Trends in Frost Dates, Frost Free Duration and Seasonal Temperature on the Canadian Prairie

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1 1 Trends in Frost Dates, Frost Free Duration and Seasonal Temperature on the Canadian Prairie Herb Cutforth 1, Ted O Brien 2, Jason Tuchelt 2 and Rick Rickwood 2 1 SPARC, Agriculture and Agri-Food Canada, Swift Current, SK 2 PFRA, Agriculture and Agri-Food Canada, Regina, SK Introduction Generally, the Canadian Prairie has become warmer over the past 4-5 decades. However, some regions of the prairies have warmed faster than others. The largest temperature increase has generally occurred during January through April (JFMA) whereas the change in September through December (SOND) temperatures has been relatively small. Objective To look at meteorological records from weather stations across the Canadian prairies for evidence of trends towards increased frost free durations, earlier dates of last spring frosts and later dates of first fall frosts ( C) in response to climate warming. Methods We gathered meteorological records from numerous weather stations across the agricultural portion of the Canadian prairies. On a yearly basis, we determined the last date for spring frost, the first date for fall frost, and the frost free duration ( C) between 194 and 1997 at each weather station. On a yearly basis, we determined the average seasonal minimum temperature (Tmn) for spring (April, May, June), fall (August, September, October), and the growing season (April through October). We analyzed the data to determine: trends in last spring frosts, first fall frosts, and frost free durations; and trends in average seasonal minimum temperature at each location. We plotted the seasonal distribution of trends in frost dates and frost free duration, and the seasonal temperature trends. We compared the seasonal frost trends with the seasonal temperature trends to determine the degree of correlation. Results Generally, from 194 to 1997, across the prairies, the last date of spring frost ( C) occurred progressively earlier and frost free duration increased with time. The change towards later first fall frosts was not as rapid nor as consistent across the prairies. The trends in frost dates were significantly correlated with the trends in the average seasonal minimum temperatures. This suggests that trends in frost dates should follow somewhat the warming trends on the Canadian prairies. Trends for earlier last spring frost dates were not significant over much of the agricultural area of Manitoba and of southern Alberta, whereas trends for earlier last spring frost dates were significant over much of the agricultural area of Saskatchewan and of central and northern Alberta (Fig. 1, top). Correspondingly, zero or significant cooling trends (negative trends) in the average spring minimum temperature occurred over much of the agricultural area of Manitoba and over a large area of southern Alberta (Fig. 1, bottom). However, significant warming trends in the average spring minimum temperature occurred over much of the agricultural area of Saskatchewan and of central and northern Alberta. Trends for later first fall frost dates were not significant over much of the agricultural area of

2 2 Manitoba, Saskatchewan and southern Alberta (Fig. 2, top). Significant trends for later first fall frost dates occurred over a large area in northern Alberta-northeastern British Columbia, in northcentral Alberta, and within smaller areas scattered throughout the prairies. Correspondingly, significant warming trends occurred in central Alberta and to a lesser extent in northern Alberta - northeastern BC, and within small areas scattered throughout southern Alberta, and southern Saskatchewan (Fig. 2, bottom). Trends for increasing frost free durations were not significant over large portions of the agricultural area of Manitoba and of southern Alberta, whereas trends for increasing frost free duration were significant over much of the agricultural area of Saskatchewan and of central and northern Alberta (Fig. 3, top). Correspondingly, zero or significant cooling trends in the growing season average minimum temperature occurred over much of the agricultural area of Manitoba, over a large area of southern Alberta, and within a small area in southwestern Saskatchewan (Fig. 3, bottom). Significant warming trends in the growing season average minimum temperature occurred over much of the agricultural area of Saskatchewan and of central and northern Alberta. The largest warming trends tended to occur along a northwest-southeast diagonal extending across the prairies from northwestern Alberta to southeastern Saskatchewan. Conclusions The warming trend on the prairies has not been uniform in time, area, or rate, and neither has the change in frost dates or frost free duration. Generally, the spacial distribution in frost dates and frost free durations have roughly followed the regional warming patterns on the prairies. The largest trends for earlier last spring frost dates, later first fall frost dates and increasing frost free durations occurred in northern Alberta whereas some of the smallest trends occurred over much of southern Alberta and in southern Manitoba; northern Alberta is a region with one of the largest warming trends (especially on an annual basis) whereas southern Alberta and southern Manitoba are regions with some of the smallest warming trends. Future Research Compare changes in the spacial distribution of actual frost dates and frost free durations that have occurred over the past 6 years across the Prairies.

3 Figure 1: Spacial distribution across the prairies of linear trends for last spring frost ( C) dates (top) and for the average seasonal spring (April, May, June) minimum temperature (Tmn) (bottom) from 194 to Shaded areas represent the extent of the agricultural land on the Canadian prairie. Contour interval of trends in frost dates is.2 days yr -1 and of trends in seasonal Tmn is.3 C yr -1. Negative trends indicate progressively earlier last spring frost dates from 194 to For temperature, negative trends indicate areas of cooling, positive trends indicate areas of warming.

4 Figure 2: Spacial distribution across the prairies of linear trends for first fall frost ( C) dates (top) and for the average seasonal fall (August, September, October) minimum temperature (Tmn) (bottom) from 194 to Shaded area represents the extent of the agricultural land on the Canadian prairie. Contour interval of trends in frost dates is.2 days yr -1 and of trends in seasonal Tmn is.3 C yr -

5 Figure 3: Spacial distribution across the prairies of linear trends for frost free duration ( C) (top) and for the average growing season (April through October) minimum temperature (Tmn) (bottom) from 194 to

6 1997. Shaded area represents the extent of the agricultural land on the Canadian prairie. Contour interval of trends in frost dates is.2 days yr -1 and of trends in growing season Tmn is.3 C yr -1. 6