Background
The authors note that most prior studies of snowmelt runoff (SMR) in the western United States have depended on trend analyses to identify changes in timing, but they indicate that "trend analyses are unable to determine if a trend is gradual or a step change." This fact is crucial, they say, because when "changes in SMR timing have been identified by linear trends, there is a tendency to attribute these changes to global warming because of large correlations between linear trends in SMR timing and the increasing trend in global temperature."

What was done
Using daily streamflow data for 84 stations in the western United States, each with complete water-year information for the period 1950-2003, McCabe and Clark conducted a number of analyses that enabled them to determine each station's mean streamflow trend over the last half century plus any stepwise changes that may have occurred in each data series.

What was learned
As others had previously learned, the two researchers found that "the timing of SMR for many rivers in the western United States has shifted to earlier in the snowmelt season." However, they discovered that "the shift to earlier SMR has not been a gradual trend, but appears to have occurred as a step change during the mid-1980s," which shift was in turn "related to a regional step increase in April-July temperatures during the mid-1980s."

What it means
After discussing various possible reasons for what they had discovered, McCabe and Clark give their final assessment of the phenomenon in their concluding comments: "the observed change in the timing of SMR in the western United States is a regional response to natural climatic variability and may not be related to global trends in temperature."