Tree Rings and Climate: Sharpening the Focus - Dendroclimatology - page 334

period at the hemispheric level 'prior to the twentieth century very likely occurred
between 950 and 1100, but temperatures were probably between 0.1 and 0.2 ◦ C
below the 1961-1990 mean and significantly below the level shown by instrumen-
tal data after 1980.' Further, 'The evidence currently available indicates that NH
mean temperatures during medieval times (950-1100) were indeed warm in a 2-
kyr context and even warmer in relation to the less sparse but still limited evidence
of widespread average cool conditions in the 17th century However, the evi-

dence is not sufficient to support a conclusion that hemispheric mean temperatures
were as warm, or the extent of warm regions as expansive, as those in the twenti-
eth century as a whole, during any period in medieval times

...

...

'. In summarizing
all the reconstructions they considered (see their Fig. 6.10c), AR4 concluded that
for the Northern Hemisphere there were 'relatively cool conditions in the 17th and
early nineteenth centuries and warmth in the 11th and early 15th centuries, but the
warmest conditions are apparent in the twentieth century.' These features are con-
sistently represented in most of the published reconstructions (see AR4 Fig. 6.10c ),
and their scale in the last 3 or 4 centuries seems to be remarkably consistent, even
when quite different types of records have been used; for example, documentary
evidence, early instrumental measurements, or glacier dynamics (e.g., Luterbacher
et al. 2004 ; Oerlemans 2005 ) . As will be discussed below, the global climate of the
period between roughly AD 500 and AD 1500 turns out to be much more interesting
than a simple 'Warm Period,' and that finding owes much to dendroclimatology.

11.4 Causes of Climate Variability in the Past Millennium

Using statistical comparisons, Mann et al. ( 1998 ) attributed much of the
hemispheric-scale patterns of temperatures of the last 400 years to solar and vol-
canic forcing, with greenhouse gas concentrations 'emerging as the dominant factor
in the twentieth century.' By the time of the writing of the AR4 report (Jansen
et al. 2007 ) , a number of climate model runs had been performed forced by esti-
mates of natural and anthropogenic forcing factors and building on earlier modeling
work by, for example, Crowley ( 2000 ) . The forcing factors included solar radi-
ation, volcanic gases and ash, and greenhouse gases in every case, and some
combination of tropospheric sulfate aerosols, tropical and/or stratospheric ozone
changes, and/or halocarbons, and land use changes. A variety of models were used,
including coupled global climate models (GCMs), energy balance models (EBMs),
and earth system models of intermediate complexity (EMICS). Remarkably, a
fairly consistent picture of simulated temperature over the past 1000 years or so
emerges. 'Despite differences in the detail and implementation of the different forc-
ing histories, there is generally good qualitative agreement between the simulations
regarding the major features' (Jansen et al. 2007 ) . Further, most of the common
major features of the simulations are seen in the 'ensemble view' of the then avail-
able reconstructions, and the simulations largely fall within the envelope of the
various reconstructions and their uncertainties (AR4, Fig. 6.13d ). These results