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Climatological Estimates of Greenland Ice Sheet Sea Level Contributions: Recent Past and Future

Climatological Estimates of Greenland Ice Sheet Sea Level Contributions: Recent Past and Future. J. E. Box Byrd Polar Research Center. Understanding Sea-level Rise and Variability WCRP Workshop Global Earth Observation System of Systems (GEOSS)

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Climatological Estimates of Greenland Ice Sheet Sea Level Contributions: Recent Past and Future

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  1. Climatological Estimates of Greenland Ice Sheet Sea Level Contributions: Recent Past and Future J. E. Box Byrd Polar Research Center Understanding Sea-level Rise and Variability WCRP Workshop Global Earth Observation System of Systems (GEOSS) UNESCO, Place Fontenoy - Paris, France (Room XII)

  2. Spatial/Temporal Reconstructions • Temperature and Accumulation • Seasonal and Annual • Empirical functions based on correlation of Polar MM5 24 km gridded results with: • Coastal station temperature records; • Annual accumulation rates from ice cores • PARCA • Outlet Glacier Discharge • Annual • Empirical functions based on seasonal coastal temperature correlation with outlet glacier discharge • Rignot, E., and P. Kanagaratnam (2006), Changes in the velocity structure of the Greenland Ice Sheet, Science, 311, 986-990.

  3. Temperature Records • Coastal • Monthly, Seasonal, Annual * Vinther, B. M., K. K. Andersen, P. D. Jones, K. R. Briffa and J. Cappelen, Extending Greenland Temperature Records into the late 18th Century, doi:10.1029/2005JD006810, JGR, in press 2006. ** GISTEMP (NASA), Hansen et al.

  4. Polar MM5 - Bromwich D.H., J. Cassano, T. Klein, G. Heinemann, K. Hines, K. Steffen, and J. E. Box, 2001: Mesoscale modeling of katabatic winds over Greenland with the Polar MM5, Monthly Weather Review, 129(9), 2290-2309. - Cassano, J., J.E. Box, D.H. Bromwich, L. Li, and K. Steffen, 2001: Verification of Polar MM5 simulations of Greenland's atmospheric circulation, Journal of Geophysical Research, 106(D24), 33867-33890. - Box, J.E., D. H. Bromwich, L-S Bai, 2004: Greenland ice sheet surface mass balance for 1991-2000: application of Polar MM5 mesoscale model and in-situ data, J. Geophys. Res., Vol. 109, No. D16, D16105, 10.1029/2003JD004451 - Box, J.E., D.H. Bromwich, B.A. Veenhuis, L-S Bai, J.C. Stroeve, J.C. Rogers, K. Steffen, T. Haran, S-H Wang, Greenland ice sheet surface mass balance variability (1988-2004) from calibrated Polar MM5 output, J. Climate, accepted Sept 27 2005.

  5. Polar MM5 Domains

  6. Polar MM5 Mass Budget Results (1988-2005)

  7. NEW • Box, J.E., D.H. Bromwich, B.A. Veenhuis, L-S Bai, J.C. Stroeve, J.C. Rogers, K. Steffen, T. Haran, S-H Wang, 2006, Greenland Ice Sheet Surface Mass Balance Variability (1988-2004) from Calibrated Polar MM5 Output, J. Climate, 19 (12), 2783-2800

  8. Methods: Statistical Regression • Explanatory (‘independent’) variables • long term record, such as coastal station temperatures or ice core accumulation rates • ‘dependent’ variables • Polar MM5 temperature • Polar MM5 accumulation rate • Polar MM5 training period • 1988-2005, (N = 18), Currently • 1958-2005, (N = 48), before July 2006 • Determine highest correlation rank for each grid cell vs explanatory data • Store regression coefficients (slope and intercept) for the top two ranked sites for each grid cell • Use full instrumental record to reconstruct dependent variables • Temperature, Accumulation, outlet glacier discharge • ice sheet temperatures • 1873-2004 available for annual means, (N = 132) • seasonal means ultimately

