4.4.3.4 Model-Based Estimates of Change

Physically based sea ice models, forced with winds and temperatures from atmospheric reanalyses and sometimes constrained by observed ice concentration fields, can provide continuous time series of sea ice extent and thickness that can be compared to the sparse observations, and used to interpret the observational record. Models such as those described by Rothrock et al. (2003) and references therein are able to reproduce the observed interannual variations in ice thickness, at least when averaged over fairly large regions. In particular, model studies can elucidate some of the forcing agents responsible for observed changes in ice thickness.

A comparison of various model simulations of historical arctic ice thickness or volume is shown in Figure 4.11. All the models indicate a marked reduction in ice thickness of 0.6 to 0.9 m starting in the late 1980s, but disagree somewhat with respect to trends and/or variations earlier in the century. Most models indicate a maximum in ice thickness in the mid-1960s, with local maxima around 1980 and 1990 as well. There is an emerging suggestion from both models and observations that much of the decrease in thickness occurred between the late 1980s and late 1990s.

Figure 4.11. Comparison of model-based time series of the annual mean average sea ice thickness anomaly (computed relative to the mean of the entire period) in the Arctic Basin, obtained from a variety of models (redrafted from Rothrock et al., 2003; see this paper for identification of the individual models and their attributes), along with the sea ice volume anomalies in the Arctic Basin (grey curve and right-hand scale; computed by Koeberle and Gerdes, 2003).

It is not possible to attribute the abrupt decrease in thickness inferred from submarine observations entirely to the (rather slow) observed warming in the Arctic, and some of the dramatic decrease may be a consequence of spatial redistribution of ice volume over time (e.g., Holloway and Sou, 2002). Low-frequency, large-scale modes of atmospheric variability (such as interannual changes in circulation connected to the Northern Annular Mode) affect both wind-driving of sea ice and heat transport in the atmosphere, and therefore contribute to interannual variations in ice formation, growth and melt (e.g., Rigor et al., 2002; Dumas et al., 2003).

For the Antarctic, Fichefet et al. (2003) conducted one of the few long-term simulations of ice thickness using observationally based atmospheric forcing covering the period 1958 to 1999. They noted pronounced decadal variability, with area-average ice thickness varying by ±0.1 m (over a mean thickness of roughly 0.9 m), but no long-term trend.