Abstract
Land-use changes until the beginning of the 20th century made the terrestrial biosphere a net source of atmospheric carbon. Later, burning of fossil fuel surpassed land use changes as the major anthropogenic source of carbon. The terrestrial biosphere is at present suggested to be a carbon sink, but the distribution of excess anthropogenic carbon to the ocean and biosphere sinks is highly uncertain. Our modeling suggest that land-use changes can be tracked quite well by the carbon isotopes until mid-20th century, whereas burning of fossil fuel dominates the present-day observed changes in the isotope signature. The modeling indicates that the global carbon isotope fractionation has not changed significantly during the last 150 years. Furthermore, increased uptake of carbon by the ocean and increasing temperatures does not yet appear to have resulted in increasing the global gross ocean-to-atmosphere carbon fluxes. This may however change in the future when the excess carbon will emerge in the ocean upwelling zones, possibly reducing the net-uptake of carbon compared to the present-day ocean.
Highlights
Of carbon being taken up by the terrestrial biosphere can be found from the residual[16,18]
We propose a new simple box model solved in forward mode to evaluate the sensitivity of the atmospheric carbon isotope signature on natural and anthropogenically induced carbon fluxes
In order to use the forward model to constrain parameters of the global carbon cycle, such as the major natural carbon fluxes, the airborne fraction, and amount of excess carbon being stored in the ocean and terrestrial biosphere, we performed a sensitivity study where we changed parameters one by one and observed the effect on the modelled carbon isotope signatures
Summary
Of carbon being taken up by the terrestrial biosphere can be found from the residual (difference between carbon accumulated in the atmosphere and amount taken up by the global oceans)[16,18]. A forward model is a numerical algorithm where fossil fuel emissions, carbon-emissions from land-use changes, base (at time zero) natural fluxes, isotope fractionation, and partitioning of excess carbon is used as input, and the carbon isotope signature of the atmosphere is calculated. One main aim is to tune the global natural carbon fluxes, but the forward model allows us to see if the main human sources of carbon emissions; fossil fuel burning and land-use changes, can be observed in the recent data of the atmosphere carbon isotopes. Tans et al.[35] solved the model explicitly for the fluxes given the isotope disequilibria as input, whereas we varied carbon cycle parameters (fluxes, ocean/terrestrial biosphere sink/source strengths, etc.) and compared the model fit to the measured atmospheric carbon inventory. Some further discussion on challenges using the results from the inverse models is provided at the end of the results section
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