Greenland high-elevation mass balance: inference and implication of reference period (1961-90) imbalance

We revisit the input-output mass budget of the high-elevation region of the Greenland ice sheet evaluated by the Program for Arctic Regional Climate Assessment (PARCA). Our revised reference period (1961-90) mass balance of 54 +/- 48 Gt a(-1) is substantially greater than the 0 +/- 21 Gt a(-1) assessed by PARCA, but consistent with a recent, fully independent, input-output estimate of high-elevation mass balance (41 +/- 61 Gt a(-1)). Together these estimates infer a reference period high-elevation specific mass balance of 4.8 +/- 5.4 cm w.e.a(-1). The probability density function (PDF) associated with this combined input-output estimate infers an 81 % likelihood of high-elevation specific mass balance being positive (>0 cm w.e.a(-1)) during the reference period, and a 70% likelihood that specific balance was >2 cm w.e.a(-1). Given that reference period accumulation is characteristic of centurial and millennial means, and that in situ mass-balance observations exhibit a dependence on surface slope rather than surface mass balance, we suggest that millennial-scale ice dynamics are the primary driver of subtle reference period high-elevation mass gain. Failure to acknowledge subtle reference period dynamic mass gain can result in underestimating recent dynamic mass loss by similar to 17%, and recent total Greenland mass loss by similar to 7%.