Carbon Dioxide and Earth's Future: Pursuing the Prudent Path

4. Rising Sea Levels Inundating Coastal Lowlands

The claim: Anthropogenic-induced global warming will lead to rapidly melting polar ice sheets, rapidly rising sea levels and catastrophic coastal flooding.

With respect to the melting of earth's polar ice sheets, we begin in the Northern Hemisphere with the review of Alley et al. (2005), who claimed that "the Greenland Ice Sheet may melt entirely from future global warming," which contention they 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." Between the periods of 1993-94 and 1998-99, for example, they report that "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," adding that despite excess snowfall in the southeast in 2002 and 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."

Just one day before Alley et al.'s paper appeared in print, however, Johannessen et al. (2005), working with satellite-altimeter data from Greenland, reported in a Sciencexpress paper posted online that although below 1500 meters the mean change of the ice sheet height with time was a decline of 2.0 0.9 cm/year over the 11-year period 1992-2003, above 1500 meters there was a positive growth rate of fully 6.4 0.2 cm/year due to snow accumulation; and averaged over the entire ice sheet the mean result was also positive, with a mean growth rate 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, a result that was just the opposite of that suggested by Alley et al.

Then, like a pendulum changing direction yet again, came 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's mass deficit in the last decade and, therefore, to 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." Hard on the heels of their paper, however, came the satellite radar altimetry study of Zwally et al. (2005), which once again sent the pendulum swinging in the opposite direction in response to their finding that "the Greenland ice sheet is thinning at the margins (-42 2 Gt/year below the equilibrium-line altitude) and growing inland (+53 2 Gt/year above the equilibrium-line altitude) with a small overall mass gain (+11 3 Gt/year; -0.03 mm/year sea-level equivalent)."

But Chen et al. (2006) soon after pushed the pendulum way back in the other direction with their Gravity Recovery and Climate Experiment (GRACE) study, wherein they concluded that satellite-measured gravity variations suggested that the Greenland Ice Sheet was currently disappearing at a rate of some 240 cubic kilometers per year. However, the many problems with which they had to contend in reaching this conclusion were complex enough to make the effort nearly intractable and render their end result highly questionable.

In ruminating about this confusing situation, Cazenave (2006) described the many knotty problems that have beset the GRACE technique and led to the disturbingly large scatter in Greenland ice loss calculations (50 to 250 Gt/year) among the several studies that have employed it. Almost contemporaneously, however, a new approach to the analysis of GRACE data was developed by Luthcke et al. (2006); and it would appear to have greatly improved the fidelity of their findings, which suggested that there has been a mean ice mass loss of only 101 16 Gt/year from Greenland over the period 2003 to 2005. Nevertheless, because of the short time span involved, and the fact that "over Greenland," as Cazenave describes it, "ice mass varies widely from year to year," little could be concluded from the GRACE data that had been accumulated to date; and she stated that because the different analyses "do not overlap exactly in time, different trend estimates are to be expected."

Two years later, Das et al. (2008) established observation sites at two large supraglacial lakes on the western margin of the Greenland Ice Sheet atop approximately 1000-meter-thick sub-freezing ice. One of the lakes rapidly drained on 29 July 2006 in a dramatic event that was monitored by local GPS, seismic and water-level sensors, which indicated that the entire lake drained in approximately 1.4 hours, with a mean drainage rate exceeding the average rate of water flow over Niagara Falls. One consequence of this event was a westward surface displacement of 0.5 meter in excess of the average daily displacement of 0.25 meter. However, pre- and post-drainage lateral speeds did not differ appreciably, leading the researchers to conclude that "the opening of a new moulin draining a large daily melt volume (24 m3/sec) had little apparent lasting effect on the local ice-sheet velocity."

But what might be the effect of multiple lake drainages?

In a second study that addressed this question, Joughin et al. (2008) assembled a comprehensive set of interferometric synthetic aperture radar (InSAR) and GPS observations over the period September 2004 to August 2007. These data allowed the construction of 71 InSAR velocity maps along two partially overlapping RADARSAT tracks that included Jakobshavn Isbrae (western Greenland's largest outlet glacier), several smaller marine-terminating outlet glaciers, and a several-hundred-kilometer-long stretch of the surrounding ice sheet. The data thereby obtained revealed summer ice-sheet speedups of 50+% in some places. However, the researchers noted that "the melt-induced speedup averaged over a mix of several tidewater outlet glaciers is relatively small (<10 to 15%)." And when factoring in the short melt-season duration, they found that "the total additional annual displacement attributable to surface melt amounts to a few percent on glaciers moving at several hundred meters per year." In addition, they reported that "the limited seasonal observations elsewhere in Greenland suggest a low sensitivity to summer melt similar to that which we observe."

