OBSERVED CHANGES OF MEAN MAXIMUM, MINIMUM, AND DIURNAL RANGE
Spatial and Seasonal Patterns of Contemporary Trends

Daily maximum and minimum temperatures from over 2,000 stations were available for analysis in the countries shaded in Figure 1 during the period 1951 to 1990 (except for Sudan and the former Soviet Union, which had data through 1987 and 1989, respectively). Selected subsets of these data were averaged within various regions of each country. The definition of each region represents a compromise between climatic homogeneity and ensuring an adequate number of stations within its boundaries to limit sampling error. The base period for calculating departures from the average included the years 1951 to 1990 (or slightly fewer years in some countries, as mentioned before). The regions are delineated in Figure 2 where, as in other large-scale studies of the change of the mean annual temperature (Jones et al., 1986a,b; Jones, 1988), the average of the trends of the mean annual maxima and minima reveals a general rise of temperature. A decrease of the minimum temperature within any region is uncommon; it is somewhat more frequent for the maximum temperature, as seen over the United States and China. The differential rate of warming between the maximum and minimum temperatures is apparent, with only a few regions reflecting an increase of the DTR. These weak exceptions occur in central Canada and south-easternmost Australia.

There are some seasonal variations of the rates of decreasing DTR, but they vary from country to country (Table 1). In Japan the decrease is not evident during summer, and it is not as strong during this time over China. In the United States the decrease is weak during the spring but quite strong during the autumn. Alaska and Canada have strong decreases during the spring, with somewhat lesser decreases during the autumn. Over the former Soviet Union the decrease in the DTR is significant throughout the year, but somewhat weaker during the winter. Over Sudan, the rate of the DTR decrease is strong in all seasons except during the summer rainy season, where rains have been very sparse over the past few decades. Over South Africa, decreases in the DTR are greatest in the Southern Hemisphere spring, and DTR actually increases during autumn. In the eastern half of Australia the decrease of the DTR is apparent throughout the year, but is weakest during the Southern Hemisphere summer.

When considered collectively, 60 percent of the trends in Table 1 reflect statistically significant decreases of the DTR. A test for a change point in the trend (Solow, 1987) indicates that for most seasons and areas there is insufficient evidence to suggest a statistically significant change point in the rate of the decrease. The Table 1 temperature trends can be area-weighted to reflect the overall rate of DTR decrease. Table 2 shows the decrease both north and south of the equator, but without any pronounced seasonal cycle in the Northern Hemisphere. The area of available data in the Southern Hemisphere is too small to permit any general statements about trends in that portion of the globe, but the decrease in the Northern Hemisphere is quite apparent. The rate of the decrease in the DTR (-1.4°C/100 years) is comparable to the increase of the mean temperature (1.3°C/ 100 years).

For all areas combined (Figure 3), a noticeable difference between the rates of warming of the minimum and the maximum temperature began in the 1960s. The minimum temperature has continued to warm relative to the maximum through the 1980s. The time series ends in 1989, the year after the major North American drought, because data from the former Soviet Union were not available past 1989. The

TABLE 2 Trends of Temperature (°C/100 years) for Annual and Three-Month Mean Maximum Temperature, Minimum Temperature, and Diurnal Temperature Range for the Areas Denoted in Figure 1 (less Pakistan, northern Finland, and Denmark)

Northern Hemisphere 1951-1990 (50% of land area)

Seasons

Max

Min

DTR

D-J-F

1.3

2.9

-1.5

M-A-M

2.0

3.2

-1.3

J-J-A

-0.3

0.8

-1.1

S-O-N

-0.4

1.3

-1.7

Annual

0.5

2.0

-1.4

Southern Hemisphere 1951-1990 (10% of land area)

Seasons

Max

Min

DTR

D-J-F

1.6

22

-0.6

M-A-M

1.7

25

-0.8

J-J-A

1.0

1 3

-0.4

S-O-N

0.8

2 1

-1.3

Annual

1.3

2.0

- 0.8

Globe 1951-1990 (37% of land area)

Seasons

Max

Min

DTR

D-J-F

1.3

2.9

-1.6

M-A-M

1.9

3.1

-1.2

J-J-A

-0.2

0.8

-1.1

S-O-N

-0.3

1.4

-1.7

Annual

0.7

2.1

-1.4



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