Increasing CO2 seasonal cycle amplitude in the north: analysis with an eco-evolutionary optimality model

Land-atmosphere carbon exchanges and feedbacks constitute one of the largest uncertainties in future climate projections. A large increase in the seasonal cycle amplitude (SCA) of CO₂ has occurred since the 1950s, especially in northern high latitudes, reflecting enhanced vegetation activity. However, global land-surface and dynamic vegetation models have produced a very wide range of magnitudes for the SCA, and have generally (sometimes drastically) underestimated its increase. We explored the controls of the SCA using a parameter-sparse eco-evolutionary optimality (EEO) model for gross primary production, the ‘P model’, combined with simple, generic representations of plant and decomposer respiration, to simulate seasonal cycles and decadal trends of net ecosystem exchange (NEE). Simulated NEE fields were used to generate near-surface CO₂ concentrations with the help of the atmospheric chemistry-transport model TM5. Modelled CO₂ SCA and SCA trends were similar to those observed at CO₂ monitoring stations in northern high latitudes, outperforming state-of-the-art Earth System Models. Rising CO₂ was shown to be the primary driver of increasing SCA. Climate showed a mixed but overall positive impact; however, the influence of climate shifted from positive to negative in the late 1990s, resulting in a slight reduction in SCA amplitude over the satellite era.