We estimate this relationship separately below and above the equilibrium line altitude (ELA, separating negative and positive SMB) and for regions north and south of 77° N, from a set of MAR simulations in which we alter the ice sheet surface elevation. These give four "SMB lapse rates", gradients that relate SMB changes to elevation changes. We assess uncertainties within a Bayesian framework, estimating probability distributions for each gradient from which we present best estimates and credibility intervals (CI) that bound 95% of the probability. Below the ELA our gradient estimates are mostly positive, because SMB usually increases with elevation: 0.56 (95% CI: ?0.22 to 1.33) kg m?3 a?1 for the north, and 1.91 (1.03 to 2.61) kg m?3 a?1 for the south. Above the ELA, the gradients are much smaller in magnitude: 0.09 (?0.03 to 0.23) kg m?3 a?1 in the north, and 0.07 (?0.07 to 0.59) kg m?3 a?1 in the south, because SMB can either increase or decrease in response to increased elevation.
Our statistically founded approach allows us to make probabilistic assessments for the effect of elevation feedback uncertainty on sea level projections (Edwards et al., 2014).
- Greenland ice sheet
- Sea level change
- Surface mass balance