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 future climate. Previous research has shown the dominance of CO₂ forcing in driving Pliocene surface air temperature change but little is understood about the drivers of other climate parameters.

We implement a novel, simple linear factorisation method to seven models in the PlioMIP2 ensemble to assess the relative influence of CO₂ forcing in the Pliocene. Outputs are termed “FCO₂” and reflect the relative influence of CO₂, where 1 represents wholly dominant CO₂ forcing.

CO₂ forcing is found to be the dominant driver of surface air temperature change in six of the seven models (global mean FCO₂ of ensemble = 0.56), and five of the seven models for sea surface temperature (global mean FCO₂ of ensemble = 0.56). FCO₂ varies latitudinally for both, with CO₂ forcing less dominant at high latitudes.

FCO₂ shows the most variation between models for precipitation change (individual model global mean FCO₂ range = 0.30-0.69), though CO₂ remains the most dominant driver in the ensemble (global mean FCO₂ = 0.51). Spatial change in precipitation is predominantly driven by changes in orography and ice sheets in individual models.

The accuracy of our FCO₂ method is evidenced by comparison to an energy balance analysis, which also adds nuance to the results and highlights feedbacks that arise from CO₂ forcing.

Our results go some way to better understanding the drivers of Pliocene climate and have implications for the interpretation of proxy data and data-model comparison, which is to be investigated further. Our results could also be expanded to include additional climate parameters, such as surface pressure.

That CO₂ forcing is the dominant driver of surface air temperature, sea surface temperature and precipitation change suggests that the Pliocene is a relevant climate analogue for the future, but attention must also be paid to the significant effect of non-CO₂ forcing in the Pliocene, the equivalent of which is not expected to be seen for hundreds or thousands of years in the future.