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Unformatted text preview: nge is 0.32°C and 0.36°C for the land–ocean index and
meteorological station analyses, respectively.
Warming rates in the model are 0.35, 0.19, and 0.24°C per decade
for scenarios A, B. and C, and 0.19 and 0.21°C per decade for the
observational analyses. Forcings in scenarios B and C are nearly the
same up to 2000, so the different responses provide one measure of
unforced variability in the model. Because of this chaotic variability,
a 17-year period is too brief for precise assessment of model
predictions, but distinction among scenarios and comparison with
the real world will become clearer within a decade.
Close agreement of observed temperature change with simulations for the most realistic climate forcing (scenario B) is accidental,
given the large unforced variability in both model and real world.
Indeed, moderate overestimate of global warming is likely because
the sensitivity of the model used (12), 4.2°C for doubled CO2, is
larger than our current estimate for actual climate sensitivity, which
is 3 1°C for doubled CO2, based mainly on paleoclimate data (17).
More complete analyses should include other climate forcings and
PNAS September 26, 2006 vol. 103 no. 39 14289 ENVIRONMENTAL
SCIENCES Fig. 1.
Surface temperature anomalies relative to 1951–1980 from surface air measurements at meteorological stations and ship and satellite SST
measurements. (A) Global annual mean anomalies. (B) Temperature anomaly for the ﬁrst half decade of the 21st century. A SST Change (°C) from 1870-1900 to 2001-2005 B Western and Eastern Pacific Temperature Anomalies (°C) Fig. 3. Comparison of SST in West and East Equatorial Paciﬁc Ocean. (A) SST in 2001–2005 relative to 1870 –1900, from concatenation of two data sets (5, 6),
as described in the text. (B) SSTs (12-month running means) in WEP and EEP relative to 1870 –1900 means. cover longer periods. Nevertheless, it is apparent that the first
transient climate simulations (12) proved to be quite accurate,
certainly not ‘‘wrong by 300%’’ (14). The assertion of 300% error
may have been based on an earlier arbitrary comparison of 1988–
1997 observed temperature change with only scenario A (18).
Observed warming was slight in that 9-year period, which is too
brief for meaningful comparison.
Super El Niños. The 1983 and 1998 El Niños were successively
labeled ‘‘El Niño of the century,’’ because the warming in the
Eastern Equatorial Pacific (EEP) was unprecedented in 100 years
(Fig. 3). We suggest that warming of the Western Equatorial Pacific
(WEP), and the absence of comparable warming in the EEP, has
increased the likelihood of such ‘‘super El Niños.’’
In the ‘‘normal’’ (La Niña) phase of El Niño Southern Oscillation
the east-to-west trade winds push warm equatorial surface water to
the west such that the warmest SSTs are located in the WEP near
Indonesia. In this normal state, the thermocline is shallow in the
EEP, where upwelling of cold deep water occurs, and deep in the
WEP (figure 2 of ref. 20). Associated with this tropical SST gradient
is a longitudinal circulation pattern in the atmosphere, the Walker
cell, with rising air and heavy rainfall in the WEP and sinking air and
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- Spring '08