5. Past Changes in Climate and Tree Growth in the Western Americas 83
The EI Nino-Southern Oscillation
ENSO is a key forcing factor for climate variability along the west coasts
of the Americas. Because each warm/cold event can differ in phasing ,
amplitude, and spatial signature, proxy data coverage from different
ENSO-sensitive regions around the globe is essential in order to determine
how each paleo event has varied in time and space. Historical records of
regional-scale EI Nifios from alongthe west coast of South America, for
example , show differences when compared to broader-scale ENSO listings
for the equatorial Pacific (Quinn & Neal, 1992).
Tree-ring records from western North America have been shown to
reflect past ENSO variations (Cleaveland, Cook, & Stahle, 1992; D' Arrigo
& Jacoby, 1991; Lough & Fritts, 1985; Swetnam & Betancourt, 1990).
Much of South America is also under the strong influence of ENSO (e.g .,
Aceituno , 1988; Quinn & Neal, 1992). Although chronologies have been
produced from Chile and Argentina, there is virtually no coverage in
Ecuador and Peru, where some of the strongest ENSO effects are found.
Lough and Fritts (1985) attempted a reconstruction of ENSO based on
Southern Hemisphere chronologies, although the results were less encouraging than reconstructions based on western North American
chronologies. However, additional research is needed to more fully
evaluate the ENSO signal in the Southern Hemisphere tree-ring data.
Comparison of the chronologies from western North and South America
could be useful for studying the differential respon se of tree growth to
drought and ENSO variables in these two settings .
High-Latitude Climatic Response
It is anticipated that the AMIGO project can aid the detection of globalwarming effects on the biosphere (Fuentes et al. , 1991). Climate models
project that increasing levels of atmospheric CO 2 will result in pronounced
warming at high latitudes of both hemispheres. Yet due to the smaller
Southern Hemisphere land-sea ratio, the Southern Hemisphere warming
might be delayed due to the buffering effect of oceans and ice (Fuentes et
al., 1991; Gordon, 1992). The instrumental record does not support this
projection, however, since SH temperatures have recently been increasing
at a faster rate (Gordon, 1992).
Tree rings are one source of high-resolution data that can help determine whether climate and forest growth during the period of anthropogenic change exceed levels of past natural variability . One possible
AMIGO project would be to compare tree-ring records from the high
latitudes of western North and South America to determme if recent
growth exceeds prior levels and to compare the trends for the two regions.
The tree-ring records shown in Figures 5.2 and 5.3 represent examples of
relevant data. Comparison of the standard errors of recent levels of tree
growth or inferred warming as compared to the long-term reconstructed
The EI Nino-Southern Oscillation
ENSO is a key forcing factor for climate variability along the west coasts
of the Americas. Because each warm/cold event can differ in phasing ,
amplitude, and spatial signature, proxy data coverage from different
ENSO-sensitive regions around the globe is essential in order to determine
how each paleo event has varied in time and space. Historical records of
regional-scale EI Nifios from alongthe west coast of South America, for
example , show differences when compared to broader-scale ENSO listings
for the equatorial Pacific (Quinn & Neal, 1992).
Tree-ring records from western North America have been shown to
reflect past ENSO variations (Cleaveland, Cook, & Stahle, 1992; D' Arrigo
& Jacoby, 1991; Lough & Fritts, 1985; Swetnam & Betancourt, 1990).
Much of South America is also under the strong influence of ENSO (e.g .,
Aceituno , 1988; Quinn & Neal, 1992). Although chronologies have been
produced from Chile and Argentina, there is virtually no coverage in
Ecuador and Peru, where some of the strongest ENSO effects are found.
Lough and Fritts (1985) attempted a reconstruction of ENSO based on
Southern Hemisphere chronologies, although the results were less encouraging than reconstructions based on western North American
chronologies. However, additional research is needed to more fully
evaluate the ENSO signal in the Southern Hemisphere tree-ring data.
Comparison of the chronologies from western North and South America
could be useful for studying the differential respon se of tree growth to
drought and ENSO variables in these two settings .
High-Latitude Climatic Response
It is anticipated that the AMIGO project can aid the detection of globalwarming effects on the biosphere (Fuentes et al. , 1991). Climate models
project that increasing levels of atmospheric CO 2 will result in pronounced
warming at high latitudes of both hemispheres. Yet due to the smaller
Southern Hemisphere land-sea ratio, the Southern Hemisphere warming
might be delayed due to the buffering effect of oceans and ice (Fuentes et
al., 1991; Gordon, 1992). The instrumental record does not support this
projection, however, since SH temperatures have recently been increasing
at a faster rate (Gordon, 1992).
Tree rings are one source of high-resolution data that can help determine whether climate and forest growth during the period of anthropogenic change exceed levels of past natural variability . One possible
AMIGO project would be to compare tree-ring records from the high
latitudes of western North and South America to determme if recent
growth exceeds prior levels and to compare the trends for the two regions.
The tree-ring records shown in Figures 5.2 and 5.3 represent examples of
relevant data. Comparison of the standard errors of recent levels of tree
growth or inferred warming as compared to the long-term reconstructed