Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 208

Table 6.2 Pre-1850 periods ( p) of extreme temperatures in the Northern and Southern Hemispheres
based on proxy data, for summers, decades, and centuries. The value ( v) is the difference in 8C with the
1961-90 baseline period. (After Jones et al. 1988 )

Northern Hemisphere Southern Hemisphere

Warmest Coldest Warmest Coldest

Period of time p v p v p v p v

Summers 1106 0.54 1601

1.21

1469 0.86 1257

0.85

1074 0.49 1641

1.16

1468 0.78 1750

0.84

1103 0.27 1032

0.96

1356 0.72 1302

0.83

1342

0.94

1396 0.70

1695

0.93

Decades 1101-10 0.02 1691-1700 0.68 1461-70 0.19 1691-1700 0.45

1831-40 0.68 1391-1400 0.16 1751-60 0.41

1601-10 0.64 1171-80 0.04 1051-60 0.39

1641-50 0.61 1011-20 0.35

1811-20 0.61 1441-50 0.35

Centuries 11th

0.16

17th

0.52

14th

0.12

17th

0.25

14th

0.23

19th

0.48

12th

0.13

18th

0.22

12th

0.25

16th

0.40

15th

0.15

19th

0.20

13th

0.30

15th

0.31

13th

0.16

11th

0.19

18th

0.30

16th

0.16

for the SH by averaging. The data are still too sketchy to make any firm
conclusions, and regional influences may dominate some results. The timing
of the two periods, both in extent and in start and finish, seems to vary spatially,
both between and within hemispheres. The values in Table 6.2 suggest that the
climate variations are stronger overall in the NH, which perhaps is not surprising
given the much stronger ocean mitigating influences in the SH.

6.4.4 Reasons for Holocene climate variations in
the Southern Hemisphere
Cook et al.( 1995 ) state that there are no proven explanations for the Holocene
climate variations in the SH. A combination of several possibilities is likely, and
the discussion by Zielinski in Section 6.3.4 is applicable. During colder periods,
the expansion and contraction of the circumpolar vortex, leading to more
meridional circulation and larger wave numbers, can cause important climate
changes. Expansion of the westerlies and shifts in the subtropical anticyclone
can create drier climates in central Chile (Petit et al. 1999 ) but wetter climates in