Temperature and Precipitation Trends in Canada During the 20th Century
Xuebin Zhang, Lucie A. Vincent, W. D. Hogg, Ain Niitsoo
AtmosphereOcean (2000)Volume: 38, Issue: 3, Pages: 395–429
What is interesting in this paper is the conclusion (best to show that first):
Like other parts of the world, Canada has not become hotter (no increase in higher quantiles of maximum temperature), but has become less cold.
Well, isn’t that interesting. Exactly what I’m seeing. Let’s continue as there is more interesting stuff here.
Based upon the gridded datasets, the time series of annual mean temperature anomalies relative to the 1961–1990 mean, are computed and shown for southern Canada (1900–1998) in Fig. 3. There is a statistically significant positive trend, which accounts for an increase of 0.98C, for the region during the period. The linear trend is not exactly monotonic. The rises of temperature prior to the 1940s and after the 1970s account for the significant trend. There is a modest decrease during 1940–1970. Trends differ for different regions, and for different seasons, as well as for daily maximum and minimum temperatures.
Yep, that’s what I see in the all the stations. That 1940-1970’s pause and/or drop in the average. But there is more:
The greatest warming, which is in the Prairies, is about 1.58C over the 99-yr period. The spatial patterns of the trends differ from season to season. The mean daily maximum temperature has increased over all of southern Canada in both
winter and spring. However, it has increased in some areas but decreased in other
areas during summer and fall. Among the four seasons, spring shows the greatest
warming. The spatial pattern in this season is similar to the annual one, except that
spring warming is stronger and the area with significant upward trend has expanded
from the Prairies to include northern B.C. and Manitoba. The greatest warming during spring is well over 28C for the 1900–1998 period in the Prairies. Warming during winter is more than 1.58C during 1900–1998 in western Canada, however, the trends are significant only in southwestern B.C. Summer maximum temperature
shows significant positive trends in Quebec and the Prairies, and significant negative trends in southwestern B.C. Fall shows no significant warming or cooling trends. It is apparent that warming in spring maximum temperature contributed the
most to the positive trend in the annual mean of daily maximum temperature.
Strong warming is the sole characteristic of the minimum temperature. This is
clearly shown in Fig. 5, where no negative trends can be found. Annual mean minimum temperature has increased from about 1 to 2.58C during the last 99 years, with strongest warming in the Prairies and southern Quebec. The trends are statistically significant over all of southern Canada. Spring minimum temperatures have warmed the most, with a rate over 38C during 1900–1998 in the northern Prairies. Winter has the second highest warming rate with some areas in the Prairies and B.C. reaching as high as 38C, however, some of the warming in Manitoba, Ontario and along the east coast is not significant. In summer, the spatial pattern is quite uniform with significant trends from 1.3 to 2.08C. During the fall, the minimum temperatures have warmed with significant increase in eastern Canada and along the west coast. Overall, these results clearly show that daily minimum temperatures, indicators of nighttime temperatures, have significantly increased throughout southern Canada over the past century.
Significant positive trends were also found in the annual and seasonal daily mean
temperatures. The spatial patterns (not shown) are similar to those of minimum temperatures but the trends are of a lesser magnitude. This suggests that it is the strong and positive trends of the minimum temperatures, especially during spring and summer, that contributed the most to the trend in the mean temperatures.
Greater warming in the minimum than in the maximum temperatures results in
significant decrease in the daily temperature range since the beginning of this century (Fig. 6). Significant decrease in the DTR is observed from coast to coast and for all seasons, with a trend of –0.5 to –2.08C during 1900–98. We can conclude that night-time temperature has increased more than daytime temperature in all seasons in the southern part of Canada during the last century. Most of the decrease in the DTR occurred prior to the 1950s, especially late in the first half of the 20th century, coinciding with an increase in total cloud amount in Canadian mid-latitudes during the first half of the 20th century (Henderson-Sellers, 1989; McGuffie and Henderson-Sellers, 1988). Henderson-Sellers (1989) did not propose any specific reasons for the cloud increase. Significant decrease in the DTR did not occur in the second half of the century when the greatest increase in greenhouse gases took place. This suggests that trends in the DTR are closely related to changes in total cloud amount. The trends in both DTR and total cloud cover differ from one season to the other. Future investigation into the relationships between the changes in DTR and cloud cover is needed.
It needs to be understood that they gridded the country as such:
To reduce the amount of calculation, trend analysis was performed on the time series of a coarse 200 × 200 km grid obtained by averaging the values of 16 grid points from the 50 × 50 km grid. This procedure further smoothes the fields. Since the climate observation network was not established in northern Canada until the late 1940s, there were large portions of the country with no data during the first half of the century. Based loosely on error analyses reported in Milewska and Hogg (unpublished manuscript) and those reported here, interpolation limits were established. No more than five stations with the shortest distance (less than 750 km for precipitation and 1000 km for temperature) to a grid point are used to interpolate the value for the grid. The distance limits are seldom reached. For example, more than 80% of grid values for precipitation have stations within 200 km. No grid values were generated for northern Canada (north of about 608N) prior to 1950s due to insufficient data. Trend analysis was performed on datasets for 1900–1998 for the southern part of the country (south of about 608N) where grid values were complete for both temperature and precipitation, and for 1950–1998 for the nation as a whole.
Ok, let’s take that on face value. Though from what I’ve seen with differences in close by stations, I’m not happy with their approach. Seems this differences between close by stations to them is “noise” that needed smoothing out.
Regardless of this methodology to smooth out the data, their conclusion is clear:
Annual mean temperature has warmed an average of 0.98C in southern Canada
over the last century. Associated with this increase in mean temperature is a relatively smaller increase in daily maximum temperature and a larger increase in daily minimum temperature. In this century, the increases have resulted in a decrease in diurnal temperature range by 0.5 to 2.08C. The bulk of decline in DTR occurred during the first half of the century, coinciding with an increase in cloud cover during that period. Both of these results are broadly consistent with greenhouse gas induced climate change, but the timing of the changes, coming prior to the most significant increase in greenhouse gases, suggests that other mechanisms may be responsible. Examining the areas affected by abnormal and extreme temperature confirmed the above analysis. It also suggested that the probability distribution of minimum temperature has shifted with a higher mean but only the left-hand side of maximum temperature distribution has been shifted upward. This indicates that southern Canada has not become hotter but less cold.
At least they are honest.
More studies are needed before we can conclude that such changes are the manifestations of anthropogenic climate change.