4.6.3.2 Ongoing Dynamic Ice Sheet Response to Past Forcing

Because some portions of ice sheets respond only slowly to climate changes (decades to thousands of years or longer), past forcing may be influencing ongoing changes (Box 4.1). Some geologic data support recent and perhaps ongoing antarctic mass loss (e.g., Stone et al., 2003). A comprehensive attempt to discern such long-term trends contributing to recently measured imbalances was made by Huybrechts (2002) and Huybrechts et al. (2004). They found little long-term trend in volume of the Greenland Ice Sheet, but a trend in antarctic shrinkage of about 90 Gt yr^–1, primarily because of retreat of the West Antarctic grounding line in response to the end of the last ice age. Models project that this trend will largely disappear over the next millennium. In tests of the sensitivity of this result to various model parameters, Huybrechts (2002) found a modern thinning trend in most simulations but an opposite trend in one; in addition, simulated trends for today depend on the poorly known timing of retreat in West Antarctica. Moreover, the ice flow model responds too slowly to some forcings owing to the coarse model grid and lack of some stresses and processes (see Section 4.6.3.3), perhaps causing the modelled long-term trend to end more slowly than it should.

The recent ice flow accelerations discussed in Section 4.6.3.3 are likely to be sufficient to explain much or all of the estimated antarctic mass imbalance, and ice flow and surface mass balance changes are sufficient to explain the mass imbalance in Greenland. This points to little or no contribution from long-term trends to modern ice sheet balance, although with considerable uncertainties.