In concluding, Joughin et al. wrote that "surface-melt-enhanced basal lubrication has been invoked previously as a feedback that would hasten the Greenland Ice Sheet's demise in a warming climate." However, their real-world observations of this phenomenon showed that "several fast-flowing outlet glaciers, including Jakobshavn Isbrae, are relatively insensitive to this process."

To the south of Jakobshavn Isbrae, however, Joughin et al. noted that the ice sheet's western flank is relatively free of outlet glaciers and that ice loss there is primarily due to melt; and they say that "numerical models appropriate to this type of sheet flow and that include a parameterization of surface-melt-induced speedup predict 10-to-25% more ice loss in the 21st Century than models without this feedback." This estimate, of course, is based on a model parameterization of surface-melt-induced speedup that may or may not be an adequate representation of reality. Nevertheless, it can probably safely be concluded, as Joughin et al. expressed it, that the phenomenon of surface-melt-enhanced basal lubrication likely will not have a "catastrophic" effect on the Greenland Ice Sheet's future evolution.

Studying the subject contemporaneously were van de Wal et al. (2008), who acquired ice velocity measurements from the major ablation area along the western margin of the Greenland Ice Sheet and determined that "the englacial hydraulic system adjusts constantly to the variable meltwater input, which results in a more or less constant ice flux over the years," such that the phenomenon "may have only a limited effect on the response of the ice sheet to climate warming over the next decades," with their data suggesting that that "limited effect" might actually be to slow rather than hasten ice flow to the sea.

Shortly thereafter, Nick et al. (2009) developed "a numerical ice-flow model that reproduced the observed marked changes in Helheim Glacier," which they described as "one of Greenland's largest outlet glaciers," after which they used the model to study the glacier's dynamics and determine what they might imply about the future mass balance of the Greenland Ice Sheet and subsequent global sea levels. The four researchers reported that their model simulations showed that "ice acceleration, thinning and retreat begin at the calving terminus and then propagate upstream through dynamic coupling along the glacier." What is more, they found that "these changes are unlikely to be caused by basal lubrication through surface melt propagating to the glacier bed." And, therefore, Nick et al. concluded that "tidewater outlet glaciers adjust extremely rapidly to changing boundary conditions at the calving terminus," stating that their results implied that "the recent rates of mass loss in Greenland's outlet glaciers are transient and should not be extrapolated into the future."

About the same time, Wake et al. (2009) reconstructed the 1866-2005 surface mass-balance (SMB) history of the Greenland ice sheet on a 5 x 5-km grid using a runoff-retention model based on the positive degree-day method that accounts "for the influence of year-on-year surface elevation changes on SMB estimates," which was "forced with new datasets of temperature and precipitation patterns dating back to 1866." This they did in order to compare "the response of the ice sheet to a recent period of warming and a similar warm period during the 1920s to examine how exceptional the recent changes are within a longer time context." And in doing so, the six scientists determined that present-day SMB changes "are not exceptional within the last 140 years." In fact, they found that the SMB decline over the decade 1995-2005 was no different from that of the decade 1923-1933. Therefore, "based on the simulations of these two periods," according to Wake et al., "it could as well be stated that the recent changes that have been monitored extensively (Krabill et al., 2004; Luthcke et al., 2006; Thomas et al., 2006) are representative of natural sub-decadal fluctuations in the mass balance of the ice sheet and are not necessarily the result of anthropogenic-related warming."

Contemporaneously, Ettema et al. (2009) applied a regional atmospheric climate model over a domain that included the Greenland Ice Sheet and its surrounding oceans and islands at what they described as an "unprecedented high horizontal resolution (~11 km)," which for use over Greenland was coupled to a physical snow model that treated surface albedo as a function of snow/firn/ice properties, meltwater percolation, retention and refreezing. The atmospheric part of this model was forced at the lateral boundaries and the sea surface by the global model of the European Centre for Medium-Range Weather Forecasts for the period September 1957 to September 2008. This work revealed the "total annual precipitation in the Greenland ice sheet for 1958-2007 to be up to 24% and surface mass balance up to 63% higher than previously thought," with the largest differences occurring in coastal southeast Greenland, where the seven scientists said that the much higher-resolution facilitates captured snow accumulation peaks that past five-fold coarser resolution regional climate models missed.

