Tree Rings and Climate: Sharpening the Focus - Dendroclimatology - page 333

However, the review did indicate that for some areas of the globe, 'temperatures
appeared to have been warmer than mid-twentieth century values, although these
warmer regional episodes were not strongly synchronous.' Even so, the authors
pointed out that 'far more high-quality records of the climate of recent millennia
would be needed to provide a definitive answer to the question: was there a Medieval
Warm Period and if so, where and when?'
More than a decade after the Climatic Change volume, the report of Working
Group I of the Fourth Assessment Report (AR4) of the Intergovernmental Panel
on Climate Change (IPCC) came to a broadly similar conclusion regarding the
question addressed by Hughes and Diaz ( 1994 ) (Jansen et al. 2007 , Box 6.4),
even though considerably more data (including many tree-ring data) were by then
available, and major progress had been made in improving methods. AR4 stated
that, 'In medieval times, as now, climate was unlikely to have changed in the
same direction, or by the same magnitude, everywhere At some times, some

regions may have experienced even warmer conditions than those that prevailed
throughout the twentieth century

...

' AR4 then went on to make the important
point that, 'Regionally restricted evidence by itself,

...

is of little practical rele-
vance to the question of whether climate in medieval times was globally as warm
or warmer than today. Local climate variations can be dominated by internal cli-
mate variability, often the result of the redistribution of heat by regional climate
processes.'
The AR4 continues, 'Only very large-scale climate averages can be expected to
reflect global forcings over recent millennia

...

To define medieval warmth in a way
that has more relevance for exploring the magnitude and causes of recent large-scale
warming, widespread and continuous palaeoclimatic evidence must be assimilated
in a homogeneous way and scaled against recent measured temperatures to allow a
meaningful quantitative comparison against twentieth-century warmth

...

A number
of studies that have attempted to produce very large spatial-scale reconstructions
have come to the same conclusion: that medieval warmth was heterogeneous in
terms of its precise timing and regional expression

...

'
It is noteworthy that most of these climate reconstructions have used tree-ring
records. Several have been based solely on tree-ring evidence (for example, Briffa
et al. 2001 ; Esper et al. 2002 ; Cook et al. 2004a ; D'Arrigo et al. 2006 ) , and others
have combined tree rings with other annual- or decadal-resolution proxy climate
records, such as varved sediments, ice cores, coral growth bands, and historical doc-
uments (for example, Kaufman et al. 2009 ; Mann et al. 1998 , 1999 , 2000 , 2008 ,
2009 ; Mann and Jones, 2003 ; Moberg et al. 2005 ) . A few have used no tree rings
(Huang et al. 2000 ; Oerlemans 2005 ) . Most of the tree-ring data used have been
derived from ring widths, but an important body of work has been based on maxi-
mum latewood density (MXD; see Briffa et al. 2001 ) . In general, tree rings provide
the largest number of proxy records available for these reconstructions, and they
have the widest geographical distribution.
While commenting that, 'The uncertainty associated with present palaeoclimate
estimates of NH mean temperatures is significant, especially for the period prior to
1600 when data are scarce

...

... ' AR4 (Jansen et al. 2007 ) noted that the warmest