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Patterns and mechanisms of early Pliocene warmth

2013; Nature Portfolio; Volume: 496; Issue: 7443 Linguagem: Inglês

10.1038/nature12003

1476-4687

Alexey V. Fedorov, C. M. Brierley, K. T. Lawrence, Zhonghui Liu, Petra Dekens, Ana Christina Ravelo,

Marine and environmental studies

About five to four million years ago, in the early Pliocene epoch, Earth had a warm, temperate climate. The gradual cooling that followed led to the establishment of modern temperature patterns, possibly in response to a decrease in atmospheric CO2 concentration, of the order of 100 parts per million, towards preindustrial values. Here we synthesize the available geochemical proxy records of sea surface temperature and show that, compared with that of today, the early Pliocene climate had substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures. Using an Earth system model, we show that none of the mechanisms currently proposed to explain Pliocene warmth can simultaneously reproduce all three crucial features. We suggest that a combination of several dynamical feedbacks underestimated in the models at present, such as those related to ocean mixing and cloud albedo, may have been responsible for these climate conditions. A synthesis of geochemical proxy records of sea surface temperature shows that the early Pliocene climate was little different from today in terms of maximum ocean temperatures but had substantially lower meridional and zonal temperature gradients. The early Pliocene — a warm period between about 5 and 3 million years ago — is the most recent time when the Earth experienced CO2 levels similar to today's. Understanding early Pliocene climate conditions and mechanisms is therefore a priority. Alexey Fedorov and colleagues have assembled the available proxy climate indicators and reveal that the early Pliocene had about the same maximum sea surface temperatures as present, but weakened temperature gradients in both north to south and, particularly in the equatorial Pacific, east to west directions. None of the mechanisms currently at work in climate models can account for the full early Pliocene climate state, implying that challenges remain for the modelling of Earth system processes in warmer-than-present climates.