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Technical Thematic Report No. 5. - Canadian climate trends, 1950-2007

Trend Analysis

In order to compare stations across different climate regions in a systematic fashion, and to make regionally averaged climate series less sensitive to changes in the spatial distribution of stations within a region over time, climate variables were expressed as anomalies with respect to a fixed 1961-1990 reference time period. In the case of temperature, the trend analysis was performed on the temperature departures (or anomalies) from the 1961-1990 reference period. For precipitation, the trend analysis was performed on the anomalies expressed as a percentage of the 1961-1990 mean. The snow cover variables were not converted to anomalies as the results were found to be insensitive to the use of anomalies or raw values.

Trends in climate series were assessed using Kendall’s tau based slope estimator (Sen, 1968) following Zhang et al. (2000). This method is less sensitive to the effect outliers in the series than conventional least-squares methods, and provides more reliable assessment of statistical significance of the trend. Throughout this report, a trend is considered significant if it is statistically significant at the 5% level, and is indicated on maps with a solid triangle. The direction of the triangle indicates the sign of the trend, that is, upward-pointing triangles represent positive trends and vice versa. Trends were computed for the period 1950 to 2007 to provide the best possible spatial coverage over the country for the longest period of data coverage. There are few observations prior to 1950 in northern Canada.

It is important to note that the national maps shown here only give the strength and direction of the average change over the 1950 to 2007 period. The temporal characteristics of variability and change in the various variables over the 1950 to 2007 period are presented and discussed in each terrestrial Technical Ecozone+ Report.

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Temperature

Mean daily air temperature trends are dominated by significant increases that are observed over most regions of Canada (Figure 1). The annual mean temperature has increased by about 1.4°C over the country as a whole, though the amount of temperature change differs between ecozones+. The strongest warming (>1.5°C) has occurred over western and northwestern Canada, with the lowest warming (<0.5°C) over eastern Canada. Other regions of Canada have typically experienced warming of mean annual air temperatures by 1 to 2°C.

Figure 1. Change in mean annual temperature, 1950-2007

Long Description for Figure 1

This map of Canada shows change in mean annual temperature from 1950 to 2007. Mean daily air temperature trends are dominated by significant increases that are observed over most regions of Canada. The annual mean temperature has increased by about 1.4° C over the country as a whole, though the amount of temperature change differs between ecozones+. The strongest warming (>1.5° C) has occurred over western and northwestern Canada, with the lowest warming (<0.5° C) over eastern Canada. Other regions of Canada have typically experienced warming of mean annual air temperatures by 1 to 2° C.

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Temperature change also differs from one season to another (Figure 2). Significant warming trends are most frequently observed in winter and spring, with significant warming concentrated over western Canada. A large number of stations show evidence of cooling in the fall but none are significant; the only stations showing significant fall trends are warming over northern Canada. Similarly, in summer the only significant trends are warming and these stations tend to be located in southern Canada. Analyses of daily temperature extremes indicate trends consistent with warming including fewer cold nights, cold days, and frost days, but more frequent warm nights and warm days (Bonsal et al., 2001; Vincent and Mekis, 2006). Zhang et al. (2006) found evidence that increases in atmospheric concentrations of greenhouse gases from human activities were making a contribution to temperature increases in Canada.

Figure 2. Change in mean temperature, 1950-2007 for a) spring (March, April, May), b) summer (June, July, August), c) fall (September, October, November), and d) winter (December, January, February).

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Long Description for Figure 2

This map of Canada shows change in mean annual temperature from 1950 to 2007. Mean daily air temperature trends are dominated by significant increases that are observed over most regions of Canada. The annual mean temperature has increased by about 1.4° C over the country as a whole, though the amount of temperature change differs between ecozones+. The strongest warming (>1.5° C) has occurred over western and northwestern Canada, with the lowest warming (<0.5° C) over eastern Canada. Other regions of Canada have typically experienced warming of mean annual air temperatures by 1 to 2° C.

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Precipitation

Precipitation has generally increased over Canada since 1950 with the majority of stations with significant trends showing increases (Figure 3). The increasing trend is most coherent over northern Canada where many stations show significant increases. There is not much evidence of clear regional patterns in stations showing significant changes in seasonal precipitation (Figure 4) except for significant decreases which tend to be concentrated in the winter season over southwestern and southeastern Canada. Also, increasing precipitation over the Arctic appears to be occurring in all seasons except summer. The trend toward increasing precipitation has been accompanied by increases in extreme daily precipitation amounts during the growing season (Qian et al., 2010).

Figure 3. Change in the amount of annual precipitation, 1950-2007.

Long Description for Figure 3

This map of Canada shows change in annual precipitation from 1950 to 2007. Precipitation has generally increased over Canada since 1950 with the majority of stations with significant trends showing increases. The increasing trend is most coherent over northern Canada and the Western Interior Basin Ecozone+, where many stations show significant increases.

