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ENSO Activity Over the Mid to Late Holocene
McGregor, H.V. and Gagan, M.K.  2004.  Western Pacific coral δ18O records of anomalous Holocene variability in the El Niņo-Southern Oscillation.  Geophysical Research Letters 31: 10.1029/2004GL019972.

Spurred on by "the strong El Niņos of 1982/1983 and 1997/1998, along with the more frequent occurrences of El Niņos during the past few decades," Timmermann et al. (1999) developed a global climate model that operates with sufficient resolution to address the issue of "whether human-induced 'greenhouse' warming affects, or will affect, ENSO," finding that "when the model is forced by a realistic future scenario of increasing greenhouse-gas concentrations, more frequent El-Niņo-like conditions and stronger cold events in the tropical Pacific Ocean result," much like what they report has been observed in nature.

What was done
In a study that provides additional real-world evidence with which model simulations of climate evolution in response to greenhouse-gas-induced warming may be evaluated, McGregor and Gagan used several annually-resolved fossil Porites coral δ18O records to investigate the characteristics of paleo-ENSO events that occurred during the middle to late Holocene, over which period of time the earth cooled substantially.

What was learned
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 over the past fifty years.  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.  In addition, McGregor and Gagan note that "the emerging picture of reduced El Niņo frequency for the mid-Holocene western Pacific is consistent with the paleo-ENSO record from storm deposits in lake Laguna Pallcacocha, Ecuador (Rodbell et al., 1999; Moy et al., 2002)."

For the period 2.5-1.7 ka, however, the results were quite different, with all of the coral records pertaining to this period 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."

What it means
Taken together, these diverse sets of results portray what McGregor and Gagan describe as a "mid-Holocene El Niņo suppression and late Holocene amplification," during the former of which periods the earth was significantly warmer than it is currently, while in transiting to the latter period it grew a lot colder.  Hence, as is indicated in many of the other studies of this topic that we have reviewed and archived in our Subject Index -- which can be found under the heading El Niņo (Relationship to Global Warming) -- it would appear that real-world ENSO events respond to global warming in exactly the opposite fashion to what is suggested by the model of Timmermann et al. (1999).

To be fair, however, we note that some subsequent models have produced results that are more in line -- qualitatively, at least -- with what has been observed in nature (Clement et al., 2000; Liu et al., 2000), although, of course, it can always be questioned whether they do so for the right reasons ? as they should be questioned, for McGregor and Gagan say there are other aspects of the climate-El Niņo linkage that still differ significantly from what has been observed in proxy climate data.

Clement, A.C., Seager, R. and Cane, M.A.  2000.  Suppression of El Niņo during the mid-Holocene by changes in the Earth's orbit.  Paleoceanography 15: 731-737.

Liu, Z., Kutzbach, J. and Wu, L.  2000.  Modeling climate shift of El Niņo variability in the Holocene.  Geophysical Research Letters 27: 2265-2268.

Moy, C.M., Seltzer, G.O., Rodbell, D.T. and Anderson D.M.  2002.  Variability of El Niņo/Southern Oscillation activity at millennial timescales during the Holocene epoch.  Nature 420: 162-165.

Rodbell, D.T., Seltzer, G.O., Anderson, D.M., Abbott, M.B., Enfield, D.B. and Newman, J.H.  1999.  An ~15,000-year record of El Niņo-driven alluviation in southwestern Ecuador.  Science 283: 516-520.

Timmermann, A., Oberhuber, J., Bacher, A., Esch, M., Latif, M. and Roeckner, E.  1999.  Increased El Niņo frequency in a climate model forced by future greenhouse warming.  Nature 398: 694-696.

Tudhope, A.W., Chilcott, C.P., McCuloch, M.T., Cook, E.R., Chappell, J., Ellam, R.M., Lea, D.W., Lough, J.M. and Shimmield, G.B.  2001.  Variability in the El Niņo-Southern Oscillation through a glacial-interglacial cycle.  Science 291: 1511-1517.

Woodroffe, C.D., Beech, M.R. and Gagan, M.K.  2003.  Mid-late Holocene El Niņo variability in the equatorial Pacific from coral microatolls.  Geophysical Research Letters 30: 10.1029/2002GL 015868.

Reviewed 28 July 2004