Pliocene tropical oceans were warmer after all

by Timothy Oleson
Tuesday, November 4, 2014

Credit: ©

Scientists may have overturned the idea that Earth’s tropical oceans were the same temperature during the Early to Middle Pliocene — between about 5 million and 3 million years ago — as they are today, despite the world being a far warmer place then.

Reconstructions of Earth’s Pliocene climate have suggested that atmospheric carbon dioxide concentrations hovered around 400 parts per million, similar to current levels, warming air temperatures by 3 to 4 degrees Celsius compared to pre-industrial modern times. Mid- and high-latitude oceans are also thought to have been several degrees warmer. Curiously, though, studies of the Pliocene tropical oceans — relying mostly on magnesium-calcium (Mg/Ca) ratios preserved in fossilized foraminifera — have suggested these oceans were no warmer than today. This observation has led to talk of an unexplained climatic thermostat that regulated tropical temperatures then, and which could potentially act again to mitigate future warming.

Now, Charlotte O’Brien of the University of Bristol in England (now at Oxford University) and colleagues have found that such a thermostat probably didn’t exist. The researchers analyzed new foraminiferal Mg/Ca data, along with records of two other temperature proxies, from ocean sediment cores collected in the South China and Caribbean seas. Although their Mg/Ca record agreed with earlier analyses, the two other proxies indicated that the tropical Pacific and Atlantic Oceans were 2 to 3 degrees Celsius higher. The discrepancy likely arose, O’Brien’s team noted in Nature Geoscience, because of a systematic bias in Mg/Ca ratios caused by long-term changes in the relative concentrations of magnesium and calcium in seawater. Correcting for the bias brought temperature estimates from all three proxies into better agreement.

Records of seemingly unchanged tropical ocean temperatures and talk of a tropical thermostat have helped fuel “overly optimistic perspectives of future warming,” noted Mark Pagani of Yale University in a commentary also published in Nature Geoscience. The conclusion of the new study represents “a fundamental improvement rather than a small step forward,” he wrote.

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