Expressed as a percentage of the 1961-1990 mean.

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Figure 4. Change in the amount of precipitation, 1950-2007 for, a) spring (March-May), b) summer (June-August), c) fall (September-November), and d) winter (December-February).

Long Description for Figure 4

This graphic presents four maps of Canada showing the amount of annual precipitation from 1950 to 2007 by season; spring (March, April, May), summer (June, July, August), fall (September, October, November), and winter (December, January, February). There is not much evidence of a clear regional pattern in stations showing significant changes in seasonal precipitation except for significant decreases which tend to be concentrated in the winter season and significant increases in the spring over southwestern and southeastern Canada. Also, increasing precipitation over the Arctic appears to be occurring in all seasons except summer.

Expressed as a percentage of the 1961-1990 mean.

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Trends in the annual number of days with measurable precipitation (Figure 5) have a similar pattern to trends in total annual precipitation but with larger numbers of stations showing significant increases and decreases. This is particularly evident in summer where significant increases in the number of days with precipitation are observed over most regions of Canada (Figure 6).

Figure 5. Change in the number of days with precipitation, 1950-2007.

Long Description for Figure 5

This map of Canada shows change in the number of days with precipitation from 1950 to 2007. Trends in the annual number of days with measurable precipitation have generally increased over Canada since 1950 with the majority of stations with significant trends showing increases. The increasing trend is most coherent over northern Canada, Western Interior Basin and Atlantic Maritime ecozones+ where many stations show significant increases.

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It is difficult to generalize a spatial pattern in precipitation trends apart from a tendency for stations with significant increases in precipitation amount and number of days with precipitation to be located over southern coastal and northern regions of Canada. While it is not yet clear what is responsible for the precipitation changes in Canada, a recent study found evidence of anthropogenic influences in observed precipitation increases over Northern Hemispheric land areas north of 55°N including Canada (Min et al., 2008). In addition, the tendency for Arctic stations to show significant increases is consistent with climate model projections of future changes in high latitude precipitation.

Figure 6. Change in the number of days with precipitation from 1950-2007 for a) spring (March-May), b) summer (June-August), c) fall (September-November) and d) winter (December-February).

Long Description for Figure 6

This graphic presents four maps of Canada showing the change in the number of days with precipitation from 1950 to 2007 for a) spring (March, April, May), b) summer (June, July, August), c) fall (September, October, November), and d) winter (December, January, February). There is not much evidence of a clear regional pattern in significant changes in the number of days with precipitation. Significant trends for an increase in the number of days with precipitation are evident in both spring and fall seasons, particularly over the Arctic, Western Interior Basin, and Atlantic Maritime ecozones+. Also, an increase in the number of days with precipitation over the Arctic appears in all seasons.

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Drought

The Palmer Drought Severity Index (PDSI) trend results for the summer season for 1950 to 2007 show that decreases dominate indicating a shift towards drier conditions (Figure 7). However, only 8 of the 80 stations analyzed showed significant changes with decreases occurring mainly over western Canada, and increases over eastern Canada. The PDSI is sensitive to both temperature and precipitation and the negative trends over western Canada are consistent with the observed summer warming seen in Figure 2.

Figure 7. Change in the Palmer Drought Severity Index (PDSI) for summer (June-August), 1950-2007.

Long Description for Figure 7

This map of Canada shows change in the Palmer Drought Severity Index (PDSI) for summer (June to August) from 1950 to 2007. PDSI trends show that decreases dominate indicating a shift towards drier conditions. However, only 8 of the 80 stations analyzed showed significant changes with decreases occurring mainly over western Canada, and increases over eastern Canada.

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Snow and ice conditions

Interannual variability and trends in the duration of snow and ice cover are closely linked to air temperatures in the fall and spring periods. Analysis of trends in snow cover duration in the first and second halves of the snow year (defined from August to July) (Figure 8) are consistent with the fall and spring temperature trends shown in Figure 2 with little change in snow cover in the fall but widespread decreases (39% of stations) in spring snow cover over western and northern Canada in response to warmer spring temperatures. The significant trend to earlier snow melt was previously documented by Brown and Braaten (1998) and is part of a hemispheric-wide trend of earlier melt of snow and ice (Lemke et al., 2007). The maximum depth of snow cover also shows a general tendency to smaller values (Figure 9), but less significantly (19% of stations) and less spatially coherent than spring snow cover duration. Over southern Canada the decrease in maximum snow depths is being driven by less winter precipitation (Figure 4d) and a lower fraction of precipitation falling as snow from winter warming (Figure 2d and Figure 10). The decreases in maximum depth reported at some Arctic stations is difficult to explain as this region experienced increasing precipitation and an increase in the fraction of precipitation falling as snow from 1950 to 2007, also documented in Vincent and Mekis (2006), and climate models suggest that maximum snow accumulation will increase over northern high latitudes in response to global warming (Brown and Mote, 2009).

