Sharply increased mass loss from glaciers and ice

LETTER

doi:10.1038/nature10089

Sharply increased mass loss from glaciers and ice caps in the Canadian Arctic Archipelago

Alex S.Gardner 1,2,Geir Moholdt 3,4,Bert W outers 5,Gabriel J.W olken 6,David O.Burgess 7,Martin J.Sharp 1,J.Graham Cogley 8,Carsten Braun 9&Claude Labine 10

Mountain glaciers and ice caps are contributing significantly to pre-sent rates of sea level rise and will continue to do so over the next century and beyond 1–5.The Canadian Arctic Archipelago,located off the northwestern shore of Greenland,contains one-third of the glo-bal volume of land ice outside the ice sheets 6,but its contribution to sea-level change remains largely unknown.Here we show that the Canadian Arctic Archipelago has recently lost 6167gigatonnes per year (Gt yr 21)of ice,contributing 0.1760.02mm yr 21to sea-level rise.Our estimates are of regional mass changes for the ice caps and glaciers of the Canadian Arctic Archipelago referring to the years 2004to 2009and are based on three independent approaches:surface mass-budget modelling plus an estimate of ice discharge (SMB 1D ),repeat satellite laser altimetry (ICESat)and repeat satellite gra-vimetry (GRACE).All three approaches show consistent and large mass-loss estimates.Between the periods 2004–2006and 2007–2009,the rate of mass loss sharply increased from 3168Gt yr 21to 92612Gt yr 21in direct response to warmer summer temperatures,to which rates of ice loss are highly sensitive (64614Gt yr 21per 1K increase).The duration of the study is too short to establish a long-term trend,but for 2007–2009,the increase in the rate of mass loss makes the Canadian Arctic Archipelago the single largest contri-butor to eustatic sea-level rise outside Greenland and Antarctica.Several long-term records (about 50years)of the surface mass budget (surface accumulation minus surface ablation)of individual glaciers and ice caps exist for the Canadian Arctic Archipelago (CAA,see Fig.1)7,8,but extrapolation of these records to estimate the mass budget of the entire region introduces a large uncertainty.Repeat airborne laser alti-metry surveys have been used to estimate that the glaciers of the CAA lost 23Gt yr 21of ice between spring 1995and spring 2000(ref.9).This represents 0.063mm yr 21of sea-level rise if we take the global area of the ocean to be 362.53106km 2(ref.10).Since 2000the CAA has experi-enced some of the warmest summer temperatures on record,with four of the five warmest years since 1960occurring after 2004(Supplemen-tary Information).Between 2005and 2009all CAA glaciers with long-term monitoring programmes 7,8experienced their most negative five-year period of surface mass budget since measurements began in the early 1960s.Here we present three independent estimates of change in total glacier mass between autumn 2003and autumn 2009for the northern CAA (Fig.1;area 106,400km 2)and two independent estimates for the southern CAA (Fig.1;area 42,000km 2).

The first estimate is derived using a numerical model that simulates the regional mass change resulting from the surface mass budget.Ice discharge due to the calving of icebergs from glaciers that terminate in the sea,denoted D ,is added to the surface mass-budget model results to account for the total regional ice loss (model SMB 1D )(Supplemen-tary Information).The model is not applied to the southern CAA because there are too few records of glacier mass budget and near-surface temperature with which to calibrate the model.The second

estimate derives mass change from the change in land-ice volume measured using repeat laser altimetry from the Ice,Cloud and Land Elevation Satellite (ICESat)11.The third estimate is derived using repeat gravity observations collected by the Gravity Recovery and Climate Experiment (GRACE)satellites.The three methods are inde-pendent and produce consistent estimates of changes in glacier mass for the years 2004to 2009(Fig.2),where each year refers to the mass-budget year starting in the autumn of the previous calendar year.All estimates are given as the mean 62s (95%confidence interval).

In general,the CAA receives low amounts of precipitation (100–300kg m 22yr 21)with locally higher rates (300–1,000kg m 22yr 21)

1

Department of Earth and Atmospheric Sciences,University of Alberta,Edmonton,Alberta,T6G 2E3,Canada.2Department of Atmospheric,Oceanic and Space Science,University of Michigan,Ann Arbor,Michigan 48109,USA.3Department of Geosciences,University of Oslo,N-0316Oslo,Norway.4Institute of Geophysics and Planetary Physics,Scripps Institution of Oceanography,La Jolla,California 92093,USA.5The Royal Netherlands Meteorological Institute,NL-3730AE De Bilt,Netherlands.6Division of Geological and Geophysical Surveys,Alaska Department of Natural Resources,Fairbanks,Alaska 99709,USA.7Geological Survey of Canada,Ottawa,Ontario,K1A 0E8,Canada.8Department of Geography,Trent University,Peterborough,Ontario,K9J 7B8,Canada.9Department of Geography and Regional Planning,Westfield State University,Westfield,Massachusetts 01086,USA.10Campbell Scientific Canada Corp.,Edmonton,Alberta,T5M 1W7,Canada.

