On the climatic influence of CO2 forcing in the Pliocene

Abstract

<p>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 CO<sub>2</sub> forcing in driving Pliocene surface air temperature change but little is understood about the drivers of other climate parameters.</p> <p>We implement a novel, simple linear factorisation method to seven models in the PlioMIP2 ensemble to assess the relative influence of CO<sub>2</sub> forcing in the Pliocene. Outputs are termed “FCO<sub>2</sub>” and reflect the relative influence of CO<sub>2</sub>, where 1 represents wholly dominant CO<sub>2</sub> forcing.</p> <p>CO<sub>2</sub> forcing is found to be the dominant driver of surface air temperature change in six of the seven models (global mean FCO<sub>2</sub> of ensemble = 0.56), and five of the seven models for sea surface temperature (global mean FCO<sub>2</sub> of ensemble = 0.56). FCO<sub>2</sub> varies latitudinally for both, with CO<sub>2</sub> forcing less dominant at high latitudes.</p> <p>FCO<sub>2</sub> shows the most variation between models for precipitation change (individual model global mean FCO<sub>2</sub> range = 0.30-0.69), though CO<sub>2</sub> remains the most dominant driver in the ensemble (global mean FCO<sub>2</sub> = 0.51). Spatial change in precipitation is predominantly driven by changes in orography and ice sheets in individual models.</p> <p>The accuracy of our FCO<sub>2</sub> method is evidenced by comparison to an energy balance analysis, which also adds nuance to the results and highlights feedbacks that arise from CO<sub>2</sub> forcing.</p> <p>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.</p> <p>That CO<sub>2</sub> 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-CO<sub>2</sub> forcing in the Pliocene, the equivalent of which is not expected to be seen for hundreds or thousands of years in the future.</p>