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Greenland Ice Sheet Mass Balance
E. Hanna1, J. Box2, P. Huybrechts3
1Department of Geography, University of Sheffield, UK
Recent relatively high summer temperatures (1995-2005) are associated with increased net ice loss over Greenland. Recent warm events are about the same magnitude, if not smaller, than those of the early 20th century warm period (1918-1947). 2006 was not as warm as other recent years such as 2003 or 2005. Physical response mechanisms, such as hydraulic acceleration of the ice sheet from continued warming, remain incompletely understood.
The Greenland ice sheet (GrIS) contains 7.4m global sea-level equivalent and is vulnerable to mass loss by climate warming (Gregory et al. 2004). Observational studies have provided many insights into recent GrIS mass balance changes. Airborne and satellite laser-altimetry data analyses indicate a volume loss of about 60 km3 yr-1 in the 1993/4 - 1998/9 period, that increased to about 80 km3 yr-1 in 1997-2003 (Krabill et al. 2000, 2004, Thomas et al. 2006). Various recent analyses of gravimetric (GRACE) satellite data suggest greater mass (volume) losses in the 101-226 Gt yr-1 (111-248 km3 yr-1) range within the recent few years, that is, 2002-2006 (Chen et al. 2006, Luthcke et al. 2006, Ramillien et al. 2006, Velicogna and Wahr 2006). The largest mass losses are generally indicated as being from low elevations (<~2000 m) and especially in southeast Greenland, with partly compensating mass gains at higher elevations (>2000 m) (Luthcke et al. 2006). Other altimetry data suggests that there appears to have been substantial growth of the GrIS interior from 1992-2003 (Johannessen et al. 2005, Thomas et al. 2005, Zwally et al. 2005), which may be partly attributable to increased atmospheric moisture and precipitation and/or shifting storm tracks forced by enhanced greenhouse gases (IPCC 2007).
Satellite radar interferometry (InSAR) reveals widespread acceleration of Greenland glaciers, a pattern progressing northward since 1996, with an accompanying doubling of the ice sheet's volume deficit from approximately 90 to 220 km3 yr-1 (Rignot and Kanagaratnam 2006), although it is as yet too early to say just how exceptional are these changes. The recent accelerations of Kangerdlussuaq and Helheim (outlet) glaciers appear to be driving the recent (2001-2006) concentration of mass loss in south-east Greenland (Stearns and Hamilton 2007); however, the velocities of these glaciers appear to have decreased again in 2006 to near their previous rates (Howat et al. 2007). Some of these margin changes may signal a response to recent climatic warming through infiltration of surface meltwater in a basal dynamic positive feedback mechanism (Zwally et al. 2002, Parizek and Alley 2004). Alternatively, thinning and breakup of Jakobshavn Glacier's (SW Greenland) floating ice tongue and acceleration of the glacier itself—as well as that of Helheim Glacier in south-east Greenland—may have been induced, or at least influenced, by forcing from the ocean (Thomas et al. 2003, Howat et al. 2005, Bindschadler 2006).
Taking all approaches together, the general consensus is an accelerating mass loss of the GrIS over the past decade 1995-2005 (IPCC 2007 Ch. 4). A recent survey concludes that the GrIS is currently losing ~100 Gt yr-1 (Shepherd and Wingham 2007). However, there remains considerable discrepancy among these pioneering observational estimates. Most of the observational studies have data spans of less than a decade, which also means that the interpretation of their results may be seriously affected by large year-to-year variability in GrIS mass turnover, e.g. sudden glacier accelerations (Rignot and Kanagartnam 2006). Since the Helheim and Kangerdlugsuaq glaciers have been shown to accelerate and decelerate over just a few years (Howat et al. 2007), these 'speed-ups' may just represent flow variability on interannual time scales and therefore represent the 'weather' rather than the 'climate' of the GrIS (Shepherd and Wingham 2007, Howat et al. 2007). All in all, we cannot be certain that the apparently accelerating GrIS mass loss and unexpected, rapid changes in its outlet glaciers, represent a profound change in ice-sheet behaviour (Murray 2006). The above studies alone, therefore, have yet to provide a convincing broad temporal perspective on how the GrIS might be responding to long-term climatic change, most notably the evident global and regional warming since the 1970s (IPCC 2007).
Surface Mass Balance
Surface mass balance (SMB), essentially net snow accumulation minus meltwater runoff, time series are available (e.g., Hanna et al. 2005; Box et al. 2006), which can help place the remotely sensed results into a longer, multi-decadal, climatic perspective. These series are based on meteorological models that assimilate observations for calibration and verification. There is good agreement of respective annual precipitation and runoff values from the two independently-derived SMB series for the period of published overlap (1988-2004). However, by definition, neither SMB series takes into account the mass losses from iceberg calving and basal melting, for which only crude estimates can currently be given. Iceberg calving is roughly equivalent to the amount of annual runoff whereas the basal melting is probably relatively small for Greenland.
Polar atmospheric model calculations after Box et al. (2006) suggest that 2006 was a normal year for snow accumulation for the ice sheet as a whole (Table 1), accumulation being the difference of solid plus liquid precipitation and surface-water-vapor exchanges. A positive 2006 meltwater production anomaly, however, exceeded one standard deviation of the 1988-2004 mean. The extreme year 2005 was excluded to facilitate comparison with Box et al. (2006) and does not significantly change our conclusion that 2006 had likely abnormally negative SMB. Runoff rates were apparently 57% above this recent 17-year mean. The accumulation-area ratio was below the value of recent decades, owing to slightly below normal precipitation and an apparent expansion of the ablation zone. Enhanced 2006 melt rates are also suggested by above normal temperatures, in particular for the southern part of the ice sheet south of 64° N, where MODIS satellite data processed using the Liang et al. (2005) method suggest low-albedo (dark surface) anomalies relative to the 2000-2006 base period.
