The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 169

There is evidence that the MCA was not regional (confined to Europe) and was
to some degree at least hemispheric and, if not in terms of temperature but precipit-
ation, more global. Preserved tree stumps have been found in marshes and lakes in
California's Sierra Nevada that have been radiocarbon dated and analysed by dendro-
chronology (see section 2.1.1) to determine the climate. This has revealed that the
Sierra experienced severe drought for over two centuries before ad 1112 and for

more than 140 years before 1350 (Steine, 1994). Diatom (common members

of algal flora) analysis of sediment cores from Moon Lake, in the Upper Colorado
River, infer some level of salinity. This suggests that during the MCA, further to the
north west, the northern great plains that cover much of the US states of Montana,
Wyoming and North and South Dakota saw increased drought frequency compared
to more recent, hemispherically if not globally, cooler times. Today the area is one
of considerable agricultural productivity, but which was famously undermined by
droughts in the 1890s and 1930s (Laird et al., 1996). The implications for today
are that should 21st-century emissions of greenhouse gases continue then that area
could again suffer from severe and prolonged drought. (It is interesting to note that
since the late 1980s part of western North America has experienced a marked decline
in rainfall.)
There is, however, an absence of evidence for the MCA from some parts of the
northern hemisphere (such as dendrochronological evidence from the Urals). Further,
it may be that the warming was confined to certain seasons. The picture is unclear
and readers interested in the MCA are advised to read the references and do their own
literature searches. It might be that there was a general warming but accompanied
with some circulation changes that counteracted the warming in some areas. This is
not too unsurprising as climate change is never completely uniform all over the globe.
Given this, it is somewhat surprising that the MCA northern hemisphere temperatures
were in part reflected in the southern hemisphere. Using silver pine ( Lagarostrobos
colensoi ) in 2002 Edward Cook, Jonathan Palmer and Rosanne D'Arrigo reported the
first dendrochronological temperature analysis of past temperatures in New Zealand's
summer (January-March) that extended back through the proposed MCA

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1000-
1300 interval. They found that its expression in terms of the New Zealand summer
temperatures is composed of two periods of generally above-average warmth -

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1137-1177 and 1210-1260 - that are punctuated by years of below-average temper-
atures and a middle period that is near average. Overall, this translates to a MWP that
was probably 0.3-0.5 ◦ C warmer than the overall 20th-century average at Hokitika
and, for the 1210-1260 period, comparable to the warming that has occurred since

1950. However, to date (2012) the emerging picture of the MCA is not yet as coherent
as climate researchers would like. In 2009 US researchers, led by Michael Mann and
one British team member, reporting in Science , used a global climate proxy network
of tree-ring samples, ice cores, coral and sediments to reconstruct surface temper-
ature patterns over this interval. The medieval period was found to display warmth
that matches or exceeds that of the early first decade of the 21st century in some
regions, but which falls well below recent levels globally. This period was marked by
a tendency for conditions like the La Nina (part of the ENSO) in the tropical Pacific.
The authors suggest that the North Atlantic Oscillation may be involved in the MCA

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