Historical trends and controlling factors of isoprene emissions in CMIP6 Earth system models

Terrestrial isoprene, a biogenic volatile organic compound emitted by many plants, indirectly influences Earth's radiative balance through its interactions with atmospheric oxidants, affecting ozone formation, methane lifetime, and secondary aerosol production. Elucidating its historical changes is therefore important for predicting climate change and air quality. Isoprene emissions can respond to climate (e.g. temperature, shortwave radiation, precipitation), land use and land cover change (LULCC), and atmospheric CO2 concentrations. However, historical trends of isoprene emissions and the relative influences of the respective drivers of those trends remain highly uncertain. This study addresses uncertainty in historical isoprene emission trends and their influential factors, particularly the roles of climate, LULCC, and atmospheric CO2 (via fertilization and inhibition effects). The findings are expected to reconcile discrepancies among different modelling approaches and to improve predictions of isoprene emissions and their climate change effects. To investigate isoprene emission trends, controlling factors, and discrepancies among models, we analyzed long-term (1850–2014) global isoprene emissions from online simulations of CMIP6 Earth system models and offline simulations using the Vegetation Integrative SImulator for Trace gases (VISIT) dynamic vegetation model driven by climate reanalysis data. Mean annual global present-day isoprene emissions agree well among models (434–510 TgC yr−1) with a 5 % inter-model spread (24 TgC yr−1), but regional emissions differ greatly (9 %–212 % spread). All models show an increasing trend in global isoprene emissions in recent decades (1980–2014), but their magnitudes vary (+1.27 ± 0.49 TgC yr−2, 0.28 ± 0.11 % yr−1). Long-term trends of 1850–2014 show high uncertainty among models (−0.92 to +0.31 TgC yr−2). Results of emulated sensitivity experiments indicate meteorological variations as the main factor of year-to-year fluctuations, but the main drivers of long-term isoprene emission trends differ among models. Models without CO2 effects implicate climate change as the driver, but other models with CO2 effects (fertilization only/fertilization and inhibition) indicate CO2 and LULCC as the primary drivers. The discrepancies arise from how models account for CO2 and LULCC alongside climate effects on isoprene emissions. Aside from LULCC-induced reductions, differences in CO2 inhibition representation (strength and presence or absence of thresholds) were able to mitigate or reverse increasing trends because of rising temperatures or in combination with CO2 fertilization. Net CO2 effects on global isoprene emissions show the highest inter-model variation (σ=0.43 TgC yr−2), followed by LULCC effects (σ=0.17 TgC yr−2), with climate change effects exhibiting more or less variation (σ=0.06 TgC yr−2). The critical drivers of isoprene emission trends depend on a model's emission scheme complexity. This dependence emphasizes the need for models with accurate representation of CO2 and LULCC effects alongside climate change influences for robust long-term predictions. Important uncertainties remain in understanding the interplay between CO2, LULCC, and climate effects on isoprene emissions, mainly for CO2. More long-term observations of isoprene emissions across various biomes are necessary, along with improved models with varied CO2 responses. Moreover, instead of reliance on the current models, additional emission schemes can better capture isoprene emissions complexities and their effects on climate.