Zhang, X., Hogg, W.D. and Mekis, E. 2001. Spatial and temporal characteristics of heavy precipitation events over Canada. Journal of Climate 14: 1923-1936.
What was done
The authors analyzed the spatial and temporal characteristics of extreme precipitation events over Canada for the period 1900-1998, using what they describe as "the most homogeneous long-term dataset currently available for Canadian daily precipitation."
What was learned
Decadal-scale variability was found to be a dominant feature of both the frequency and intensity of the annual number of extreme precipitation events, but there was "no evidence of any significant long-term changes" in these indices during the 20th century. When the annual data were broken down into seasonal data, however, an increasing trend in the number of extreme autumn snowfall events was noted. Finally, an investigation into precipitation totals (extreme and non-extreme) revealed a slightly increasing trend across Canada during the period of study, which was attributed to increases in the number of non-heavy precipitation events.
What it means
Observed trends and variability of climate extremes are often utilized as indicators of climate change; and modeling results consistently project increases in both the frequency and intensity of precipitation in a CO2-enriched world. Yet real-world data, in the words of the authors, demonstrate that "increases in the concentration of atmospheric greenhouse gases during the twentieth century have not been associated with a generalized increase in extreme precipitation over Canada."
Clearly, the models have failed once again in predicting more and stronger high-end precipitation events. There has been a slight increase in total precipitation over Canada during this time period; but it is due to an increase in the number of low precipitation events, which is something for which all Canada can be thankful; for ecosystems have likely benefited from this additional moisture supply.