Was There a 'Medieval Warm Period', and if so, Where and When?
Malcolm K. Hughes
Laboratory of Tree Ring Research, University of Arizona, Tucson, AZ
Henry F. Diaz
NOAA/ERL/CDC, Boulder, CO
Abstract
It has frequently been suggested that the period encompassing the ninth
to the fourteenth centuries A.D. experienced a climate warmer than that
prevailing around the tum of the twentieth century. This epoch has
become known as the Medieval Warm Period, since it coincides with the
Middle Ages in Europe. In this review a number of lines of evidence are
considered, (including climate-sensitive tree rings, documentary
sources, and montane glaciers) in order to evaluate whether it is
reasonable to conclude that climate in medieval times was, indeed, warmer
than the climate of more recent times. Our review indicates that for
some areas of the globe (for example, Scandinavia, China, the Sierra
Nevada in Califomia, the Canadian Rockies and Tasmania), temperatures,
particularly in summer, appear to have been higher during some parts of
this period than those that were to prevail until the most recent decades
of the twentieth century. These warmer regional episodes were not
strongly synchronous. Evidence from other regions (for example, the
Southeast United States, southem Europe along the Mediterranean, and
parts of South America) indicates that the climate during that time was
little different to that of later times, or that wamming, if it
occurred, was recorded at a later time than has been assumed. Taken
together, the available evidence does not support a global Medieval Warm
Period, although more support for such a phenomenon could be drawn from
high-elevation records than from low-elevation records. The available
data exhibit significant decadal to century scale variability throughout
the last millennium. A comparison of 30-year averages for various
climate indices places recent decades in a longer term perspective.