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Precipitation Extremes Along North America's West Coast
Reference
Mass, C., Skalenakis, A. and Warner, M. 2011. Extreme precipitation over the west coast of North America: Is there a trend? Journal of Hydrometeorology 12: 310-318.

Background
The authors write that "a number of articles in the media and reports by some non-governmental organizations have suggested an increasing number of heavy precipitation events over portions of the western United States and have proposed that anthropogenic global warming could be the cause."

What was done
In a review of the subject designed to evaluate this proposal, Mass et al. analyzed "trends in heavy precipitation for the period 1950-2009 by examining the decadal distributions of the top 60, 40 and 20 two-day precipitation events for a collection of stations along the coastal zone of the United States and British Columbia [Canada], as well as the decadal distribution of maximum daily discharge for unregulated rivers from northern California to Washington State."

What was learned
The three researchers from the University of Washington's Department of Atmospheric Sciences report that "during the past 60 years there has been a modest increase in heavy precipitation events over southern and central coastal California, a decline in heavy events from northern California through the central Oregon coast, a substantial increase in major events over Washington, and a modest increase over coastal British Columbia," although they note that "most of these trends are not significantly different from zero at the 95% level." In addition, they found that "trends in maximum daily discharge of unregulated rivers are consistent with the above pattern, with increasing discharges over the past three decades over Washington and northern Oregon and declines over the remainder of Oregon and northern California.

So how "consistent" are these results with what climate models suggest should occur in response to rising temperatures?

Mass et al. report that the results of the two climate models analyzed by Chen et al. (2003) suggest "a pattern quite different from the one described above," and they say that the model employed by Kim (2005) also produced "a pattern quite distinct from that observed since 1950." In addition, they note that the models studied by Tebaldi et al. (2006) produced "a pattern closer, but not identical to, that observed over the past 60 years," while they say that Duffy et al. (2006) "analyzed the precipitation produced over the western United States by four regional climate models," finding the spatial distributions of precipitation they produced to "vary substantially," even among themselves.

What it means
In their concluding paragraph, Mass et al. write that "considering the large variability in precipitation trends among the various general circulation models in the above studies and their associated regional climate models, and the differences between the simulated trend distributions and the observed trend patterns found in this study and others, it is unclear whether anthropogenic global warming is the source of past spatial patterns of extreme precipitation trends along the west coast of North America."

References
Chen, M., Pollard, D. and Barron, E.J. 2003. Comparison of future climate change over North America simulated by two regional climate models. Journal of Geophysical Research 108: 4348-4367.

Duffy, P.B., Arritt, R.W., Coquard, J., Gutowski, W., Han, J., Iorio, J., Kim, J., Leung, L.-R., Roads, J. and Zeledon, E. 2006. Simulations of present and future climates in the western United States with four nested regional climate models. Journal of Climate 19: 873-895.

Kim, J. 2005. A projection of the effects of the climate change induced by increased CO2 on extreme hydrologic events in the western U.S. Climatic Change 68: 153-168.

Tebaldi, C., Hayhoe, K., Arblaster, J.M. and Meehl, G.A. 2006. Going to the extremes: An intercomparison of model-simulated historical and future changes in extreme events. Climatic Change 79: 185-211.

Reviewed 22 June 2011