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Figure 1|Glaciers and ice caps of the Canadian Arctic Archipelago.Black dashed lines delineate the northern and southern study regions.The main panel is an enlargement of the red rectangle superimposed on the map of the Arctic (inset).

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concentrated on the east-facing slopes flanking Baffin Bay (Fig.1).Surface air temperatures over ice masses in the region exceed the freezing point during only two to three months of the year.Because there is generally low interannual variability in precipitation and high variability in melt production,interannual variability in the regional surface mass budget is largely governed by changes in the summer surface energy budget 7.These are strongly correlated with summer surface air temperatures 12–14,which are,in turn,highly dependent on local synoptic conditions 15,16.In this study we apply a surface mass-budget model that determines surface melt using the temperature-index method 17,18.The model is forced with downscaled 19and bias-corrected temperature and precipitation fields from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis (Supplementary Information).For the years 2004to 2009the modelled mass loss from the surface mass budget (SMB)plus ice dis-charge (D ),where D 54.661.9Gt yr 21(Supplementary Information),of the northern CAA was 34613Gt yr 21(Fig.3).The average mass loss from the northern CAA was 7618Gt yr 21for the years 2004to 2006,increasing to 61618Gt yr 21for the years 2007to 2009with a peak loss of 79630Gt yr 21in 2008.The difference between the two periods is primarily due to a 42Gt yr 21increase in melt production,which resulted from regionally warmer summer air temperatures in the lower troposphere.Warmer temperatures also contributed to a 7%decrease in snow fraction.A slight decrease in annual precipitation amount,and changes in the amount of meltwater retained by the annual snowpack,contributed another 12Gt yr 21to the increased mass loss.

For both the northern and southern CAA,we derived elevation changes from ICESat’s Geoscience Laser Altimeter System (GLAS)for the period 2003–2009(ref.20).Elevation changes are estimated relative to rectangular planes that are fitted to 700-m-long segments of near-repeat-track data 21.The planes represent a simplified surface topography such that multi-temporal elevation measurements that are slightly offset in location can be compared.We then extrapolate elevation changes to volume changes and convert them to mass

changes using a plausible range of firn and ice densities (Supplemen-tary Information).For the years 2004to 2009,ICESat results show that the northern CAA lost 3767Gt yr 21and that the southern CAA lost 2466Gt yr 21.ICESat results show increases in mass loss between 2004–2006and 2007–2009of 39Gt yr 21and 14Gt yr 21for the northern and southern CAA,respectively.Recent observations in both Alaska 22and Greenland 23have found that marine-terminating glaciers are thinning more rapidly than land-terminating glaciers.To assess whether the same phenomenon is occurring in the CAA,we separately determined elevation changes for marine-and land-terminating glacier basins (Supplementary Information).Our results show no dif-ference in the area-averaged rate of elevation change between the two basin types,suggesting that total ice discharge from marine-termin-ating glaciers has not accelerated in recent years.This gives increased confidence in both the extrapolation of ICESat elevation changes and our estimate of ice discharge.

C u m u l a t i v e m a s s c h a n g e (G t )

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Figure 2|Cumulative change in glacier mass between autumn 2003and autumn 2009.Separate estimates are provided for the northern (a )and southern (b )CAA.Error bars represent the 95%confidence interval.

–2,000–1,500–1,0000500

–500Mass budget (kg m –2 yr –1)

Figure 3|Modelled surface mass budget of the northern CAA between autumn 2003and autumn 2009.The model resolution of 0.5km allows us to resolve the highly negative surface mass budgets of the outlet-glacier tongues.

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Lastly,we derived mass changes for both the northern and southern CAA from GRACE gravity measurements.Mass-change estimates from GRACE agree very well with the other two data sets for the northern CAA,with an average mass loss between2004and2009of 3969Gt yr21.The observations confirm the sharp increase in northern CAA mass loss between2004–2006and2007–2009,with an increase in the average mass loss of60Gt yr21.The southern CAA is estimated to have lost ice at an average rate of2467Gt yr21over the six-year study period,with a16Gt yr21increase in the rate of loss between the first three and last three years,and is in very good agreement with ICESat. The most likely sources of the disagreement between the three methods are:uncertainties in constraining the terrestrial water storage in the GRACE estimates,the identification of the appropriate end-of-season mass change in the GRACE signal,and fewer ICESat elevation retrievals in2009(Supplementary Information).

