A number of studies have examined long-term (century-scale) records of climate variables over the North American region. Most of this work has pertained to analyses of near-surface air temperature and precipitation. Gridded analyses of annual near-surface North American air temperatures for the period 1901-96 (see Figure A-2 in Annex A) show trends toward increasing temperatures over most of the continent. Temperature increases over land are greatest over an area extending from northWestern Canada, across the southern Canada/northern United States region, to southeastern Canada and the northeastern United States. These increases range mainly from 1-2°C/100 years. Decreases in annual temperature on the order of 1°C/100 years are observed along the Gulf coast and on the order of 0.5°C/100 years off the northeast coast of Canada (Environment Canada, 1995). Sea-surface temperatures appear to have warmed off both the west and east coasts of the continent, especially in the Gulf of Alaska.
The time series of anomalies in mean annual temperature for the entire North American region is depicted in Figure A-10 in Annex A. The record reveals temperatures increasing through the 1920s and 1930s, peaking around 1940, and then gradually decreasing through the early 1970s. From this point through the late 1980s, temperatures increased to levels similar to the 1940 era; they have remained mainly above normal since, with the exception of 1996. The more recent warmth has been accompanied by relatively high amounts of precipitation (see below), unlike the dry and warm 1930s. The value of the overall linear trend for 1901-96 is 0.57°C/100 years, a trend significant at better than the 99% confidence level.
The generally increasing temperatures of recent decades, both around the globe
and across North America, have been found to result mainly from increases in
daily minimum temperature (Tmin); increases in
daily maximum temperature (Tmax) have less influence
on the observed increase in the daily mean temperature (Karl et al., 1993; Horton,
1995). This trend has caused the diurnal temperature range (DTR) to decrease
in many areas. Over North America, Karl et al. (1993) found that Tmin increased
greatly over the western half of the continent from 1951 to 1990-in many locales
by as much as 2-3°C/100 years. Increases in Tmax
were smaller, for the most part, with Tmax actually
decreasing somewhat in the desert SouthWest. The combined effect of these changes
resulted in decreases in DTR of 1-3°C/100 years for much of western North America
over the period 1951-90. [Trends are reported in Karl et al. (1993) as degrees
per century to allow for direct comparison between regions with slightly different
periods of record and should not be construed as representing actual trends
over the past century.] Environment Canada (1995) also found that, over a longer
period of record (1895-1991), maximum and minimum temperatures for Canada have
been changing at different rates (Figure 8-4), with
the minimum temperatures rising more than twice as much as maximum temperatures
for the country as a whole.
|Figure 8-4: Canadian annual average maximum and minimum temperature trends for 1895-1991 (adapted from Environment Canada, 1995).|
Annual precipitation amounts from 1901 to 1995 over North America as a whole show evidence of a gradual increase since the 1920s, reaching their highest levels in the past few decades (see Figure A-10 in Annex A). Figure A-1 (Annex A) indicates that the regions experiencing the largest increases are portions of northWestern Canada (>20%), eastern Canada (>20%), and the Gulf coast of the United States (10-20%). The increases in eastern Canada shown in Figure A-1 are corroborated by Groisman and Easterling (1994) and Environment Canada (1995). The analysis of U.S. Historical Climatology Network data by Karl et al. (1996) for 1900-94 reveals the increases along the U.S. Gulf coast and also shows 10-20% increases over the central and northern Plains states, much of the Midwest and Northeast, and over the desert SouthWest. Decreases of 10-20% are apparent over California and the northern Rocky Mountain states (Figure 8-5).
|Figure 8-5: Conterminous U.S. precipitation trends for 1900-94 (converted to %/century), centered within state climatic divisions. The trend magnitude for each climatic division is reflected by the diameter of the circle. Solid circles represent increases, and open circles decreases.|
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