Climate change - Atmosphere, Weather and Climate - page 386

conditions in Europe and North America. Although
temperatures since have not equalled those of the
thermal maximum, a warmer interval or intervals
occurred between the ninth and mid-fifteenth centuries
AD . Summer temperatures in Scandinavia, China, the
Sierra Nevada (California), Canadian Rocky Mountains
and Tasmania exceeded those that prevailed until the
late twentieth century.

3 The past 1000 years

Temperature reconstructions for the northern hemi-
sphere over the past millennium are based on several
types of proxy data, but especially dendrochronology,
ice cores and historical records. Figure 13.6 shows such
reconstructions for the past millennium. Given that the
range of twice the standard error is ±0.5°C until about
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. Documentary records for western and
central Europe show a warm phase around AD 1300.
Icelandic records indicate mild conditions up to 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 Innuit.
Deteriorating conditions followed, and severe winters
between AD 1450 and 1700 gave a 'Little Ice Age'
with extensive Arctic pack-ice and glacier advances in
some areas to maximum positions since the end of the
Ice Age. 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 and
regional variability. The coldest interval of the Little Ice
Age in the northern hemisphere was AD 1570 to 1730.
Climate records for the past 500 years suggest fluc-
tuations on three time intervals: fifteen to thirty-five
years with a peak-to-peak amplitude of 0.3°C; fifty to a
hundred years with a 1.0°C amplitude over the North
Atlantic-Arctic; and 100 to 400-year global oscillations
of about 0.75°C. The shortest interval is apparently
linked to ENSO/PNA circulation modes and the fifty- to
hundred-year scale to low-frequency changes in the
thermohaline circulation. Interdecadal variability seems
to be mostly induced by atmospheric dynamics. Much
of the variance in winter temperatures is associated with
the varying strength of land/ocean contrasts in the 1000
to 500 mb thickness field.

Figure 13.6 Trends of air temperature for the northern hemi-
sphere over the past millennium. The reconstructed forty-year
smoothed values are plotted for 1000 to 1880 together with the
linear trend 1000 to 1850, and observed temperatures for 1902
to 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 to 1600. The values are plotted as
anomalies relative to 1961 to 1990.
Source : Adapted from Mann et al . (1999), courtesy of M.E. Mann,
University of Virginia, and the American Geophysical Union.

Long instrumental records for stations in Europe
and the eastern United States indicate that the warming
trend that ended the 'Little Ice Age' had begun at least
by the mid-nineteenth century (Figure 13.7). Global
records since 1881 (Figure 13.8) show a significant, but
irregular, temperature rise of between 0.3 and 0.6°C,
probably closer to the upper estimate. This trend was
least in the tropics and greatest in cloudy, maritime
regions of high latitudes. Winter temperatures were
most affected. The general temperature rise has not been
continuous, however, and four phases can be identified:

1 1881 to 1920 , during which there was mean annual
oscillation within extreme limits of 0.4°C but no
consistent trend.
2 1920 to mid-1940s , during which there was con-
siderable warming averaging 0.4°C.
3 The mid-1940s to early 1970s , during which there
were oscillations within extreme limits of less than
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
experienced a mean lowering of winter temperatures
by 2 to 3°C between 1940 and 1949 and 1950 and