The error-weighted mean of all mass-change estimates gives a total mass loss for the CAA of368641Gt or1.0160.11mm sea-level rise for the years2004to2009.Most of the mass loss came from the northern CAA,which lost224630Gt,with the remaining144628Gt coming from the southern CAA(see Supplementary Figs1–3for a further subdivision of the mass losses within the northern and southern CAA).We estimate that the majority of the mass loss(about92%)is due to meltwater runoff,with a much smaller contribution coming from ice discharge from marine-terminating glaciers(about8%). Three-quarters of all mass loss occurred in the last three years of the observation period with an average loss of92612Gt yr21,or 0.2560.03mm yr–1sea-level rise.This rate is four times greater than the estimated mass loss for CAA over the period1995to2000(ref.9). This increase in mass loss is in direct response to warmer surface air temperatures in summer,to which the glaciers of the CAA have a high sensitivity.Over the six-year period of our study an additional 64614Gt yr21of ice was lost to the oceans for every1K rise in mean summer surface air temperature.Dividing by the total glacier area gives an area-averaged temperature sensitivity of2430690kg m22yr21K21, which is two times larger than estimated from glacier surface mass-budget records2,24,25and is close to sensitivities estimated from regional climatology2.The sensitivity to precipitation is much smaller;a10% increase in precipitation would result in a mass gain of only about 5Gt yr21.Such a low sensitivity to precipitation is in contrast to gla-ciers located in wet maritime regions.For example a10%increase in precipitation over the Patagonia icefields,which have a combined ice area that is one-tenth the size of the CAA,would result in a12Gt yr21 gain of mass26.

To put the mass losses occurring in the CAA into a global per-spective,the Patagonia icefields lost ice at an average rate of 28611Gt yr21between April2002and December2006(ref.27)with little change in the ice-loss trend for the years2007to2009(J.Chen, personal communication).The glaciers of the Gulf of Alaska lost mass at an average rate of88615Gt yr21for the years2004to2006,slow-ing to70611Gt yr21for the years2007to2009(update to ref.28). The sharp increase in mass loss from the CAA and the slowdown in loss from the Gulf of Alaska makes the CAA the largest contributor to eustatic sea level rise outside Greenland and Antarctica for the years 2007–2009.Because of the high sensitivity to temperature and low sensitivity to precipitation,the CAA is expected to continue to be one of the largest contributing regions to eustatic sea level rise well into the next century and beyond5.

METHODS SUMMARY

The surface mass-budget model was run at a resolution of500m by500m for the period1949to2009(Supplementary Information).Model results are validated against observations and agree well with in situ point surface mass-budget measure-ments(Supplementary Fig.4:r50.86,N53,717,standard error5350kg m22). For the four regions with well-established surface mass-budget measurement pro-grammes(Agassiz Ice Cap,north-western Devon Ice Cap,Meighen Ice Cap and White Glacier7,8)the model has a very low bias(218kg m22yr21)in the glacier-averaged surface mass budget(Supplementary Information).To be consistent with the other data sets presented in this study,we discuss only mass changes modelled over the ICESat and GRACE operational period between autumn2003and autumn2009.

To recover mass changes from the GRACE measurements we use forward model-ling of mass changes in predefined basins,minimizing the least-squares difference between GRACE observations and the forward model in an iterative method (Supplementary Information and refs29and30).To avoid biases from surrounding areas(Supplementary Fig.1)as a result of the limited spatial resolution and integral character of the GRACE observations,mass changes are modelled for the Greenland Ice Sheet and other areas surrounding the CAA.GRACE measurements were made available by the Center for Space Research(CSR version RL04)and were down-loaded from https://www.wendangku.net/doc/e69278985.html,/DATA_CATALOG/graceinfo.html. More details about the data and methods can be found in the Supplementary Information.

Received23November2010;accepted4April2011.

Published online20April2011.

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Supplementary Information is linked to the online version of the paper at

https://www.wendangku.net/doc/e69278985.html,/nature.

Acknowledgements We thank A.Arendt for reviewing the manuscript and

S.Luthcke and A.Arendt for providing the updated glacier mass anomalies for Alaska.We thank H.Blatter,W.Colgan,E.Dowdeswell,M.Huss,S.Marshall and D.Mueller for contributing observational data sets.We thank R.Riva and P.Stocchi for providing glacial isostatic adjustment models.This work was supported by funding to A.S.G.from NSERC Canada and the Alberta Ingenuity Fund,funding to G.M.by the European Union7th Framework Program(grant number226375) through the ice2sea programme(contribution number017),and funding to M.J.S. from NSERC and CFCAS(through the Polar Climate Stability Network).The SMB modelling was conducted using the infrastructure and resources of AICT of the University of Alberta.

Author Contributions A.S.G.developed the study and wrote the paper.A.S.G,G.M.and B.W.all contributed equally to the analysis,using SMB1D,ICESat and GRACE, respectively.G.J.W.provided ice and basin outlines,model topography and created Fig.

1.The remaining authors provided in situ measurements.All authors discussed and commented on the manuscript at all stages.

Author Information Reprints and permissions information is available at

https://www.wendangku.net/doc/e69278985.html,/reprints.The authors declare no competing financial interests. Readers are welcome to comment on the online version of this article at

https://www.wendangku.net/doc/e69278985.html,/nature.Correspondence and requests for materials should be addressed to A.S.G.(alexsg@https://www.wendangku.net/doc/e69278985.html,).

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