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Global Warming and El Niņos
Volume 9, Number 19: 10 May 2006

In a study of the relationship between global warming and El Niņo activity that builds upon the earlier work of Rein et al. (2004), Rein et al. (2005) developed high-resolution marine proxy data for El Niņo variability over the last 20,000 years, which they obtained from a sediment core that was retrieved from a sheltered basin situated on the edge of the Peruvian shelf (12°03'S, 77°39.8'W) about 80 km west of Lima. So what did they find?

The most well-defined feature of the entire record was a dramatic depression of El Niņo activity between about 5.6 and 8 thousand years ago, which coincided with the major warmth of the Holocene Climatic Optimum. The next most significant feature was "the medieval period of low El Niņo activity," which the researchers describe as "the major anomaly during the late Holocene," i.e., the Medieval Warm Period, when their data indicate that "El Niņos were persistently weak between 800 and 1250 AD."

When Rein et al. studied the past millennium in more detail, however, they found signs of just the opposite behavior, but on a much shorter timescale, noting that, on average, "temperature reconstructions show higher temperatures with increased El Niņo activity." Such was only to be expected, however, for that is what El Niņos do; they create significant spikes in mean global air temperature, which was dramatically demonstrated by the 1997/98 El Niņo that produced the highest mean annual temperature (1998) of the entire satellite record, and which gave the record its slightly upward-trending slope. Consequently, it can be appreciated that although El Niņos may significantly impact short-term climate, periodically nudging global temperatures upward, they in turn are even more strongly impacted by long-term climate, as demonstrated by the findings discussed in the preceding paragraph, where it is readily seen that long-term warmth depresses El Niņo activity.

In providing some additional examples of this phenomenon - i.e., the overriding power of centennial- to millennial-scale climate variability compared to decadal to multidecadal variability (in terms of what drives what when it comes to changes in temperature and El Niņo activity) - Rein et al. report that (1) during the Medieval Warm Period, when "Northern Hemisphere temperatures peaked," there was "extraordinarily weak El Niņo activity," (2) in the late 13th and the early 17th centuries, "temperatures in the Northern Hemisphere were rather cool but El Niņo activity was high," and (3) during the 19th century, when the Northern Hemisphere began to warm as the planet commenced its recovery from the global chill of the Little Ice Age, El Niņo activity began to decline, as Medieval-Warm-Period-like conditions were once again in process of being established as the Modern Warm Period was coming into existence.

In light of these several observations, we suggest that the slight global warming evident in the satellite record of the past quarter-century was simply a natural consequence of El Niņo activity, which will likely subside somewhat as the non-CO2-induced Modern Warm Period (which we believe to be a product of earth's natural millennial-scale oscillation of climate) becomes more firmly entrenched at a slightly higher temperature commensurate with that of the Medieval Warm Period.

Sherwood, Keith and Craig Idso

PS: For more information pertinent to the warming that produced the Little Ice Age to Modern Warm Period transition, see Climate Oscillations (Millennial Variability) in our Subject Index.

Rein, B., Luckge, A., Reinhardt, L., Sirocko, F., Wolf, A. and Dullo, W.-C. 2005. El Niņo variability off Peru during the last 20,000 years. Paleoceanography 20: 10.1029/2004PA001099.

Rein, B., Luckge, A. and Sirocko, F. 2004. A major Holocene ENSO anomaly during the Medieval period. Geophysical Research Letters 31: 10.1029/2004GL020161.