A holistic view of the Pliocene climate

Debates continue on various aspects of the Pliocene climate, and Pliocene warmth in general, but there are several characteristics of this climate state on which we seem to agree, including the reduced equatorial (east-west) and reduced meridional SST gradients in the Pacific ocean relative to the modern. The actual magnitude of these reductions (in the range of 1 to 4^oC) is however strongly debated, as it depends on the way the gradients are defined, particular calibrations used to estimate temperature, and specific time intervals considered. In contrast to the equatorial Pacific SST gradient, the pH gradient was probably stronger during the Pliocene, and biological productivity in the eastern equatorial Pacific was higher. In the subtropics, coastal upwelling regions were much warmer (up to 10^oC). Subtropical land regions were wetter and had extensive lake systems and lush  vegetation. Arctic inland annual-mean temperatures were much warmer (by 10-20^oC). The subarctic North Pacific may have sustained deep water formation leading to an active Pacific meridional overturning circulation (PMOC) at least during parts of the Pliocene, with more evidence becoming available recently. The early Pliocene was probably 1-2^oC warmer, and some of these effects were more pronounced, compared to the mid-Pliocene. The Pliocene climate had atmospheric CO₂ concentrations around 400ppm (with a large uncertainty and slightly higher concentrations during the early Pliocene). Several of the Pliocene climatic features appear to be consistent with each other, such as warmer coastal upwelling regions and stronger precipitation in the subtropics. Other features seem to contradict, as the reduced equatorial SST gradient, and hence a weaker Walker circulation and equatorial upwelling, versus increased productivity in the eastern equatorial Pacific. The question then arises whether these climate features are independent or closely linked and therefore have to be explained together. Here, I attempt to combine these disparate features into one holistic picture of the Pliocene climate that puts all these elements in a consistent framework, based on changes in both atmospheric and oceanic circulations, and incorporating models and data.