Improving representation of photosynthesis in Earth System Models.

Earth System Models (ESMs) provide the complex simulations of past, current and future climate that are required to inform policy decisions. Because climate change is driven principally by rising atmospheric carbon dioxide concentration ([CO2]), model estimation of future [CO2] will strongly influence climate projections. The inability of ESMs to confidently simulate the enormous CO2 fluxes associated with the global carbon cycle translates to greater uncertainty in projections of the onset, frequency and severity of the increasingly inevitable high impact consequences of our changing climate (IPCC, 2013). The CO2 fluxes associated with the terrestrial biosphere – in comparison to the CO2 fluxes between the atmosphere and oceans – are poorly constrained (Friedlingstein et al., 2014). Current understanding and model representation of the terrestrial carbon cycle, and the response of the terrestrial carbon cycle to rising atmospheric [CO2] and temperature, and changing precipitation patterns, are among the greatest uncertainties in ESMs in terms of both scientific understanding and model representation (Booth et al., 2012). In July 2013, the 8 New Phytologist Workshop – ‘Improving representation of leaf level respiration in large-scale predictive climate-vegetation models,’ held in Canberra and Kioloa, Australia – addressed uncertainty in carbon cycle projections associated with model representation of plant respiration (Atkin et al., 2014). The 9 New Phytologist Workshop complemented the preceding workshop on autotrophic respiration by addressing model uncertainty associated with the representation of photosynthesis in the terrestrial carbon cycle. Unlike respiration, photosynthetic CO2 uptake is already well described by a model – the Farquhar, von Caemmerer and Berry model of photosynthesis (Farquhar et al., 1980) – and many ESMs use a derivation of this formulation coupled to models of stomatal control (Ball et al., 1987) to estimate gross primary production (GPP). However, ESMs differ in the way this model is implemented and parameterized, as well as how photosynthesis responds to temperature, with unclear consequences for ESM output. A road map for new science