Loading the dice: Climate change and recent Northwest droughts

Distinguishing natural variability from human influence remains a difficult and important challenge to which novel approaches are needed. Unfortunately, using conventional techniques the signal-to-noise ratio of a climate event is approximately inversely related to its societal relevance: scientific confidence in the detection of human influence on global mean temperature is very high, but global mean temperature has very little societal relevance, whereas local extreme events are much more relevant but statistically much more difficult to attribute to anthropogenic warming. For future changes simulated by climate models, it is important to understand whether those changes could already be emerging. Using a massive ensemble of climate model simulations generated through the distributed computing project climateprediction.net, we investigate the role of climate change in the 2000-01 Northwest drought. We compare 1-year simulations using observed sea surface temperatures (SSTs) with various estimates of anthropogenic SST influence removed. Low winter precipitation, the primary cause of the drought, is actually found to be less likely in the anthropogenic simulations. Assuming the statistics to be broadly similar for 2001 conditions as for other recent years, the results suggest that instead of drier winters, anthropogenic influence favors an increase in cumulative winter precipitation and extreme high single day winter precipitation. Summer drought, conversely, is more likely in the anthropogenic simulations, which show a greater likelihood of low cumulative summer precipitation and longer stretches without rain. Given a large enough sample size, this new method permits an assessment of anthropogenic influence on extreme weather events. Our results suggest that anthropogenic warming is already impacting the likelihood of Pacific Northwest droughts.