Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 198

Figure 6.6 Recent
reconstructions of NH
temperature for the last 1000
years. All time series have
been smoothed to highlight
variations on the 50-year time
scale. References for each
time series in box on the
figure. The time period
usually given for the Medieval
Warm Period (MWP, AD
900-1300) and the Little Ice
Age (LIA, AD 1450-1850)
has been changed for this
figure given the likely
boundary at AD 1400 in the
records shown. (Modified
from Jones et al. 2001 )

0.2

MWP? LIA?

0.0

-0.2

-0.4

Jones et al. (1998)
Mann et al. (1999)
Briffa et al. (2000)
Crowley & Lowery (2000)
Observation.

-0.6

1000 1200 1400 1600 1800 2000

Year (AD)

there is no evidence for any exceptional warmth in southern Europe until the
mid fourteeth century. Interestingly, if one looks more at western Europe,
there is evidence of warm springs and dry summers throughout most of the
thirteenth century, evidence that may be used to contribute to the idea that
there is a lack of temporal synchroneity in the MWP. However, even within
northern parts of Europe there are contrasting climatic conditions. For
instance, the positive summer temperature anomalies over the millennium
mean in the Polar Urals from 1110 to 1350, as derived from tree-ring records,
is opposite to the negative anomalies shown by summer temperatures in
Fennoscandia during the same time period. On the other hand, there are
positive anomalies in the Fennoscandian records from 971 to 1100 and
1350 to 1540.
Looking at tree-ring evidence in North America shows a similar disconti-
nuity in regional climatic conditions for the MWP. For instance, high-altitude
studies in the Canadian Rockies indicate climatic conditions favorable for
glacial retreat in the tenth to thirteenth centuries, as well as the advance of
forests into higher elevations of the Canadian Rockies. Summer temperatures
in the Sierra Nevada show positive temperature anomalies from the millen-
nium mean over a longer time frame than in the Canadian Rockies, that is,
warm summers are prevalent from 1090 to 1450. The southwestern United
States may have experienced climatic conditions that would favor greater
crop growth (i.e. an increase in effective precipitation compared to other time
periods) during the tenth to thirteenth centuries. This improved crop growth
could be related to an enhanced Arizona monsoon between 700 and 1350.
Summer enhanced monsoons could be related to an increase in solar receipt.
However, in the southeastern United States, there does not appear to be any
evidence of either prolonged wet spells or prolonged drought in the ninth
through fourteenth centuries.
Overall, a consistent pattern of continuous and spatially consistent warm-
ing between about 900 and 1450 is lacking, but this does not preclude the