* unless otherwise indicated
A 49-year (1958-2006) SMB series, updated and recalibrated from Hanna et al. (2005), reveals 1998, 2003 and 2006 as, respectively, the first, second and third highest runoff years (Fig. 1). There is a statistically significant underlying trend-line increase in runoff from 1958-2006 of 113.0 km3 yr-1 (equivalent to 40.0% of mean 1958-2006 runoff, compared with a standard deviation of 24.3% of the mean). Moreover, the five highest runoff years have all occurred since and including 1995, and five of the nine highest runoff years since 2001 inclusive. Greenland ice sheet precipitation follows a significantly increasing trend of 90.9 km3 or +14.9% over this 49 year period, compared with a standard deviation of 69.7 km3 yr-1 or 11.4% (Fig. 1). Additional precipitation, mainly in the form of snow accumulation, therefore largely (about three quarters) offsets rising Greenland runoff in terms of the SMB. There is thus a relatively small and insignificant negative trend in SMB of -22.1 km3 yr-1 ( = 104.8 km3 yr-1) from 1958-2006, highlighting the sensitive balance between increased snow accumulation in the interior of the ice sheet and increased runoff around the edges. The further mass loss from ice dynamics due to accelerated flow of outlet glaciers was at least several times larger for the recent few warmest years (Rignot and Kanagaratnam 2006).
Recent high snow accumulation events occurred in winter 2004/05, concentrated in west Greenland (Nghiem et al. 2007), and winter-spring 2002/03 in SE Greenland (Krabill et al. 2004; Box et al. 2005; Hanna et al. 2006). On the other hand, 2006 was the sixth lowest precipitation year in the 49-year ECMWF Greenland record after Hanna et al. (2006), which together with the high 2006 runoff, resulted in the second-lowest annual ice sheet net mass input (SMB) since 1958.
Notably, our SMB data do not agree with model predictions that suggest increased Greenland accumulation may be outweighed by rising runoff in a warmer climate (Huybrechts et al. 2004). However, according to the mechanism proposed by Zwally et al. (2002), the additional meltwater that we observe may already be more readily reaching the bed of the GrIS and prompting accelerated flow of Greenland outlet glaciers—likely a substantial fraction of the enhanced flow detected by Rignot and Kanagaratnam (2006). Such amplification might explain the significant increases in overall mass loss strongly suggested by the consensus of GrIS mass balance estimates for the past decade (IPCC 2007 Ch. 4) but is far from proven. However, since some recent key glacier accelerations in east Greenland have already subsided (Howat et al. 2007), the sustainability of the enhanced flow is questionable. If the Zwally effect is dominant (which very much remains to be shown), then flow variability is of course directly connected to surface-runoff variability.
Surface Air Temperature
Southern Greenland temperature changes since the early 1990s in summer reflect general northern hemisphere and global warming. This differs from the Greenland warming phase between 1918 and 1947 when there was less apparent linkage between Greenland and the northern hemisphere average. According to a composite record of seven coastal Greenland stations south of 70° N latitude, summer 2003 was the warmest since 1958 (1958 marks the start of the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-40 climate reanalysis and hence the Hanna et al. (2005, 2007) modelled surface mass balance series). The second warmest summer, in 2005, had the most extensive anomalously warm conditions over the ablation zone of the ice sheet. Spring (MAM) 2006 was the third warmest in coastal southern Greenland since 1958 (Table 2, data from Cappelen et al. 2007). 2006 statistics for the southeast Greenland station Tasiilaq do not suggest abnormally warm air temperatures and may be affected by above normal spring and summer sea-ice concentration anomalies (http://nsidc.colorado.edu/data/seaice_index/).
Anomalies and Z-scores are computed with respect to the 1971-2000 base period.
Extreme cold years including 1884, 1992, 1993, 1983, 1984, are associated with volcanic cooling (Box, 2002; Hanna et al. 2005). Other record-setting cold years, e.g. 1887, or 1899, may result from other factors than volcanism, such as positive sea ice concentration anomalies.
Significant increases 1958-2006 in Greenland margin summer temperatures and runoff, record-high 2003 summer temperature and 2005 snowmelt records, and a highly significant correlation of recent Greenland with Northern Hemisphere temperatures, collectively suggest that an expected signal of the GrIS to global warming may be emerging (Hanna et al. 2007). This signal now appears to be distinct from natural/regional climatic fluctuations, such as those related to changes in the North Atlantic Oscillation (e.g. Hanna & Cappelen 2003).
The new Greenland summer warmth and snowmelt records are consistent in timing with recent increased losses of summer Arctic sea ice (e.g. Comiso 2006, Richter-Menge et al. 2006). Indeed, reduced extent and duration of winter sea-ice should expose Greenland to enhanced warm air advection from surrounding seas, lengthening snowmelt and runoff seasons and possibly enhance snow accumulation, the latter a negative feedback for ice sheet response to climate warming.
Considering available station data that are continuous and begin before 1900 (Table 3), the year 2006 is not outstanding. In this longer perspective, only 2003 at Tasiilaq is outstanding in recent decades. Over the past century, years in Greenland that register as abnormally warm, 1929, 1932, 1941, 1947, and 1960 are outstanding, having temperatures warmer than observed recently. Increases in GrIS melt and runoff during this past century warm period must have been significant and were probably even larger than that of the most recent last decade (1995-2006).
Anomalies and Z-scores are computed with respect to the 1971-2000 base period.
John Cappelen kindly provided updated Greenland temperature data in advance of the relevant publication listed below.
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