Computer model simulations have given rise to three climate-alarmist claims regarding the influence of global warming on ENSO events: (1) global warming will increase the frequency of ENSO events, (2) global warming will increase the intensity of ENSO events, and (3) weather-related disasters will be exacerbated under El Niņo conditions. In this summary, we describe materials related to the first two of these assertions. For materials on the latter item, see ENSO (Relationship to Extreme Weather) in our Subject Index).
To begin, then, we note that all of the claims described above are derived from climate model simulations. Timmermann et al. (1999), for example, developed a global climate model which, according to them, operates with sufficient resolution to address the issue of whether "human-induced 'greenhouse' warming affects, or will affect, ENSO." When running this model with increasing greenhouse-gas concentrations, more frequent El-Niņo-like conditions do indeed occur. However, this is not what observational data reveal to be the case. The frequent and strong El Niņo activity of the recent past is in actuality no different from that of a number of other such episodes of prior centuries, when it was considerably colder than it is today, as described in several of the papers highlighted below. And in many instances, the El Niņo activity of the recent past is shown to be vastly inferior to that of colder times.
Evans et al. (2002) reconstructed gridded Pacific Ocean sea surface temperatures from coral stable isotope (ð18O) data, from which they assessed ENSO activity over the period 1607-1990. The results of their analysis showed that a period of relatively vigorous ENSO activity over the colder-than-present period of 1820-1860 was "similar to [that] observed in the past two decades." Likewise, in a study that was partly based upon the instrumental temperature record for the period 1876-1996, Allan and D'Arrigo (1999) found four persistent El Niņo sequences similar to that of the 1990s; and using tree-ring proxy data covering the period 1706 to 1977, they found several other ENSO events of prolonged duration. In fact, there were four or five persistent El Niņo sequences in each of the 18th and 19th centuries, which were both significantly colder than the final two decades of the 20th century, leading them to conclude there is "no evidence for an enhanced greenhouse influence in the frequency or duration of 'persistent' ENSO event sequences."
In a somewhat different type of study, Brook et al. (1999) analyzed the layering of couplets of inclusion-rich calcite over inclusion-free calcite, and darker aragonite over clear aragonite, in two stalagmites from Anjohibe Cave in Madagascar, comparing their results with historical records of El Niņo events and proxy records of El Niņo events and sea surface temperatures derived from ice core and coral data. This exercise revealed that the cave-derived record of El Niņo events compared well with the historical and proxy ice core and coral records; and these data indicated, in Brook et al.'s words, that "the period 1700-50 possibly witnessed the highest frequency of El Niņo events in the last four and a half centuries while the period 1780-1930 was the longest period of consistently high El Niņo occurrences," both of which periods were considerably cooler than the 1980s and 90s.
In another multi-century study, Meyerson et al. (2003) analyzed an annually-dated ice core from the South Pole that covered the period 1487-1992, specifically focusing on the marine biogenic sulfur species methanesulfonate (MS), after which they used orthogonal function analysis to calibrate the high-resolution MS series with associated environmental series for the period of overlap (1973-92). This procedure allowed them to derive a five-century history of ENSO activity and southeastern Pacific sea-ice extent, the latter of which parameters they say "is indicative of regional temperatures within the Little Ice Age period in the southeastern Pacific sea-ice sector."
In analyzing these records, Meyerson et al. noted a shift at about 1800 towards generally cooler conditions. This shift was concurrent with an increase in the frequency of El Niņo events in the ice core proxy record, which is contrary to what is generally predicted by climate models. On the other hand, their findings were harmonious with the historical El Niņo chronologies of both South America (Quinn and Neal, 1992) and the Nile region (Quinn, 1992; Diaz and Pulwarty, 1994), which depict, in their words, "increased El Niņo activity during the period of the Little Ice Age (nominally 1400-1900) and decreased El Niņo activity during the Medieval Warm Period (nominally 950-1250)," as per Anderson (1992) and de Putter et al., 1998).
