Strong chemistry-climate feedbacks in the Pliocene

Abstract

The Pliocene epoch was the last sustained interval when global climate was significantly warmer than today but has been difficult to explain fully based on the external forcings from atmospheric carbon dioxide and surface albedo. Here we use an Earth system model to simulate terrestrial ecosystem emissions and atmospheric chemical composition in the mid-Pliocene (about 3 million years ago) and the preindustrial (∼1750s). Tropospheric ozone and aerosol precursors from vegetation and wildfire are ∼50% and ∼100% higher in the mid-Pliocene due to the spread of the tropical savanna and deciduous biomes. The chemistry-climate feedbacks contribute a net global warming that is +30-250% of the carbon dioxide effect and a net aerosol global cooling that masks 15-100% of the carbon dioxide effect. These large vegetation-mediated ozone and aerosol feedbacks operate on centennial to millennial timescales in the climate system and have not previously been included in paleoclimate sensitivity assessments.