In the introduction to their study of the surface mass balance of the Greenland Ice Sheet (GrIS), Bougamont et al. (2007) write that "for the 21st century, the predicted sea level contribution from the GrIS is +0.5 ± 0.4 mm/year, for all climate scenarios and a range of climate models," but they say that "these predictions use a Positive Degree Day Model (PDDM) to determine the surface mass balance (SMB), which determines the amount of melt using a temperature threshold only," while "a more physically-based approach is to use an energy balance and snowpack model (EBSM), which takes into account all the fluxes of heat at the surface but requires considerably more inputs to drive it." Hence, to investigate the importance of these differences, the eight researchers compared "a PDDM [Reeh, 1991] and an EBSM [Bougamont et al., 2005] to calculate the SMB of the GrIS in a warming climate as prescribed by a transient run of the Hadley Centre Climate Model version 3 (HadCM3)," with each model "first tuned to produce annual runoff rates close to estimates by Hanna et al. (2005)."

When all was said and done, Bougamont et al. (2007) found that "the PDDM has a larger response to the simulated climate warming than the EBSM (concurring with van de Wal, 1996), translating into more than a factor 2 difference [our italics] in the cumulative net surface mass," and "generating annual runoff rates almost twice as large [our italics]." Why? Because (1) the PDDM does not include parameterizations for changes in "lapse rates, specific humidity, winds and cloud cover associated with climate change," because (2) "important albedo feedbacks are not explicitly incorporated into a PDDM," and because of (3) the larger EBSM refreezing rates compared to those of the PDDM, which represents "an important component of the SMB (concurring, for example, with Janssens and Huybrechts, 2000)." With respect to this latter point, in fact, the research team writes that roughly half the difference in annual runoff rates generated by the two models "was due to differences in refreezing rates in the snowpack."

What are the implications of these findings?

Bougamont et al. (2007) write that, in view of their results, "large uncertainties in estimates of the future surface mass balance response of the ice sheet remain," and that "our ability to predict the future behavior of the GrIS is constrained not only by uncertainties in modeling ice dynamics but equally by our ability to adequately model the surface mass balance." In this regard, they further state that "assessing which model performs 'best'," was "not the purpose of this study." Nevertheless, their comments about the differences between the PDDM and EBSM models leave little doubt that the former (which predict greater meltwater runoff into the sea) are significantly inferior to the latter, which conclusion suggests that even the paltry 0.5 ± 0.4 mm/year of sea level rise predicted for the 21st-century by today's climate models (5 ± 4 cm, or about 2 ± 1.6 inches over the full hundred years) may be much greater than what might actually occur.

Sherwood, Keith and Craig Idso

References
Bougamont, M., Bamber, J.L., Ridley, J.K., Gladstone, R.M., Greuell, W., Hanna, E., Payne, A.J. and Rutt, I. 2007. Impact of model physics on estimating the surface mass balance of the Greenland ice sheet. Geophysical Research Letters 34: 10.1029/2007GL030700.

Bougamont, M., Bamber, J.L. and Greuell, W. 2005. A surface mass balance model for the Greenland ice sheet. Journal of Geophysical Research 110: 10.1029/2005JF000348.

Hanna, E., Huybrechts, P., Janssens, I., Cappelen, J., Steffen, K. and Stephens, A. 2005. Runoff and mass balance of the Greenland ice sheet: 1958-2003. Journal of Geophysical Research 110: 10.1029/2004JD005641.

Janssens, I. and Huybrechts, P. 2000. The treatment of meltwater retention in mass-balance parameterizations of the Greenland ice sheet. Annals of Glaciology 31: 133-140.

Reeh, N. 1991. Parameterization of melt rate and surface temperature on the Greenland ice sheet. Polarforschung 59: 113-128.

van de Wal, R.S.W. 1996. Mass-balance modeling of the Greenland ice sheet: A comparison of an energy-balance and a degree-day model. Annals of Glaciology 23: 36-45.