Chemistry-driven changes strongly influence climate forcing from vegetation emissions

Abstract

Emissions of biogenic volatile organic compounds (BVOCs) affect climate via formation of organic aerosols and their influence on cloud properties, and via atmospheric oxidation changes influencing the greenhouse gases ozone and methane. BVOCs also exhibit dependence on climate (leading to a feedback), and land use change (including afforestation/reforestation as a CO2 removal strategy). Despite recent scientific advances, there remains considerable uncertainty between model simulations in the net impact BVOCs have on climate. One contributor to this uncertainty is the description of BVOC chemistry, hitherto minimally assessed in a climate context. Using the Earth system model UKESM1 we quantify the influence of chemistry by comparing the climate response to a doubling of BVOC emissions in a pre-industrial atmosphere with standard and state-of-science chemistry mechanisms, with both using interactive oxidant fields. The net feedback from BVOCs is positive in UKESM1, regardless of the mechanism used. The negative feedback from enhanced aerosol scattering is outweighed by positive feedbacks from increases in ozone and methane, and changes to aerosol-cloud interactions (ACI). Contrary to prior studies, we show the ACI response is driven by reductions in cloud droplet number concentration (CDNC) via oxidant-driven suppression of gas phase SO2 oxidation. However, with the state-of-science scheme the feedback is 43% smaller due to lower oxidant depletion yielding smaller methane increases and smaller CDNC decreases. This illustrates the significant influence of chemistry and oxidants on gas and aerosol responses to BVOC emission changes and the more complex pathways by which BVOCs influence climate than are currently recognised.