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and regional variability. The coldest interval of
the Little Ice Age in the Northern Hemisphere
was AD 1570-1730. What caused the Little Ice
Age is not entirely clear. Reduced solar output
associated with the Maunder Minimum in
sunspot activity (1645-1715) likely played a role,
as did increased volcanic activity.
Long instrumental records for stations in
Europe and the eastern United States indicate that
the warming trend that ended the Little Ice Age
began at least by the mid-nineteenth century. The
time series of global annual averaged surface air
temperature from instrumental records shows a
significant temperature rise of about 0.7°C from
1880 through 2007. Both hemispheres have
participated in this warming, but it is most
pronounced in the Northern Hemisphere (
Figure
13.7
). Warming in turn encompasses both land
and ocean regions, being stronger over land
(
Figure 13.8
). Warming has been smallest in the
tropics and largest in northern high latitudes.
Warming is in turn strongest during winter. The
general temperature rise has not been continuous,
however, and four basic phases may be identified
in the global record:
0.8
0.2
-0.2
-0.8
1000
1200
1400
1600
1800
2000
Ye a r
Figure 13.6
Variation in surface air temperature
for the Northern Hemisphere over the past
millennium. The reconstructed 40-year smoothed
values are plotted for 1000-1880 together with the
linear trend 1000-1850, and observed tempera-
tures for 1902-1998. The reconstruction is based
on estimates from ice cores, tree rings and
historical records, and has two standard error limits
of about ±0.5°C during 1000-1600. The values are
plotted as anomalies relative to 1961-1990.
Source: Adapted from Mann et al. (1999). Courtesy of M. E.
Mann, Pennsylvania State University.
records.
Figure 13.6
shows a reconstruction based
on such proxies for the past millennium. Until
about
AD
1600 there is still considerable disparity
in different estimates of decadal mean values and
their range of variation. Conditions appear to have
been slightly warmer between AD 1050 and 1330
than between 1400 and 1900. There is evidence in
Western and Central Europe for a warm phase
around AD 1300. Icelandic records indicate mild
conditions up until the late twelfth century, and
this phase was marked by the Viking colonization
of Greenland and the occupation of Ellesmere
Island in the Canadian Arctic by the Inuit.
Deteriorating conditions followed. This cool
period, known as the 'Little Ice Age', was
associated with extensive Arctic sea ice and glacier
advances in some areas to maximum positions
since the end of the last glacial cycle. These
advances occurred at dates ranging from the mid-
seventeenth to the late nineteenth century in
Europe, as a result of the lag in glacier response
1
1880-1920, during which there was an oscilla-
tion within extreme limits of about 0.3°C but
no trend.
2
1920-mid-1940s, during which there was
considerable warming of approximately 0.4
C;
this warming was most strongly expressed in
northern high latitudes.
3
Mid-1940s-early 1970s, during which there
were oscillations within extreme limits of
about 0.4
°
C, with the Northern Hemisphere
cooling slightly on average and the Southern
Hemisphere remaining fairly constant in
temperature. Regionally, northern Siberia, the
eastern Canadian Arctic and Alaska experi-
enced a mean lowering of winter temperatures
by 2-3
°
C between 1940 and 1949 and 1950
and 1959; this was partly compensated by a
slight warming in the western United States,
Eastern Europe and Japan.
°
