Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 203

stronger atmospheric mixing. Polar circulation was weak (Mayewski et al.
2004 ). Changes to ocean circulation and terrestrial conditions lead to an expan-
sion of the sea ice surrounding Antarctica, and a shift in the circumpolar vortex
toward the equator. The extended ice volume enhanced the ice-ocean tempera-
ture gradient, further strengthening the meridional circulation. Impacts in other
areas of the SH included increased aridity and seasonal rainfall patterns in the
tropics, weaker summer monsoons, and some spatial changes to mid-latitude
precipitation patterns, such as increased precipitation in Chile. The details and
reasons for the recovery after 7800 BP are not known.

6.4.2 Southern Hemisphere temperature reconstructions
after AD 900
Figure 6.7 presents a composite of SH temperature variations from AD 900
onward, based on the average of seven widely distributed proxy records from
various seasons. The SH results are compared to those for the NH. A 50-year
Gaussian filter emphasizes any overall trend and variations. There are several
interesting aspects to the temperature patterns. Aside from the strong warm-
ing in both hemispheres beginning in the twentieth century, most likely
caused by human impacts (Jones et al. 1998 ), the variability associated
with natural forcing consists of small fluctuations (within 1 8C) around the
1901-50 mean used as a baseline comparison. Overall the NH temperatures
remain somewhat lower, and vary more, compared to the baseline, while the
SH temperatures are slightly higher than the standard, suggesting a greater
ocean influence. Higher NH temperatures around AD 1100 suggest evidence

1100 1200 1300 1400 1500 1600 1700 1800 1900

Figure 6.7
Reconstructions of average
temperatures from various
proxy data sets for the NH
(10 data sets) and the SH
(7 data sets). The heavy line
is a 50-year Gaussian filter
showing broad trends.
( 1998 , Edward Arnold
Publishers Ltd., Jones et al.,
Holocene, 8: 455-471, www.
hodderarnoldjournals.com)

NH10 (averages of normalized series)

1.0

0.0

-1.0

1.0

0.0

-1.0

SH7 (averages of normalized series)

1100 1200 1300 1400 1500 1600 1700 1800 1900