Averaged over the entire study period, the total ice sheet's SMB was 469 41 Gt per year; and before 1990 none of the mass balance components exhibited a significant trend. Since 1990, however, there has been a slight downward trend in Greenland's SMB of 12 4 Gt per year, which is probably not all that significant, considering the fact that over the one-year-period 1995 to 1996 its SMB rose by a whopping 250%. With respect to the stability/longevity of the Greenland Ice Sheet, therefore, Ettema et al. state that "considerably more mass accumulates on the Greenland Ice Sheet than previously thought, adjusting upwards earlier estimates by as much as 63%," which suggests that the Northern Hemisphere's largest ice sheet may well hang around a whole lot longer than many climate alarmists have been willing to admit.

In the Southern Hemisphere, Cofaigh et al. (2001) analyzed five sediment cores from the continental rise west of the Antarctic Peninsula and six from the Weddell and Scotia Seas for their ice rafted debris (IRD) content in an attempt to determine if there are Antarctic analogues of the Heinrich layers of the North Atlantic Ocean, which testify of the repeated collapse of the eastern margin of the Laurentide Ice Sheet and the concomitant massive discharge of icebergs. This they did because if such IRD layers exist around Antarctica, they reasoned they would be evidence of "periodic, widespread catastrophic collapse of basins within the Antarctic Ice Sheet," which could obviously occur again. After carefully analyzing their data, however, they concluded that "the ice sheet over the Antarctic Peninsula did not undergo widespread catastrophic collapse along its western margin during the late Quaternary," and that this evidence "argues against pervasive, rapid ice-sheet collapse around the Weddell embayment over the last few glacial cycles." Therefore, if there was no dramatic break-up of the Antarctic Ice Sheet "over the last few glacial cycles," there's a good chance there will also be none before the current interglacial ends. And since the data of Petit et al. (1999) indicate that each of the last four interglacials were warmer than the current one -- and by an average of more than 2C -- we can make that good chance an extremely good chance.

Two years later, Stone et al. (2003) collected and determined cosmogenic 10Be exposure dates of glacially-transported cobbles in elevation transects on seven peaks of the Ford Ranges that are located between the present grounding line of the West Antarctic Ice Sheet (WAIS) and the Clark Mountains some 80 km inland. Based on these ages and the elevations at which the cobbles were found, they determined a history of ice-sheet thinning over the past 10,000-plus years. This work revealed, in their words, that "the exposed rock in the Ford Ranges, up to 700 meters above the present ice surface, was deglaciated within the past 11,000 years" and that "several lines of evidence suggest that the maximum ice sheet stood considerably higher than this." They also report that the consistency of the exposure age versus elevation trends of their data "indicates steady deglaciation since the first of these peaks emerged from the ice sheet some time before 10,400 years ago" and that the mass balance of the region "has been negative throughout the Holocene."

Continuing, Stone et al. remarked that their results "add to the evidence that West Antarctic deglaciation continued long after the disappearance of the Northern Hemisphere ice sheets and may still be under way," and they report that the ice sheet in Marie Byrd Land "shows the same pattern of steady Holocene deglaciation as the marine ice sheet in the Ross Sea," where ice "has thinned and retreated since 7000 years ago," noting further that "there is strong evidence that the limit of grounded ice in both regions -- and in Pine Island Bay -- is still receding." Thus, they concluded that "the pattern of recent change is consistent with the idea that thinning of the WAIS over the past few thousand years is continuing," and in a commentary on Stone et al.'s work, Ackert (2003) makes this point even plainer, when he says that "recent ice sheet dynamics appear to be dominated by the ongoing response to deglacial forcing thousands of years ago, rather than by a recent anthropogenic warming or sea level rise."

After three more years, using measurements of time-variable gravity from the GRACE satellites, Velicogna and Wahr (2006) determined mass variations of the Antarctic Ice Sheet for the 34 months between April 2002 and August 2005. This work suggested that "the ice sheet mass decreased significantly, at a rate of 152 80 km3/year of ice, equivalent to 0.4 0.2 mm/year of global sea level rise," all of which mass loss came from the West Antarctic Ice Sheet, since they calculated the East Antarctic Ice Sheet mass balance to be 0 56 km3/year. What these results imply about the real world, however, is highly dependent upon their ability to truly represent what they presume to describe; and in this regard Velicogna and Wahr say there is "geophysical contamination ... caused by signals outside Antarctica," including "continental hydrology ... and ocean mass variability." And in addition to these problems, they 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)."

Estimates and adjustments to deal with these several problems are convoluted and complex, as well as highly dependent upon various models. In addition, the estimates and adjustments concern some huge entities, as Velicogna and Wahr acknowledge that "the PGR contribution is much larger than the uncorrected GRACE trend." In fact, their calculations indicate that the PGR contribution exceeds that of the signal being sought by nearly a factor of five! And they are forced to admit that "a significant ice mass trend does not appear until the PGR contribution is removed."

