This workshop centered around the recognition that human emissions of greenhouse gases were (and are) likely to lead to abrupt changes in climate, with severe, widespread ecological consequences if certain thresholds of climate are crossed. Topics of special interest included: bleaching of corals, disintegration of the West-Antarctic ice sheet, and changes in ocean circulation patterns. Discussions of abrupt change in the past were considered in forecasting abrupt changes forced by anthropogenic emissions. The workshop also considered potential early warning signs that a threshold was being approached.
Keywords: climate change and variability, paleoclimate, abrupt change, thresholds
The Problem: Anthropogenic greenhouse gas emissions may cause abrupt climate change with a nonlinear hysteresis response once a certain limit, often referred to as climate threshold, is crossed. Potential examples of abrupt climate changes include: (i) a widespread bleaching of corals [Hughes et al., 2003], (ii) a disintegration of the West-Antarctic ice sheet [Oppenheimer, 1998], (iii) a collapse of the North Atlantic thermohaline circulation [Broecker, 1997], and (iv) changes in ENSO properties [Timmermann et al., 1999]. These climate thresholds pose unique challenges to the design of sound climate strategies. For one, the scientific mechanisms driving these responses are highly uncertain [Cubasch and Meehl, 2001; Lempert and Schlesinger, 2000]. Furthermore, the impacts may well exceed the marginal analysis approach inherent of typical economic analysis [Tol, 2003]. In addition, decisions have to be made under deep uncertainty (i.e., no consensus exists about priors regarding model parameters or even the model structure). Last but not least, early warning signs may be detected only after the bifurcation point has already been passed [Santer et al., 1995].
Why a workshop? A recent workshop on abrupt climate change [Alley et al., 2002] broke important new ground. The need for a follow up workshop arises from important additional questions and from a substantial increase in the published work addressing this issue. For example, Alley et al. (2002) focused predominantly on changes in the North Atlantic thermohaline circulation (MOC). Recent work suggests, however, that other climate thresholds may be crossed before a MOC collapse [O’Neill and Oppenheimer, 2002] and that assumptions about available warning signs [Keller et al., 2004; Nordhaus and Popp, 1997] may have been overly optimistic [Deutsch et al., 2002]. Furthermore, the economic analyses discussed in Alley et al. (2002) center on the expected utility approach. One might ask whether alternative decision criteria such as robust decision making [Lempert and Schlesinger, 2000] or reliability constraint optimal policies [Keller et al., 2000; Tol, 1998] would come to significantly different conclusions.
Workshop focus: The weeklong workshop will bring together a group of experts on (i) the mechanisms of abrupt climate changes, (ii) detection of early warning signs, (iii) impacts of abrupt climate changes, and (iv) economic analysis of climate policies under climate thresholds with particular emphasis on near-term mitigation and the value of information.
In recent years, the topic of abrupt climate change has risen to a place of prominence in both scientific and popular discourse. The subject of abrupt climate change was addressed at a meeting held at the Aspen Global Change Institute in 2005, and the proceedings of that meeting are summarized in this article. In short, the concern among scientists is that melting freshwater could drastically impact ocean circulation, leading to an abrupt change in global climate. Such an event is believed to have occurred in the last ice age when the world may have experienced up to a 10 degree shift in temperature. However, during the AGCI meeting scientists reported on the results of climate model runs that suggest that major disruption of ocean currents is not an anticipated outcome of global warming. The general consensus of scientists by the end of the meeting was that too much focus on abrupt climate change may serve as a dangerous distraction from addressing the more realistic concerns of climate change such as sea level rise.
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