Foliar temperature acclimation reduces simulated carbon sensitivity to climate

Plant photosynthesis and respiration are the largest carbon fluxes between the terrestrial biosphere and the atmosphere(1), and their parameterizations represent large sources of uncertainty in projections of land carbon uptake in Earth system models(2,3) (ESMs). The incorporation of temperature acclimation of photosynthesis and foliar respiration, commonly observed processes, into ESMs has been proposed as a way to reduce this uncertainty(2). Here we show that, across 15 flux tower sites spanning multiple biomes at various locations worldwide (10 degrees S-67 degrees N), acclimation parameterizations(4,5) improve a model's ability to reproduce observed net ecosystem exchange of CO2. This improvement is most notable in tropical biomes, where photosynthetic acclimation increased model performance by 36%. The consequences of acclimation for simulated terrestrial carbon uptake depend on the process, region and time period evaluated. Globally, including acclimation has a net effect of increasing carbon assimilation and storage, an effect that diminishes with time, but persists well into the future. Our results suggest that land models omitting foliar temperature acclimation are likely to overestimate the temperature sensitivity of terrestrial carbon exchange, thus biasing projections of future carbon storage and estimates of policy indicators such as the transient climate response to cumulative carbon emissions(1).