Why has the seasonal cycle of CO2 increased? By Rebecca Thomas

An increase in the amplitude of the seasonal cycle of CO2 has been observed in the higher latitudes of the northern hemisphere. In situ measurements between 1960-2011 show increases at Point Barrow, Alaska and Mauna Loa Observatory, Hawaii of 35% (range 30 to 49%) and 15±5% respectively, and recent aircraft campaign data have confirmed that this is a large-scale trend, with increases of 57±7% in seasonal cycle amplitude at latitudes north of 45°N (from aircraft campaigns in 1958-63 and 2009-2010). A number of processes can influence the concentration of CO2 in the atmosphere: fire, fossil fuel emissions, the ocean and the land biosphere. Using an atmospheric transport model, it has been shown that the seasonal cycle of atmospheric CO2 in the higher latitudes of the northern hemisphere is predominantly attributed to the terrestrial biosphere (Graven et al. 2013).  The terrestrial biosphere imparts its signal on atmospheric CO2 concentration through net ecosystem exchange (NEE), the balance of net primary production (NPP) and heterotrophic respiration (Rh). The phase imbalance between these fluxes creates the seasonal cycle that is observed.

Using 13 models from the Multi-synthesis Terrestrial Model Intercomparison Project (MsTMIP), the behaviour of current terrestrial biosphere models (TBMs) was analysed. The TBMs use several equations to represent NEE, with many following the set up by Collatz et al. (1991) for photosynthetic assimilation of carbon, and a variety of different equations used for respiration. To investigate influences on NEE, the models held all driving data constant at preindustrial levels and then successively added dynamic forcing from climate, LUC, CO2 and nitrogen deposition (Huntzinger et al. 2013). Figure 1 shows that none of the TBMs were able to capture the current magnitude or the change in seasonal cycle amplitude that has been observed over the last 60 years. I therefore ask, can the distribution of the MsTMIP models in Figure 1 be explained, and why do these TBMs underestimate the CO2 amplitude increase?

Figure 1. Absolute and relative change in CO2 amplitude in MsTMIP models compared to aircraft observations. Models are detrended following the methods in Graven et al. 2013, and CMIP5 output from this paper is also shown.

Changes in NEE result from changes in the timing or magnitude of NPP and/or Rh and this can occur because of warmer temperatures or the increase in atmospheric CO2. Preliminary results suggest that modelled changes in CO2 amplitude are not a response to climatic changes as neither the climate only simulations or Rh seem to explain any of the variance in the modelled CO2 amplitude change. Therefore modelled increases are likely due to CO2 fertilisation. Changes in NPP amplitude and changes in NEE amplitude are well correlated to changes in CO2 amplitude, reinforcing this idea. However, the extent of CO2 fertilisation is difficult to isolate and quantify since none of the MsTMIP experiments had only CO2 forcing.

Continued investigation into these models will attempt to further constrain the CO2 fertilisation effect on the models.

References:

Graven HD, et al. 2013. Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960. Science 341: 1085-1089.

Huntzinger DN, et al. 2013. The North American Carbon Program Multi-scale synthesis and Terrestrial Model Intercomparison Project – Part 1: Overview and experimental design. Geosci. Model Dev. Discuss. 6: 3977-4008.

Collatz et al. 1991 Physiological and environmental regulation of stomatal conductance, photosynthesis and transpiration: a model that includes a laminar bound- ary layer. Agric. For. Meteorol,, 54: 107-136.

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One Response to Why has the seasonal cycle of CO2 increased? By Rebecca Thomas

  1. CanTHeeRava says:

    You attribute the increasing trend in seasonal amplitude of atm CO2 concentration to CO2 fertilization effect. Could you also see a possibility of changing NPP due to demographic shifts in tree populations? Stephensen et al (2014; nature) argue that mass-growth rate among trees increases with tree size (i.e., metabolic scaling theory). While this theory has serious issues to deal with (e.g. plant senescence; when do trees start to die?), I wonder whether significant demographic change in some of the temperate and boreal forests in the last 3 to 4 decades has some influence on the increase in seasonal CO2 amplitude. Please also see van Mantgem et al (2009, science) who argue that increased mortality among young trees in certain forests in the US is likely due to climate shifts (drought and warming).

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