Geoscience Reference
In-Depth Information
Figure 10.12.
Arctic summer temperature anomalies for the past 2,000 years based
on a variety of proxy sources. The blue line shows a reconstruction of summer Arctic
land temperatures over the past 2,000 years, based on a composite of twenty-three
proxy records from lake sediments, ice cores, and tree rings relative to the 1961-1990
reference period. The shaded area represents variability among different Arctic sites.
The red line shows Arctic temperatures from the instrumental record (courtesy of D.
Kaufman, Northern Arizona University, Flagstaff AZ, based on Kaufman et al.
2009
).
(See plate section for color version.)
During the Medieval Warm Epoch, surface air temperatures may have been
higher than at the start of the twentieth century in some areas, such as Scandinavia
and Greenland. Historical evidence includes records of crop planting in more north-
ern latitudes than the present. However, the picture is still unclear. T. Hughes and H.
Diaz (
1994
) give a useful review.
The LIA represents the most recent major cooling (Grove,
2004
). Whereas in
Figure 10.12
, the LIA occurred from about 1600-1850, depending on the region and
the analysis, it dated anywhere from AD 1250-1920 to AD 1550-1850 (Christiansen,
1998
). It is clearly recorded in Greenland ice cores (Fischer et al.,
1998
). Records
at GISP2 indicate that temperatures reached their lowest point in the past millen-
nium between AD 1579 and 1730 (Kreutz,
1997
). Temperatures in the northern to
mid-latitudes dropped an average of 0.5 to 1.2
o
C. In Norway, advance and retreat
histories of glaciers during the LIA seem to have varied according to distance of the
glaciers from moisture sources (Benn and Evans,
1998
).
Causes of the LIA are still somewhat uncertain. Solar variability likely played a
role (e.g., Rind et al.,
2004
). The Maunder Minimum from AD 1680 to 1730 was a
period of near absence of sunspots. Although low numbers of sunspots are known to
reduce total solar output, changes in the ultraviolet part of part of the solar spectrum
are proportionately much larger, invoking potential responses in both the strato-
spheric and tropospheric atmospheric circulation, and recent work points to feed-
backs in the ocean-atmosphere system that can amplify the response to irradiance
variations. J. Lean, J. Beer, and R. Bradley (
1995
) attribute most of the temperature
