From the CO2Science Archive: Otoliths are aragonite (CaCO3) structures in fish that are used for acoustic perception and balance. When they are formed, the oxygen that is incorporated into them is in isotopic equilibrium with that of seawater. Hence, the d¹⁸O compositions of both modern and ancient otoliths can be used to develop temperature proxies for surrounding ocean waters. The authors derived and analyzed the d¹⁸O histories of several large otoliths obtained from catfish living in near-shore habitats at two sites off the coast of Peru that were caught both during and after the strong 1997-98 ENSO event. They then derived and analyzed the d¹⁸O histories of several otoliths from two archaeological sites – Ostra (6010 +/- 90 yr BP), and Siches (6450 +/- 110 yr BP) – after which they compared the results from the different sites and time periods.

Paper reviewed: Andrus, C.F.T., Crowe, D.E., Sandweiss, D.H., Reitz, E.J. and Romanek, C.S. 2002. Otolith d¹⁸O record of mid-Holocene sea surface temperatures in Peru. Science 295: 1508-1511.

What was learned
The first exercise yielded a 10-year-long d¹⁸O history, which in addition to providing a link to recent sea surface temperatures, clearly revealed the occurrence of both the 1991-92 and 1997-98 El Niños. The older d¹⁸O histories suggested that (1) sea-surface temperatures some 6,000 years ago were 3 to 4 deg C warmer than what they were over the decade of the 1990s, and (2) sea-surface temperatures of that earlier time period exhibited better-defined seasonal signals and provided little evidence of any El Niño activity.

What it means
The results of this study confirm the results of several other studies that indicate the mid-Holocene was significantly warmer than it is currently. In addition, the more seasonally-distinct temperature histories of that earlier time period suggest there was much less upwelling of the cool Peru-Chile current near northern Peru before 5000 yr BP, which is also suggested by several other studies. We thus have a situation where a considerably warmer climate than that of the present was apparently unable to sustain significant El Niño activity. This finding is very important, for it demonstrates that future global warming would likely lead to fewer and less intense El Niños, which contradicts the predictions of climate alarmists who fiercely claim the opposite.

Update
The techniques and conclusions of Andrus et al. (2002) are criticized by Bearez et al. (2003). Andrus et al. (2003) respond to this criticism, concluding that their “close examination of the comments and citations of Bearez et al. strengthen our original interpretations.”

Update
Bearez, P., DeVries, T.J. and Ortlieb, L. 2003. Comment on “Otolith d¹⁸O record of mid-Holocene sea surface temperatures in Peru.” Science 299: 203a.

Andrus, C.F.T., Crowe, D.E., Sandweiss, D.H., Reitz, E.J. and Romanek, C.S. 2003. Response to comment on “Otolith d¹⁸O record of mid-Holocene sea surface temperatures in Peru.” Science 299: 203b.