Unformatted text preview: tes, e.g., the late 1870s El Niño is relatively
stronger if averages are taken over Niño 3 or a 5°
Nevertheless, ‘‘super El Niños’’ clearly were more abundant in the
last quarter of the 20th century than earlier in the century.
Global warming is expected to slow the mean tropical circulation
(24–26), including the Walker cell. Sea level pressure data suggest
a slowdown of the longitudinal wind by 3.5% in the past century
(26). A relaxed longitudinal wind should reduce the WEP–EEP
temperature difference on the broad latitudinal scale ( 10°N to
15°S) of the atmospheric Walker cell. Observed SST anomalies are
consistent with this expectation, because the cooling in the EEP
relative to WEP decreases at latitudes away from the narrower
region strongly affected by upwelling off the coast of Peru (Fig. 3A).
Averaged over 10°N to 15°S, observed warming is as great in the
EEP as in the WEP (see also Fig. 7).
We make no suggestion about changes of El Niño frequency, and
we note that an abnormally warm WEP does not assure a strong El
Niño. The origin and nature of El Niños is affected by chaotic ocean
and atmosphere variations, the season of the driving anomaly, the
state of the thermocline, and other factors, assuring that there will
always be great variability of strength among El Niños.
Will increased contrast between near-equatorial WEP and EEP
SSTs be maintained or even increase with more global warming?
The WEP should respond relatively rapidly to increasing GHGs. In
the EEP, to the extent that upwelling water has not been exposed
to the surface in recent decades, little warming is expected, and the
Hansen et al. contrast between WEP and EEP may remain large or increase in
Thus, we suggest that the global warming effect on El Niños is
analogous to an inferred global warming effect on tropical storms
(27). The effect on frequency of either phenomenon is unclear,
depending on many factors, but the intensity of the most powerful
events is likely to increase as GHGs increase. In this case, slowing
the growth rate of GHGs should diminish the probability of both
super El Niños and the most intense tropical storms.
Estimating Dangerous Climate Change
Modern vs. Paleo Temperatures. Modern SST measurements (5, 6)
are compared with proxy paleoclimate temperature (28) in the
WEP (Ocean Drilling Program Hole 806B, 0°19 N, 159°22 E; site
circled in Fig. 3A) in Fig. 4A. Modern data are from ships and buoys
for 1870–1981 (6) and later from satellites (5). In concatenation of
satellite and ship data, as shown in Fig. 8A, the satellite data are
adjusted down slightly so that the 1982–1992 mean matches the
mean ship data for that period.
The paleoclimate SST, based on Mg content of foraminifera
shells, provides accuracy to 1°C (29). Thus we cannot be sure that
we have precisely aligned the paleo and modern temperature scales.
Accepting paleo and modern temperatures at face value implies a
WEP 1870 SST in the middle of its Ho...
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