in 1997, a flood crested at 54 feet and caused catastrophic damages despite the fact that the flood crests were only 10 percent higher than the previous high. This modest difference from typical experience was sufficient to cross an impact threshold (Pielke, 1999).

Research by Pearce (2000) explored impact thresholds for migrating species, describing problems encountered by caribou on their 1,500-km-long trek from winter grounds in the mountains to the Arctic coastal plain in spring. Increased winter snowfall has led to delayed migration and increased river volume. In 1999, snowfall was 50 percent above average, snow melted a month later than usual, and none of the females in the herd made it to the coast before calving. A record low number of calves eventually reached the coast, and some were forced to swim the Porcupine River when only a few days old. These events were observed by the native people in the area, who were moved to reduce their traditional harvest of caribou. The size of the herd dropped from 178,000 in 1989 to 129,000 in 1999. Impacts on the migration of many other species are similarly dependent on boundaries linked to climate.

The Grand Forks floods also help demonstrate the interaction between societal decisions, perceptions of what constitutes “typical climate,” and impact thresholds. Following the 1997 Grand Forks floods, the community decided to relocate some properties and build additional levees to raise its threshold to catastrophic impacts. Depending on the assessment of the probabilities and consequences of future flood levels as well as the cost and benefit of flood protection, the community could have chosen 55, 60, or 65 feet as the elevation for the levees. Often, such decisions are made based on assumptions of past weather patterns and runoff. However, if climate is changing, or if the underlying climate system is itself variable, decisions based on past precipitation, runoff, and flood patterns are likely to build in thresholds that incorrectly estimate potential threats compared to decisions based on expectations that allow for changes in climatic means or climate variability. (For more information on the flooding and response in Grand Forks and along the Red River, see International Red River Basin Task Force, 2000.)

Trends in Abrupt Impacts

In the long march of human history, technology has increasingly insulated humans and economic activity from the vagaries of weather. In the preindustrial age, work and recreation were dictated by the cycles of day

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