Taking a little longer look back in time were Cobb et al. (2003), who generated multi-century monthly-resolved records of tropical Pacific climate variability over the last millennium by splicing together overlapping fossil-coral records from the central tropical Pacific, which exercise allowed them "to characterize the range of natural variability in the tropical Pacific climate system with unprecedented fidelity and detail." In doing so, they discovered that "ENSO activity in the seventeenth-century sequence [was] not only stronger, but more frequent than ENSO activity in the late twentieth century." They also found "there [were] 30-yr intervals during both the twelfth and fourteenth centuries when ENSO activity [was] greatly reduced relative to twentieth-century observations." Once again, therefore, we have a situation where ENSO activity was much greater and more intense during the cold of the Little Ice Age than the warmth of the late 20th century.
Inching still further back in time, Eltahir and Wang (1999) used water-level records of the Nile River as a proxy for El Niņo episodes over the past 14 centuries. This approach indicated that although the frequency of El Niņo events over the 1980s and 90s was high, it was not without precedent, being similar to values observed near the start of the 20th-century and much the same as those "experienced during the last three centuries of the first millennium," which latter period, according to Esper et al. (2002), was also significantly cooler than the latter part of the 20th century.
Woodroffe et al. (2003) found pretty much the same thing, but over an even longer period of time. Using oxygen isotope ratios obtained from Porites microatolls at Christmas Island in the central Pacific to provide high-resolution proxy records of ENSO variability since 3.8 thousand years ago (ka), they found, in their words, that "individual ENSO events in the late Holocene [3.8-2.8 ka] appear at least as intense as those experienced in the past two decades." In addition, they note that "geoarcheological evidence from South America (Sandweiss et al., 1996), Ecuadorian varved lake sediments (Rodbell et al., 1999), and corals from Papua New Guinea (Tudhope et al., 2001) indicate that ENSO events were considerably weaker or absent between 8.8 and 5.8 ka," which was the warmest part of the Holocene. In fact, they report that "faunal remains from archeological sites in Peru (Sandweiss et al., 2001) indicate that the onset of modern, rapid ENSO recurrence intervals was achieved only after ~4-3 ka," or during the long cold interlude that preceded the Roman Warm Period (McDermott et al., 2001).
Also concentrating on the mid to late Holocene were McGregor and Gagan (2004), who used several annually-resolved fossil Porites coral δ18O records to investigate the characteristics of ENSO events over a period of time in which the earth cooled substantially. For comparison, study of a modern coral core provided evidence of ENSO events for the period 1950-1997, the results of which analysis suggest they occurred at a rate of 19 events/century. The mid-Holocene coral δ18O records, on the other hand, showed reduced rates of ENSO occurrence: 12 events/century for the period 7.6-7.1 ka, 8 events/century for the period 6.1-5.4 ka, and 6 events/century at 6.5 ka. For the period 2.5-1.7 ka, however, the results were quite different, with all of the coral records revealing, in the words of McGregor and Gagan, "large and protracted δ18O anomalies indicative of particularly severe El Niņo events." They note specifically, for example, that "the 2.5 ka Madang PNG coral records a protracted 4-year El Niņo, like the 1991-1994 event, but almost twice the amplitude of [the] 1997-1998 event (Tudhope et al., 2001)." In addition, they say that "the 2 ka Muschu Island coral δ18O record shows a severe 7-year El Niņo, longer than any recorded Holocene or modern event." And they add that "the 1.7 ka Porites microatoll of Woodroffe et al. (2003) also records an extreme El Niņo that was twice the amplitude of the 1997-1998 event." Taken together, these several sets of results portray what McGregor and Gagan describe as a "mid-Holocene El Niņo suppression and late Holocene amplification."
That there tend to be fewer and weaker ENSO events during warm periods has further been documented by Riedinger et al. (2002). In a 7,000-year study of ENSO activity in the vicinity of the Galapagos Islands, they determined that "mid-Holocene [7130 to 4600 yr BP] El Niņo activity was infrequent," when, of course, global air temperature was significantly warmer than it is now, but that both the "frequency and intensity of events increased at about 3100 yr BP," when it finally cooled below current temperatures. More specifically, throughout the former 2530-year warm period, their data revealed the existence of 23 strong to very strong El Niņos and 56 moderate events; while throughout the most recent (and significantly colder) 3100-year period, they identified 80 strong to very strong El Niņos and 186 moderate events. These numbers correspond to rates of 0.9 strong and 2.2 moderate occurrences per century in the earlier warm period and 2.7 strong and 6.0 moderate occurrences per century in the latter cool period, suggestive of an approximate tripling of the rate of occurrence of both strong and moderate El Niņos in going from the warmth of the Holocene "Climatic Optimum" to the colder conditions of the past three millennia.