In light of the latter humungously confounding problem, Velicogna and Wahr rightly state that "the main disadvantage of GRACE is that it is more sensitive than other techniques to PGR." In fact, considering the many other adjustments they had to make, based upon estimations utilizing multiple models and databases with errors that had to be further estimated, one can have little confidence in their final result, particularly in light of the fact that it did not even cover a full three-year period. Much more likely to be much more representative of the truth with respect to Antarctica's mass balance are the findings of Zwally et al. (2005), who determined Antarctica's contribution to mean global sea level over a recent nine-year period to be only 0.08 mm/year compared to the five-times-greater value of 0.4 mm/year calculated by Velcogna and Wahr.

A few months later, Ramillien et al. (2006) derived new estimates of the mass balances of the East and West Antarctic Ice Sheets that were also based on GRACE data, but which pertained to the somewhat shorter period of July 2002 to March 2005, obtaining some significantly different ice sheet mass balances than those obtained by Velicogna and Wahr: a loss of 107 23 km3/year for West Antarctica and a gain of 67 28 km3/year for East Antarctica, which results yielded a net ice loss for the entire continent of only 40 km3/year (which translates to a mean sea level rise of 0.11 mm/year), as opposed to the 152 km3/year ice loss calculated by Velicogna and Wahr (which translates to a nearly four times larger mean sea level rise of 0.40 mm/year). Thus, the Ramillien et al. mean sea level rise of 0.11 mm/year was much less ominous and of the same order of magnitude as the 0.08 mm/year Antarctic-induced mean sea level rise calculated by Zwally et al. (2005), which was derived from ice surface elevation changes based on nine years of satellite radar altimetry data obtained from the European Remote-sensing Satellites ERS-1 and -2.

In an attempt to bring together much of this information, plus the findings of still other studies that pertain to both polar regions of the planet, as well as to determine what it all implies about sea level globally, Shepherd and Wingham (2007) reviewed what was known about sea-level contributions arising from the wastage of the Greenland and Antarctic Ice Sheets, concentrating on the results of 14 satellite-based estimates of the imbalances of the polar ice sheets that had been derived since 1998. These studies were of three major types -- standard mass budget analyses, altimetry measurements of ice-sheet volume changes, and measurements of the ice sheets' changing gravitational attraction -- and they yielded a diversity of values, ranging from a sea-level rise equivalent of 1.0 mm/year to a sea-level fall equivalent of 0.15 mm/year.

Of the three major approaches, the results of the latter technique were said by Shepherd and Wingham to be "more negative than those provided by mass budget or altimetry." And why is that? It is because, as they describe it, the gravity-based technique "is [1 ] new, and [2] a consensus about the measurement errors has yet to emerge, [3] the correction for postglacial rebound is uncertain, [4] contamination from ocean and atmosphere mass changes is possible, and [5] the results depend on the method used to reduce the data." In addition, they say that (6) the GRACE record is only three years long, and that (7) it is thus particularly sensitive to short-term fluctuations in ice sheet behavior that may not be indicative of what is occurring over a much longer timeframe. Even including these likely ice-wastage-inflating properties and phenomena, however, the two researchers concluded that the current "best estimate" of the contribution of polar ice wastage to global sea level change was a rise of 0.35 millimeters per year, which over a century amounts to only 35 millimeters or a little less than an inch and a half.

Yet even this unimpressive sea level increase may be too large, for although two of Greenland's largest outlet glaciers doubled their rates of mass loss in less than a year back in 2004, causing many climate alarmists to claim that the Greenland Ice Sheet was responding much more rapidly to global warming than anyone had ever expected, Howat et al. (2007) reported that the two glaciers' rates of mass loss "decreased in 2006 to near the previous rates." And these observations, in their words, "suggest that special care must be taken in how mass-balance estimates are evaluated, particularly when extrapolating into the future, because short-term spikes could yield erroneous long-term trends."

Consequently, the most reliable data related to losses of ice from Greenland and Antarctica suggest that the global sea level rise over the current century should be a whole lot smaller than the "meters" predicted by the U.S. National Oceanic and Atmospheric Administration's James Hansen in testimony presented to the Select Committee of Energy Independence and Global Warming of the U.S. House of Representatives on 26 April 2007, which in turn implies that resultant coastal flooding around the world may not even be considered "flooding" -- if it ever occurs at all! -- based on the best science of our day.

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