Decomposing pre-industrial to present-day land use change forcing in the UK Earth System Model

Abstract. Land use impacts climate through changes in carbon emissions, surface albedo and evapotranspiration, as well as biogenic volatile organic compound (BVOC) emissions, which influence atmospheric composition. The relative importance of changes in atmospheric composition driven by pre-industrial to present-day land use change has not been assessed for the UK Earth System Model (UKESM). Here, we decompose the pre-industrial to present-day land use change forcing in UKESM1.1 with additional process updates. We find a net simulated forcing of −0.08 ± 0.05 W m⁻² when using the standard (Strat-Trop vn1.0) chemistry, and −0.12 ± 0.04 W m⁻² for the complex (CRI-Strat 2) chemical mechanism. The simulated forcing includes the positive aerosol direct and indirect effects (around +0.06 W m⁻² and +0.085 W m⁻², respectively), alongside negative forcings from ozone (around −0.01 W m⁻²) and surface albedo change (around −0.17 W m⁻²). The forcing from the aerosol indirect effects calculated in this study is greater than in recent UKESM BVOC forcing experiments, which we attribute to using pre-industrial background conditions and increased organic matter hygroscopicity. Additional calculations show the radiative forcing from changes to methane lifetime is between −0.02 and −0.04 W m⁻², while the land use carbon emissions drive a carbon dioxide forcing of +0.87 W m⁻². Overall, the competing effects of changes in aerosols and short-lived greenhouse gases with surface albedo counter around 15 % of the carbon dioxide forcing. However, non-carbon dioxide effects have significant regional impacts.