  9. Hypotheses • Surface Mass Balance • Ice cores can represent the spatial/temporal patterns of accumulation rate on annual to century timescales • Seasonal and annual temperatures explain seasonal to interannual the spatial/temporal patterns of glacial melt water production and runoff • Glacier Discharge • “There is an ice dynamics response to short term (monthly to inter-annual) surface climate” • Glacier discharge sensitivity to temperature is a useful predictor of past and future solid ice flux to oceans • Total Ice Sheet Mass Budgets and Global Eustatic Sea Level Forcing • Exploitation of significant correlations of surface mass balance and outlet glacier discharge provide useful estimates of ice sheet sea level forcing on annual to century time scales

  10. Summer Correlation Primary Secondary

  11. Two Site Seasonal Temperature Correlations

  12. Seasonal Temperature Reconstruction Results

  13. Seasonal Temperature ReconstructionValidation

  14. Annual Temperature Reconstruction Linear Trend: +0.045 C/ decade, 0.6 K increase over 132 years,r =0.380

  15. Annual Temperature Reconstruction 1873-2004 mean: -13.16 C Linear Trend:+0.053 C/ decade, 0.7 K increase over 132 years, r = 0.472

  16. Runoff Reconstruction (1866-2005) Based on Greenland Summer (JJA) Temperature Vs. Polar MM5 Runoff

  17. Surface Mass Balance Reconstruction (1866-2005) Using summer (JJA) temperature, R2= 0.75

  18. Greenland Ice Sheet Mass Budget Implications of Climate Warming • Runoff • +384 km3 y-1 K-1sensitivity • 1988-2005 baseline = 243 Gt y-1 • R2 = 0.673 • A 1K global temperature anomaly suggests a runoff of 627 km3 y-1 • Accumulation • +102 km3 y-1 K-1sensitivity • 1988-2005 baseline = 507 km3 y-1 • R2 = 0.237 • Surface Mass Budget • -282 km3 y-1 K-1sensitivity • 1988-2005 baseline = 264 km3 y-1 • R2 = 0.380 • A 1K global temperature anomaly suggests a -18 km3 y-1 surface mass balance • Negative surface mass budget if global climate warms additional 0.33 K • 1994-2005 global T anomaly 0.442 K (with respect to 1961-1990) base period • 2005 global T anomaly 0.667 K (with respect to 1961-1990) base period

  19. Surface Mass Balance Reconstruction (1866-2005) Using summer (JJA) temperature, R2= 0.75

  20. Sea Level Contribution Reconstructions: 1873-2004 Sea Level Contribution Reconstructions: 1873-2004 D Constant 1996-2005 JJA Discharge,SMB ANN Discharge,SMB ANN Discharge,SMB nonlinear (Const. D, Ann. SMB), (nonlin. D, Ann. SMB) 1920-1930:0.66 mmy-1 , 0.82 mmy-1 1960-1970:0.58 mmy-1 , 0.59 mmy-1 1994-2004:0.64 mmy-1 , 0.74 mmy-1 1873-2004:0.64 mmy-1 , 0.74 mmy-1

  21. Conclusions • Warming over the recent decade produced large positive melt anomalies, yet, climatological reconstruction suggests recent summer melt is not unprecedented, warm summers in 1950s and 1930s • Surface mass budget sensitivity to global temperature anomalies suggest that the Greenland Ice Sheet surface mass budget would become negative for a 1K global temperature anomaly. • 1994-2005 global temperature anomaly = 0.66 K • Past warming trends suggest this threshold reached within next 30-60 years. • Overall ice sheet mass budget would be ~-300 km3y-1 • A model that accounts for temperature thresholds in outlet glacier discharge produces a ~30% larger sea level contribution that one with discharge that is invariant with melt water flux • Correlation analysis suggests a link between outlet glacier discharge and air temperature • Future sea level contribution should therefore be more than 30% greater than recent estimates from Rignot and Kanagaratnam (2006). • This model is very sensitive to the temperature threshold used. • The temperature thresholds are unknown, but can be evaluated with historic temperature and discharge data.

  22. Questions • What mechanisms are most likely to lead to outlet glacier surge in response to warming? • What temperature threshold causes outlet glacier to surge?

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