Tree Rings and Climate: Sharpening the Focus - Dendroclimatology - page 332

In the 1990s, scientific interest in the nature of decadal-scale climate variability
rose with analyses that highlighted the lower-frequency (decadal and longer) aspects
of climatic variability (e.g., Graham 1994 ; Mantua et al. 1997 ; Zhang et al. 1997 ;
Gershunov and Barnett 1998 ; Enfield and Mestas-Nuñez 2000 ; Enfield et al. 2001 ) .
This interest, in turn, led to efforts to extend the records of indices such as the
Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO),
and the North Atlantic Oscillation (NAO) using networks of tree-ring chronologies
(for example, Cook et al. 1998 ; Biondi et al. 2001 ; D'Arrigo et al. 2001 , 2005b ;
Gedalof et al. 2002 ; Villalba et al. 2001 ; Grayetal. 2004 ) .
Villalba et al. ( 2001 ; Chapter 7 , this volume) demonstrate the existence of
substantial synchrony on multiyear to multidecadal timescales in the spatial and
temporal patterns of temperature variability along the American Cordillera in con-
nection with sea surface temperature (SST) variability throughout the Pacific Ocean
(see also Dettinger et al. 2001 ) . This finding has been possible as a result of the con-
tinued development of dendroclimatic networks. The association, or teleconnection,
between SST variability in the Pacific and temperature and precipitation changes
along the entire American Cordillera has been related to forced large-scale modes
of the atmospheric circulation, such as the northern and southern annular modes
(Thompson and Wallace 2000 ; Thompson et al. 2000 ) and to planetary wave struc-
tures due to changes in diabatic heating of the atmosphere associated with the SST
changes (Horel and Wallace 1981 ; Karoly 1989 ; Hoerling and Kumar 2000 ; Diaz
et al. 2001 ) .

11.3 Reconstruction of Regional to Hemispheric Temperature
for Recent Centuries

Early efforts to constrain the range of natural variability from regional to global
scales are discussed in the volume Global Changes of the Past (Bradley 1991 ) . Webb
( 1991 ) and Wigley ( 1991 ) estimated the natural variability of global temperatures
to be in the range of 1.0-1.5 ◦ C in the past 1000 years. With regard to regional
spatial scales, Overpeck ( 1991 ) estimated that Little Ice Age (LIA) temperatures
were within about 2 ◦ C of mid-twentieth-century levels, but he also noted that 'the
exact geographic and temporal character of the LIA has not yet been established '

Attention was focused on the so-called medieval period in a workshop reported
as a series of papers in the journal Climatic Change in 1994. This workshop
represented a continuing effort to evaluate the amplitude and extent of episodes
of multidecadal to century-scale climatic variations in the historical and high-
resolution paleoclimate record. Hughes and Diaz ( 1994 ) evaluated the following
statement found in a US Department of Energy ( 1989 ) report: 'A thousand years ago,
climate in the North Atlantic regions was perhaps 1 ◦ C warmer than now

...

' Based
on an extensive review of the literature, together with the findings of the other papers
published in this special issue of Climatic Change , Hughes and Diaz concluded,
'[T]he available evidence does not support a global Medieval Warm Period

...

... '