Photosynthesis-dependent isoprene emission from leaf to planet in a global carbon-chemistry-climate model
We describe the implementation of a biochemi-cal model of isoprene emission that depends on the elec-tron requirement for isoprene synthesis into the Farquhar– Ball–Berry leaf model of photosynthesis and stomatal con-ductance that is embedded within a global chemistry-climate simulation framework. The isoprene production is calculated as a function of electron transport-limited photosynthesis, intercellular and atmospheric carbon dioxide concentration, and canopy temperature. The vegetation biophysics mod-ule computes the photosynthetic uptake of carbon dioxide coupled with the transpiration of water vapor and the iso-prene emission rate at the 30 min physical integration time step of the global chemistry-climate model. In the model, the rate of carbon assimilation provides the dominant con-trol on isoprene emission variability over canopy tempera-ture. A control simulation representative of the present-day climatic state that uses 8 plant functional types (PFTs), pre-scribed phenology and generic PFT-specific isoprene emis-sion potentials (fraction of electrons available for isoprene synthesis) reproduces 50 % of the variability across differ-ent ecosystems and seasons in a global database of 28 mea-sured campaign-average fluxes. Compared to time-varying isoprene flux measurements at 9 select sites, the model au-thentically captures the observed variability in the 30 min Published by Copernicus Publications on behalf of the European Geosciences Union. 10244 N. Unger et al.: Photosynthesis-dependent isoprene emission from leaf to planet average diurnal cycle (R 2 = 64–96 %) and simulates the flux magnitude to within a factor of 2. The control run yields a global isoprene source strength of 451 TgC yr −1 that in-creases by 30 % in the artificial absence of plant water stress and by 55 % for potential natural vegetation.