TABLE 2 Temperature Differences, 1946-1990 Average Minus 1901-1945 (land-plus-marine data)

 

Whole Year

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Northern Hemisphere

ΔT(°C)

0.23

0.24

0.28

0.30

0.30

0.28

0.23

0.19

0.18

0.17

0.14

0.17

0.28

σ(1901-90)

0.21

0.32

0.36

0.32

0.26

0.23

0.21

0.20

0.20

0.21

0.24

0.26

0.35

σ(1901-45)

0.20

0.29

0.35

0.27

0.24

0.22

0.20

0.21

0.21

0.22

0.27

0.28

0.36

σ(1946-90)

0.16

0.31

0.32

0.30

0.19

0.14

0.15

0.13

0.15

0.16

0.19

0.21

0.28

Southern Hemisphere

ΔT(°C)

0.24

0.25

0.23

0.26

0.22

0.23

0.22

0.26

0.25

0.25

0.24

0.24

0.22

Δ(1901-90)

0.20

0.24

0.23

0.23

0.22

0.23

0.23

0.24

0.25

0.23

0.21

0.20

0.23

σ(1901-45)

0.14

0.21

0.21

0.19

0.16

0.17

0.18

0.19

0.19

0.15

0.18

0.16

0.19

σ(1946-90)

0.17

0.21

0.20

0.21

0.22

0.23

0.21

0.20

0.24

0.22

0.18

0.17

0.21

Global

ΔT(°C)

0.23

0.25

0.26

0.28

0.26

0.26

0.23

0.22

0.22

0.21

0.19

0.20

0.25

σ(1901-90)

0.19

0.25

0.27

0.26

0.22

0.21

0.19

0.19

0.20

0.19

0.20

0.21

0.26

σ(1901-45)

0.16

0.22

0.24

0.20

0.19

0.18

0.17

0.17

0.18

0.16

0.21

0.20

0.24

σ(1946-90)

0.15

0.21

0.23

0.23

0.17

0.16

0.15

0.14

0.16

0.15

0.14

0.15

0.21

about 25 to 30 percent of the high-frequency (less than 20 years) variations in hemispheric temperature anomalies. Removal of the ENSO influence produces data series that may assist in the early detection of greenhouse-gas-related trends (Nicholls and Katz, 1991; Wigley and Jones, 1981).

VARIATIONS IN LONGER RECORDS

Study of the hemispheric time series since 1850 allows the variability on the 10-to-30-year time scale to be investigated. Century-time-scale variability, however, can be considered only with records that span at least several centuries. In this section we examine some of the longest instrumental records available (principally from Europe) and some of the longest annually resolved proxy climatic reconstructions.

Longer Instrumental Records

The most detailed compilation of long-term instrumental climate data (both temperature and precipitation) currently available is that of Bradley et al. (1985). This compilation extended and improved on the previously available data sets by searching meteorological and other archives for published and manuscript sources of early instrumental records. An important aspect of the resulting compilation is that it contains details of the sources of all of the station data sets and, where possible, details of their long-term homogeneity (see Bradley et al., 1985; Jones et al., 1985, 1986b). Clearly, if one is to study climatic change, it is vital to ensure homogeneity of time-series data.

Figure 3 shows annual time series for 12 stations (for seasonal time series see Jones and Bradley, 1992). The time series are smoothed using a 10-year Gaussian filter to emphasize variations on decadal and longer time scales, and expressed as anomalies from the 1901-1950 period. The longest time series are restricted to the Northern Hemisphere between 40° and 64°N. All the series, except for Toronto, were considered homogeneous by Jones et al. (1985). We have included Toronto here because the extension of its record to 1770 (Crowe, 1992) makes it the longest such time series from North America. Urban warming is clearly evident from the 1880s on.

For the European sites, the warming of the twentieth century is not unusual compared to the longer record. The period encompassing the late nineteenth and early twentieth centuries is one of the coldest for Europe. Temperatures in the late twentieth century are only marginally higher than those during parts of the eighteenth century. The North American and Asian records clearly show long-term warming, but all these records begin much later than in Europe. Evidence from tree-ring reconstructions during the seventeenth and eighteenth centuries in western North America indicates conditions as warm as today's (Briffa et al., 1992b; Fritts, 1991).

Within-Region Similarities

A comparison of the long European records shows consistency on the decadal time scale, with strongly similar cool and warm decades since 1700. Cool decades are evident



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