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This report utilizes California as a regional case study to categorize and visualize scientific uncertainties and assess the state of the art emission and climate models with an emphasis on the regional scale. The report addresses the following key questions: 1) What are the range of possible SRES scenarios; how likely are they; how can subjective probabilities be assigned, and at what levels of confidence; what methods are most promising in such estimation; and how vexing is the lack of subjective probabilities for regional analyses for the California case study? 2) How well do GCMs do at producing meteorological variables at regional scales, and which of the various strategies to improve GCMs provide the greatest direct aid in improving knowledge of and confidence in the California regional modeling efforts and assessment strategies? 3) Utilizing California as a regional case study, how can the assessment and eventual reduction of uncertainties upstream, combined with improved downscaling models, better inform regional decision-making and resource management, and what are the stakes? 4) What elements of integrated analyses are essentially unknowable? Can robust strategies (Lempert and Schlesinger, 2000) be developed in spite of those aspects of analysis?

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.