Similar results have been reported by Andrus et al. (2002) and Moy et al. (2002). According to Andrus et al., sea surface temperatures off the coast of Peru some 6000 years ago were 3 to 4°C warmer than what they were over the decade of the 1990s and provided little evidence of any El Niņo activity. Nearby, Moy et al. analyzed a sediment core from lake Laguna Pallcacocha in the southern Ecuadorian Andes, producing a proxy measure of ENSO over the past 12,000 years. For the moderate and strong ENSO events detected by their analytical techniques (weaker events are not registered), these researchers state that "the overall trend exhibited in the Pallcacocha record includes a low concentration of events in the early Holocene, followed by increasing occurrence after 7,000 cal. yr BP, with peak event frequency occurring at ~1,200 cal. yr BP," after which the frequency of events declines dramatically to the present.
With respect to the last 1,200 years of this record, however, the decline in the frequency of ENSO events is anything but smooth. In coming out of the Dark Ages Cold Period, which was one of the coldest intervals of the Holocene (McDermott et al., 2001), the number of ENSO events experienced by the earth drops by an order of magnitude, from a high of approximately 33 events per 100 yr to a low of about 3 events per 100 yr, centered approximately on the year AD 1000, which is right in the middle of the Medieval Warm Period, as delineated by the work of Esper et al. (2002). Then, at approximately AD 1250, the frequency of ENSO events exhibits a new peak of approximately 27 events per 100 yr in the midst of the longest sustained cold period of the Little Ice Age, again as delineated by the work of Esper et al. Finally, ENSO event frequency declines in zigzag fashion to a low on the order of 4 to 5 events per 100 yr at the start of the Modern Warm Period, which according to the temperature history of Esper et al. begins at about 1940.
Going even further back in time, in a study of a recently revised New England varve chronology derived from proglacial lakes formed during the recession of the Laurentide ice sheet some 17,500 to 13,500 years ago, Rittenour et al. (2000) determined that "the chronology shows a distinct interannual band of enhanced variability suggestive of El Niņo-Southern Oscillation (ENSO) teleconnections into North America during the late Pleistocene, when the Laurentide ice sheet was near its maximum extent ... during near-peak glacial conditions." But during the middle of the Holocene, when it was considerably warmer, even than it is today, Overpeck and Webb (2000) report that data from corals suggest that "interannual ENSO variability, as we now know it, was substantially reduced, or perhaps even absent."
In summing up the available evidence pertaining to the effect of temperature on the frequency of occurrence and strength of ENSO events, we have one of the most sophisticated climate models ever developed to deal with the ENSO phenomenon implying that global warming will promote more frequent El Niņo-like conditions; while we have a number of real-world observations demonstrating that El Niņo-like conditions during the latter part of the 20th century (claimed by climate alarmists to be the warmest period of the past two thousand years) are not unprecedented in terms of their frequency or magnitude and are, in fact, not much different from those that occurred during much colder times. In addition, we have a number of long-term records that suggest that when the earth was significantly warmer than it is currently, ENSO events were substantially reduced or perhaps even absent!
Clearly, the models -- and their blind-to-everything-else followers -- have a problem. They are one hundred and eighty degrees out of phase with reality.
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Andrus, C.F.T., Crowe, D.E., Sandweiss, D.H., Reitz, E.J. and Romanek, C.S. 2002. Otolith ð18O record of mid-Holocene sea surface temperatures in Peru. Science 295: 1508-1511.
Brook, G.A., Rafter, M.A., Railsback, L.B., Sheen, S.-W. and Lundberg, J. 1999. A high-resolution proxy record of rainfall and ENSO since AD 1550 from layering in stalagmites from Anjohibe Cave, Madagascar. The Holocene 9: 695-705.
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