Figure 8. Change in the number of days with ≥ 2 cm of snow on the ground, 1950-2007, in a) the first half of the snow season (August to January) which indicates changes in the start date of snow cover, and b) in the second half of the snow season (February to July) which indicates changes in the end date of snow cover

Long Description for Figure 8

This graphic presents two maps of Canada, showing the change in the number of days with ≥ 2 cm of snow on the ground from 1950 to 2007, in a) the first half of the snow season (August to January) which indicates changes in the start date of snow cover, and b) in the second half of the snow season (February to July) which indicates changes in the end date of snow cover. Trends show little change in snow cover in the fall but widespread decreases (39% of stations) in spring snow cover over western and northern Canada.

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Figure 9. Change in the maximum annual snow depth, 1950-2007.

Long Description for Figure 9

This map of Canada shows change in the maximum annual snow depth from 1950 to 2007. There is a general tendency to smaller values, with few stations reporting significant change (19% of stations) less spatial coherency.

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Figure 10. Absolute change in the ratio of snow to total precipitation in Canada, 1950-2007.

Long Description for Figure 10

This map of Canada shows absolute change in the ratio of snow to total precipitation in Canada from 1950 to 2007. There is a general decreasing trend across Canada, indicating a lower fraction of precipitation falling as snow, with the exception of Arctic Ecozone+ stations which experience an increase in the fraction of precipitation falling as snow.

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Analysis of river and lake ice freeze-up and break-up trends from the mid 1960s to the mid 1990s show contrasting seasonal responses with little change in freeze-up (with some evidence of earlier river ice formation over eastern Canada) but widespread trends for significantly earlier spring break-up (Zhang et al., 2001a; Duguay et al., 2006). These results are consistent with trends in fall and spring temperatures shown in Figure 2. A more recent analysis of trends in freeze-up and break-up at approximately 40 lake sites across Canada from 1970 to 2004 using in situ and satellite observations showed greater evidence of significantly later freeze-up at a number of lakes (Latifovic and Pouliot, 2007). The spatial pattern of trends in thaw date over the 1950 to 2005 period shows that sites with significantly earlier break-up tend to be located in western Canada (Figure 11) in agreement with the spatial pattern of climate stations showing significant spring warming (Figure 2a).

Figure 11. Trends in lake ice thaw date across Canada, 1950-2005.

Long Description for Figure 11

This map of Canada shows trends in lake ice thaw date across Canada from 1950 to 2005. Stations across western Canada through to Quebec, show significantly earlier ice thaw dates, consistent with Canadian temperature trends. The Mixedwood Plains and Atlantic Maritime ecozones+ each show one station with significantly later break-up date.

Source: IceWatch (2008a)

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Changes in hydrological regime

Analysis of streamflow trends during the second half of the 20th century (Zhang et al., 2001a) indicates a general decrease in annual mean streamflow during that period, with significant decreases detected in the southern part of the country. Monthly mean streamflow for most months also decreased, with the greatest decreases occurring in August and September. The exceptions are March and April, when significant increases in streamflow were observed. Significant increases were identified in lower percentiles of the daily streamflow frequency distribution over northern British Columbia and the Yukon. In southern Canada, significant decreases were observed in all percentiles of the daily streamflow distribution. Breakup of river ice and the ensuing spring freshet occur significantly earlier, especially in British Columbia consistent with the spring warming trends shown in Figure 2a.

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Changes in the growing season

Statistically significant increases in the length of growing season have been observed mainly in the southwest (Figure 12). Some decreases in the growing season length were seen in the Prairies, but the decreases are not significant in general. The increase in the length of growing season is largely due to an earlier start of the growing season as a result of spring warming (Figure 13). In most regions of Canada, growing season has started earlier and many stations show significant earlier starts to the growing season. The longer growing season in combination with warmer temperatures during the growing season, has resulted in significant increases in growing degree days (Figure 14). There is also evidence of a reduction in the frequency of occurrence of frost days and killing frost days during the growing season (Qian et al., 2010).

Figure 12. Change in the length of growing season, 1950-2007.

Long Description for Figure 12

This map of Canada shows change in the length of growing season from 1950 to 2007. Statistically significant increases in the length of growing season have been observed mainly in the southwest. Some decreases in the growing season length were seen in the Prairies, but the decreases are not significant in general.

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Figure 13. Change in the start date of the growing season in Canada, 1950-2007.

Long Description for Figure 13

This map of Canada shows change in the start date of the growing season from 1950 to 2007. In most regions of Canada, growing season has started earlier and many stations show significant earlier starts to the growing season.

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Figure 14. Change in the effective growing degree days (a measure of accumulated heat during the growing season), 1950-2007.

Long Description for Figure 14

This map of Canada shows change in the start date of the growing season from 1950 to 2007. In most regions of Canada, growing season has started earlier and many stations show significant earlier starts to the growing season.

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