Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 188

(7, 8, 11, and 19 years). The reconstructed spring temperature series suggests that
the recent warming exceeds temperature levels of prior centuries, extending back
to AD 1600 (Wiles at al. 1998). The three coldest intervals in the spring series
occurred in the seventeenth century. This cooling is consistent with the glacial
record from coastal Alaska, which shows a strong advance during the late seven-
teenth to mid-eighteenth centuries (Wiles and Calkin 1994 ; Wiles 1997 ; Wiles et al.
2004 ) .
A critical appraisal of surface air temperature from station records has recently
been presented for southern South America (Villalba et al. 2003 ) . Two different
spatial temperature patterns were recognized in the southern Andes during the twen-
tieth century: (1) surface cooling from 1930 to 1976 at the stations located in the
northern sector of the southern Andes by the Pacific Coast (37 ◦ − 42 ◦ S), and (2) a

remarkable surface warming in the southern stations (south of 46 ◦ S), which inten-
sifies at higher latitudes. Changes in the Pacific Decadal Mode around 1976 were
seen in summer temperature records at most stations in the Pacific domain, start-
ing a period with increased temperature across the southern Andes and at higher
latitudes. Tree-ring records from upper tree line were used to reconstruct past tem-
perature fluctuations for the two dominant patterns over the southern Andes. The
resulting reconstructions for the northern and southern sectors of the southern Andes
explain 55% and 45%, respectively, of the temperature variance over the interval
1930-1989. Cross-spectral analysis of actual and reconstructed temperatures over
the common interval 1930-1989, indicates that most of the explained variance is
at periods > 10 years in length. Consequently, these reconstructions are especially
useful for studying multidecadal temperature variations in the South American sec-
tor of the Southern Hemisphere over the past 360 years. These reconstructions show
that temperatures during the twentieth century have been anomalously warm across
the southern Andes. The mean annual temperatures for the northern and southern
sectors during the interval 1900-1990 are 0.53 ◦ C and 0.86 ◦ C above the 1640-1899
means, respectively (Villalba et al. 2003 ) .

Box 7.3 Climate signals in Patagonian upper-elevation
tree-ring records

A great deal of progress has been made in increasing the number of upper-
elevation tree-ring chronologies across the southern Andes during the past
decade. This work has involved the development of more than 90 chronolo-
gies from collections of Nothofagus pumilio , the dominant subalpine tree in
the Andes of Chile and Argentina (Villalba et al. 1997 ; Laraetal. 2001 ;
Aravena et al. 2002 ; Villalba et al. 2003 ; Laraetal. 2005 ) . These new col-
lections have increased both the spatial coverage (ca. 35 ◦ 35 to 55 ◦ S) and
the temporal span of upper-elevation records across the southern Andes (Box
Fig. 7.5 ) . The broad latitudinal distribution of N. pumilio across 2000 km in