Differing mechanisms in the CO2 seasonal cycle and its amplitude change

The net terrestrial carbon flux to the atmosphere simulated by nine TRENDY models during 1961-2012 are examined on its mean seasonal cycle and seasonal amplitude increase. While the model ensemble agree well with observations on both the trend and latitudinal pattern of flux seasonal amplitude, notable model spread is evident. Further analyses using results from TRENDY's sensitivity experiments highlight important underlying difference in mechanisms responsible for the similar amplitude increase simulated by the models. We found that CO2 fertilization effect is the prevailing mechanism for amplitude increase of net carbon flux in seven out of nine models. Models disagree on the effect of climate: even though high latitude warming contribute to a major part of amplitude increase over the boreal region in two thirds of the models, this positive impact is largely negated by the negative influence over the temperate region and the tropics, possibly related to droughts that lowered the productivity of ecosystem during peak growing season. The effect of land use plays a significant positive role in five out of the nine models, however few model is able to simulate mechanisms such as improved farming practice and crop selection. Our results suggest that using model ensemble only may hide important mechanisms from individual model.