Of primary concern to policymakers and others interested in societal responses to changing climate is the possibility of rapid or abrupt changes, as opposed to gradual and progressive changes. While general circulation models project a gradual warming of the earth's surface in response to increased levels of greenhouse gases, it is quite possible, even likely, that the system is not linear [e.g., Yiou et al., 1991], and changes could be rapid after an as-yet-unknown threshold is passed.