Summary of findings

Sea ice

A variety of indicators suggest that during the latter half of the twentieth century the Arctic has undergone substantial climate change (Serreze et al., 2000). One of the key indicators is sea-ice extent and thickness, both of which have shown a measurable and disturbing decrease during the last half of the twentieth century (see Chapter 8). Interestingly, the rate of decrease appears to be at a maximum during summer (see Figure 12.8). The pre-satellite time series (before 1972) has a larger error bar on it, but nonetheless paints a consistent and compelling picture. Extrapolation forward in time suggests that there could, potentially, be no summer sea ice in the Arctic within 50 years. This will have major impacts on energy and moisture exchange, and consequently on the climate of the northern hemisphere. The consequences of such dramatic changes in sea-ice cover are the subject of a number of general circulation model (GCM) studies, and it is currently too early to say what the implications of these changes might be.

The sea-ice record for the Southern Ocean is less temporally extensive and, essentially, limited to the satellite era. Additionally, the seasonal variation in extent is much greater than in the Arctic, increasing the noise on any long-term signal that may be present. In contrast to the Arctic, there is no measurable trend in sea-ice mass balance, although some regional variations have been noted from passive microwave data covering the last 20 years (1979 to 1999; see Parkinson (2002)).