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Hansen et al 2006 Global Temperature Changes

Sea level pressure data suggest a slowdown of the

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Unformatted text preview: tes, e.g., the late 1870s El Niño is relatively stronger if averages are taken over Niño 3 or a 5° 10° box. 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 coming decades. 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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