On the climatic influence of CO2 forcing in the Pliocene.

Understanding the dominant climate forcings in the Pliocene is crucial to assessing the usefulness of the Pliocene as an analogue for our warmer future. Here we implement a novel, yet simple linear factorisation method to assess the relative influence of CO₂ forcing in seven models of the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) ensemble. Outputs are termed "FCO₂" and show the fraction of Pliocene climate change driven by CO₂. The accuracy of the FCO₂ method is first assessed through comparison to an energy balance analysis previously used to assess drivers of surface air temperature in the PlioMIP1 ensemble. After this assessment, the FCO₂ method is applied to achieve an understanding of the drivers of Pliocene sea surface temperature and precipitation for the first time. CO₂ is found to be the most important forcing in the ensemble for Pliocene surface air temperature (global mean FCO₂ = 0.56), sea surface temperature (global mean FCO₂ = 0.56) and precipitation (global mean FCO₂ = 0.51). The range between individual models is found to be consistent between these three climate variables, and the models generally show good agreement on the sign of the most important forcing. Our results provide the most spatially complete view of the drivers of Pliocene climate to date, and have implications for both data-model comparison and the use of the Pliocene as an analogue for the future. That CO₂ is found to be the most important forcing reinforces that the Pliocene is a good palaeoclimate analogue, but the significant effect of non-CO₂ forcing at regional scale reminds us that it is not perfect, and this must not be overlooked. This comparison is further complicated when considering the Pliocene as a state in quasi-equilibrium with CO₂ forcing compared to the transient warming being experienced at present. [ABSTRACT FROM AUTHOR]