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Precipitation (Trends - Regional: North America, Canada and Mexico) -- Summary
What should have happened to precipitation around the earth as a result of the historical increase in the air's CO2 content that followed on the heels of the Industrial Revolution?  In this Summary, we consider this question as it applies to the northern and southern extremities of North America, i.e., Canada and Mexico.  For results for the United States, see Precipitation (Trends - Regional: North America, United States [soon to be posted]).

Kunkel (2003) reports that "several studies have argued that increasing greenhouse gas concentrations will result in an increase of heavy precipitation (Cubasch et al., 2001; Yonetani and Gordon, 2001; Kharin and Zwiers, 2000; Zwiers and Kharin, 1998; Trenberth, 1998)."  Consequently, Kunkel looked for such a signal in precipitation data from Canada that covered much of the past century.  His search, however, was in vain, as the data indicated, in his words, that "there has been no discernible trend in the frequency of the most extreme events in Canada."

Zhang et al. (2001) also studied the temporal characteristics of heavy precipitation events across Canada, using what they describe as "the most homogeneous long-term dataset currently available for Canadian daily precipitation."  Their efforts revealed that decadal-scale variability was a dominant feature of both the frequency and intensity of the annual number of extreme precipitation events, but they found "no evidence of any significant long-term changes."  When the annual data were divided into seasonal data, however, an increasing trend in the number of extreme autumn snowfall events was noted; and an investigation into precipitation totals (extreme plus non-extreme events) revealed a slightly increasing trend that was attributed to increases in the number of non-heavy precipitation events.  Zhang et al.'s overall conclusion, therefore, was 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."

Taking a longer view of the subject was Lamoureux (2000), who analyzed varved lake sediments obtained from Nicolay Lake, Cornwall Island, Nunavut, Canada, and compared the results with rainfall events recorded at a nearby weather station over the period 1948-1978, which comparison enabled the reconstruction of a rainfall history for the location over the 487-year period from 1500 to 1987.  This history was suggestive of a small, but statistically insignificant, increase in total rainfall over the course of the record.  Heavy rainfall was most frequent during the 17th and 19th centuries, which were the coldest periods of the past 400 years in the Canadian High Arctic, as well as the Arctic as a whole.  In addition, Lamoureux says that "more frequent extremes and increased variance in yield occurred during the 17th and 19th centuries, likely due to increased occurrences of cool, wet synoptic types during the coldest periods of the Little Ice Age."

This study, like the others discussed above, contradicts the climate-alarmist claim that extreme precipitation events become more frequent and more severe with increasing temperature.  Here in the Canadian High Arctic, in a part of the planet predicted to be most impacted by CO2-induced global warming, rising temperatures have been shown to reduce precipitation extremes, even in the face of a slight increase in total precipitation.

South of the United States, Diaz et al. (2002) created a 346-year history of winter-spring (November-April) precipitation for the Mexican state of Chihuahua, based on earlywood width chronologies of over 300 Douglas fir trees growing at four locations along the western and southern borders of Chihuahua and at two locations in the United States just above Chihuahua's northeast border.  This exercise revealed, in their words, that "three of the 5 worst winter-spring drought years in the past three-and-a-half centuries are estimated to have occurred during the 20th century."  Although this fact makes it sound like the 20th century was highly anomalous in this regard, it was not; for two of those three worst drought years occurred during a decadal period of average to slightly-above-average precipitation, so the three years were not representative of long-term droughty conditions.

Diaz et al. additionally report that "the longest drought indicated by the smoothed reconstruction lasted 17 years (1948-1964)," which again makes the 20th century look unusual in this regard.  However, for several of the years of that interval, precipitation values were only slightly below normal; and there were four very similar dry periods interspersed throughout the preceding two and a half centuries: one in the late 1850s and early 1860s, one in the late 1790s and early 1800s, one in the late 1720s and early 1730s, and one in the late 1660s and early 1670s.

With respect to the 20th century alone, there was a long period of high winter-spring precipitation that stretched from 1905 to 1932; and following the major drought of the 1950s, precipitation remained at, or just slightly above, normal for the remainder of the record.  Finally, with respect to the entire 346 years, there was no long-term trend in the data, nor was there any evidence of a significant departure from that trend over the course of the 20th century.  Consequently, Chihuahua's precipitation history did not differ in any substantial way during the 20th century from what it was over the prior quarter of a millennium, suggesting that neither 20th-century anthropogenic CO2 emissions nor 20th-century warming - whether natural or human-induced - significantly impacted precipitation in that part of North America.

References
Cubasch, U., Meehl, G.A., Boer, G.J., Stouffer, R.J., Dix, M., Noda, A., Senior, C.A., Raper, S. and Yap, K.S. 2001. Projections of future climate change.  In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K. and Johnson, C.A.  (Eds.), Climate Change 2001: The Scientific Basis. Contributions of Working Group 1 to the Third Assessment Report of the Intergovernmental Panel on Climate Change.  Cambridge University Press, Cambridge, UK.

Diaz, S.C., Therrell, M.D., Stahle, D.W. and Cleaveland, M.K.  2002.  Chihuahua (Mexico) winter-spring precipitation reconstructed from tree-rings, 1647-1992.  Climate Research 22: 237-244.

Kharin, V.V. and Zwiers, F.W.  2000.  Changes in the extremes in an ensemble of transient climate simulations with a coupled atmosphere-ocean GCM.  Journal of Climate 13: 3670-3688.

Kunkel, K.E.  2003.  North American trends in extreme precipitation.  Natural Hazards 29: 291-305.

Lamoureux, S.  2000.  Five centuries of interannual sediment yield and rainfall-induced erosion in the Canadian High Arctic recorded in lacustrine varves.  Water Resources Research 36: 309-318.

Trenberth, K.E.  1998.  Atmospheric moisture residence times and cycling: Implications for rainfall rates with climate change.  Climatic Change 39: 667-694.

Yonetani, T. and Gordon, H.B.  2001.  Simulated changes in the frequency of extremes and regional features of seasonal/annual temperature and precipitation when atmospheric CO2 is doubled.  Journal of Climate 14: 1765-1779.

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.

Zwiers, F.W. and Kharin, V.V.  1998.  Changes in the extremes of climate simulated by CCC GCM2 under CO2-doubling.  Journal of Climate 11: 2200-2222.

Last Updated 26 January 2005