Observed growing season changes in Canada

Based on a study on long-term trends in agroclimatic conditions in Canada, we present analysis on statistical differences in means and variances of agroclimatic indices during three 30yr periods in the 20th century (i.e., 1911-1940, 1941-1970 and 1971-2000). We found many occurrences of statistically significant changes in means between pairs of the three 30yr periods. The findings consistently reflect warming trends, resulting in an earlier growing season start, an earlier end to spring frost (SF), together with an extended growing season, a longer frost-free period (FFD) and more available crop heat units in the later 30yr periods. Significant changes were detected for SF and FFD at a much larger percentage of locations between the latter two 30yr periods (1971-2000 vs. 1941-1970) as compared to the earlier two periods (1941-1970 vs. 1911-1940), implying that warming in spring has become stronger in more recent decades than in the earlier decades before the 1970s. In contrast, changes in the variances of the selected agroclimatic indices were less evident than changes in their means, based on the percentage of locations showing significant differences.