High sea surface temperatures in tropical warm pools during the Pliocene

The western warm pools of the Atlantic and Pacific oceans are a critical source of heat and moisture for the tropical climate system. Over the past five million years, global mean temperatures have cooled by 3–4 °C. Yet, present reconstructions of sea surface temperatures indicate that temperature in the warm pools has remained stable during this time. This stability has been used to suggest that tropical sea surface temperatures are controlled by a thermostat-like mechanism that maintained consistent temperatures. Here we reconstruct sea surface temperatures in the South China Sea, Caribbean Sea and western equatorial Pacific Ocean for the past five million years, using a combination of the Mg/Ca-, TEX₈₆^H- and U^k'₃₇- surface-temperature proxies. Our data indicate that during the period of Pliocene warmth from about 5 to 2.6 million years ago, the western Pacific and western Atlantic warm pools were about 2 °C warmer than today. We suggest that the apparent lack of warmth seen in the previous reconstructions was an artefact of low seawater Mg/Ca ratios in the Pliocene oceans. Taking this bias into account, our data indicate that tropical sea surface temperatures did change in conjunction with global mean temperatures. We therefore conclude that the temperature of the warm pools of the equatorial oceans during the Pliocene was not limited by a thermostat-like mechanism.