and the possibility of rapid dynamic response to future warming. The Greenland ice sheet has the potential for rapid surface melting and perhaps enhanced ice flow with continued greenhouse warming. Laser-altimeter surveys in the 1990s indicated an overall negative mass balance for Greenland ice that results in a 0.13 mm per year sea level rise (Krabill et al., 2000). Since the late 1800s the margin of the Greenland ice sheet has retreated 2 km in some places (Funder and Weidick, 1991) indicating that Greenland ice is responding to twentieth century warming. The influence of the Greenland ice sheet system on potential abrupt climate change appears to be linear except for the possibility of threshold changes in ocean circulation, but the existence of dynamically controlled ice streaming at least suggests the possibility of dynamical changes (Fahnestock et al., 1993).

Much attention has focused on observations that indicate a reduction in the extent and thickness of Arctic sea ice (Cavalieri et al., 2000; Rothrock et al., 1999; Wadhams and Davis, 2000; Parkinson, 2000). Sea ice trends are less clear in the Antarctic. Published estimates from satellite measurements include no significant trend in sea ice from 1973-1996 (Jacka and Budd, 1998), and over a slightly different interval, an increase of 1.3 ± 0.2 percent per decade in mean-annual sea-ice area from 1978-1996 (Cavalieri et al., 2000). However, an interpretation of whaling records indicates a large and rapid decrease in Antarctic summertime sea ice extent between the mid 1950s and early 1970s (de la Mare, 1997). This variability, whether natural or human-induced, is important because it is large and may contribute to “surprises” that may accompany future climate change (Broecker, 1987).

Similar concerns regarding the impacts of abrupt climate change on thresholds apply to social systems. Over human history, one of the major ways humans have adapted to changing economic fortunes has been to migrate from unproductive or impacted regions to more productive and hospitable regions. Until the twentieth century, national boundaries were often open, allowing people to migrate freely in response to economic conditions. For example, as a result of the potato blight (Phytophthora infestans) in Ireland, there was a disastrous famine between 1845 and 1847 and almost one million people emigrated, mainly to America. Because national borders may be less open today, it may be difficult for people to migrate to other countries when famines or civil wars occur. These “boundary effects” could be particularly severe for small and poor countries, whose populations are often unwelcome in richer countries. To the extent that abrupt climate change may cause rapid and extensive changes of fortune for