Observations show that the amplitude of the seasonal cycle (SCA) of atmospheric CO2 in the higher latitudes of the northern hemisphere (45N-90N) has increased by 57±7% over the last 50 years. The seasonal cycle at these latitudes can be almost entirely attributed to land regions between 30-90N (Graven et al. 2013). However, terrestrial biosphere models are unable to reproduce the observed magnitude or change in seasonal cycle amplitude. Changes in the seasonal cycle have occurred as the terrestrial biosphere has responded to the rise in atmospheric CO2 (by ~70 ppm), changes in mean surface temperature (increase of ~1°C) and changes in land use. But how much have each of these changes in environmental drivers resulted in the observed increase in SCA?
Despite their shortcomings, models are doing something right. They do produce a seasonal cycle that is broadly in line with observations (in terms of timing and shape at least), the seasonal cycle does increase in amplitude in all models and they do show a greening trend. Their spread in values and variety of behaviours can be utilised because finding relationships between these models, which are all set up differently, is extremely powerful and suggestive of an underlying mechanism. This concept was demonstrated recently in Cox et al. 2013 to constrain the sensitivity of tropical land carbon storage to temperature. So, does a similar relationship exist between terrestrial biosphere models that can quantify the contribution from CO2 and temperature on the SCA? The figure below sets out a framework to analyse this question.
Initial results suggest that these inter-model relationships do exist. NPP is well correlated to SCA between models. If we then look at the CO2 only part of this, both the direct (CO2 fertilisation) and indirect (through vegetation greening) effects on NPP are also well correlated to SCA.
This is still a work in progress, and so a number of questions remain regarding (but not limited to) the regions of influence from greening and the effects of temperature.
- Graven et al. 2013 “Enhanced seasonal exchange of CO2 by Northern Ecosystems since 1960”. Science
- Cox et al. 2013 “Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability”. Nature