Geoscience Reference
In-Depth Information
Figure 10.11.
Regional pollen concentrations in the Aggasiz
Ice Cap ice core A77 (from Bradley,
1990
, by permission of
Elsevier).
10.7.2
Late Holocene Cooling, Medieval Warm
Period and the LIA
While the above evidence points to as generally warm climate in the first half of the
Holocene, albeit with a notable 8.2 ka cooling event and a highly variable timing
of the HTM depending on the region, conditions by about 5 ka were overall begin-
ning to deteriorate. Atlantic water retreated southward to the central Greenland-
Iceland-Norwegian seas and the cold East Greenland current strengthened. Average
temperatures continued to drop through 3 ka. The Atlantic ceased to have significant
influence on the GIN seas and the sea ice cover expanded (Koc et al.,
1993
).
Kaufman et al. (
2009
) synthesized a set of proxy records from lakes, tree rings
and ice cores to assess the temperature history of the Arctic region over the past
2,000
years.
Figure 10.12
, the composite temperature time series for the Arctic as a
whole, is representative of summer conditions. The key conclusion from this analy-
sis is that a progressive cooling over the first 1,900 years of the record abruptly
reversed during the twentieth century, with four of the of the five warmest decades
occurring between the years 1950 and 2000 The cooling over the first 1,900 years of
the temperature record is interpreted as a response to a steady reduction in northern
hemisphere summer insolation (Milankovitch forcing) following the HTM. Within
this 1,900-year long general cooling trends there are centennial scale fluctuations.
Although not pronounced in the Arctic proxy records, there is evidence for both the
Medieval Warm Period from about 900 to 1050 and the Little Ice Age (LIA) from
about 1600 to 1850. Proxy synthesis for the Northern Hemisphere as a whole do not
show the 1,900-year general cooling trend that is prominent in the Arctic analysis.
A reconstructed time series of summer Arctic sea ice extent by C. Kinnard et al.
(
2011
) for the past 1,450 years is broadly consistent with the reconstructed tem-
perature record but contains some puzzling features. Notably, ice extent appears
to have been reduced during the period of the LIA, which they argue may relate to
advection of warm Atlantic water into the Arctic.
