Temperature Thresholds for Carbon Flux Variation and Warming-Induced Changes

The response of net ecosystem exchange (NEE) to environmental changes is strongly nonlinear, characterized with threshold behaviors that are not well understood. Here, we investigated the threshold behaviors in the relationship between NEE and surface air temperature based on FLUXNET2015 observations, Community Land Model simulations, and Coupled Model Intercomparison Project Phase 6 (CMIP6) model outputs. An air temperature threshold of 16.4 degrees C, corresponding to the maximum carbon sink (T-sink), was identified for all 205 FLUXNET2015 sites combined. For deciduous broadleaf and mixed forests, wetlands and wheat-barley croplands, and rice-maize-soybean croplands, we identified a carbon-source threshold (T-source) of 6.8, 5.0, and 18.0 degrees C, respectively, beyond which the ecosystem becomes less of a carbon source. Five cold climate types mainly encompassing these plant functional types showed a clear carbon-source T-source of 12.2 degrees C. Six CMIP6 models project a threshold temperature increase of 1.0-2.8 degrees C by the 2090s, which results primarily from a shift of the optimum temperature for gross primary production. Not accounting for the warming-induced threshold changes may lead to an estimated time of the average summer air temperature passing T-sink that is earlier by 4.5-6.7 and 6.4-12.2 years at low (15 degrees N-15 degrees S) and high (>= 60 degrees S or >= 60 degrees N) latitudes, respectively.