Abstract
Abstract. We analyse global and regional changes in CO2 fluxes using two simple models, an airborne fraction of anthropogenic emissions and a linear relationship with CO2 concentrations. We show that both models are able to fit the non-anthropogenic (hereafter natural) flux over the length of the atmospheric concentration record. Analysis of the linear model (including its uncertainties) suggests no significant decrease in the response of the natural carbon cycle. Recent data points rather to an increase. We apply the same linear diagnostic to fluxes from atmospheric inversions. Flux responses show clear regional and seasonal patterns driven by terrestrial uptake in the northern summer. Ocean fluxes show little or no linear response. Terrestrial models show clear responses, agreeing globally with the inversion responses, however the spatial structure is quite different, with dominant responses in the tropics rather than the northern extratropics.
Highlights
The interplay of various timescales in anthropogenically forced climate change is both problematic and fascinating
We start with the decomposition used by the Global Carbon Project (GCP) (Le Quéré et al, 2013)
Where M is the mass of carbon in the atmosphere, FLUC is the flux due to land-use change (LUC) and all other fluxes have their usual meanings
Summary
The interplay of various timescales in anthropogenically forced climate change is both problematic and fascinating It is problematic since temperature responses integrate radiative forcing and radiative forcing by greenhouse gases integrates sources. For the most important greenhouse gas, CO2, this double integration gives a respectable utility to an inherently fascinating question: are there changes in the underlying processes of the carbon cycle? Cox et al (2000) combined many of these responses into a reasonably complete model of the earth system and projected a strong reduction in carbon uptake with the land becoming a net source around 2050. Friedlingstein et al (2006) showed that this was one of many possible responses Such studies naturally prompted observational tests of the important processes such as the reaction of the Amazon forest to drying (Saleska et al, 2003). Several studies have suggested sink saturation or reduction in various regions such as Schuster and Watson (2007) for the North Atlantic, Le Quéré et al (2007) for the Southern Ocean and Nabuurs et al (2013) for European forests
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