Methodology for use of isotopic climate forcings in ice sheet models.

In ice sheet models, an internally consistent calculation of climatic temperature forcing from ice-core isotopic records (delta) requires that corrections be made for an ice core site's elevation changes. Such corrections can be important for two reasons. First, elevation change at an ice divide can be large compared to elevation change at the migrating ice margin, resulting in erroneous estimates of marginal melt rate. Second, the delta of precipitation at an ice divide responds less to regional climatic temperature changes than to local temperature changes of the same magnitude that are caused by elevation changes. Significant elevation-change corrections may also be necessary for inferring the relationships used to calculate climatic temperature from delta. A simple methodology incorporating both of these elevational effects is presented here. Further, I recommend that the delta to temperature conversion relations used in ice sheet models should derive from independently reconstructed temperature and delta time-series rather than from spatial correlations, and justify this recommendation based on current understanding of the atmospheric isotopic distillation system.