Quantifying the Impact of Heterotrophic Respiration on Variability in the Global Carbon Dioxide Growth Rate.

Wednesday, December 12, 2018 - 09:30
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The atmospheric carbon dioxide (CO2) growth rate varies interannually in response to climate variations. Most of this variation arises from terrestrial ecosystems, where climate affects vegetation productivity (net primary productivity, NPP) and the rate of respiration of organic matter to CO2 (heterotrophic respiration, HR). While remote sensing observations are available to constrain variations in NPP, there are no large-scale observations that can be used to infer heterotrophic respiration rates. We therefore investigate the signature that heterotrophic respiration leaves on atmospheric CO2, which reflects both NPP and HR, at several timescales. We use a novel testbed approach to simulate HR and, in turn, an atmospheric tracer transport model to simulate the imprint of this process on atmospheric CO2. The testbed includes two models that are microbially explicit (MIMICS and CORPSE) and one that is microbially-implicit (CASA).

We quantified the interannual variability, seasonality, and climate sensitivity of HR fluxes across four ecoregions and of the fraction of CO2 variability owing to HR over six latitude bands. The microbially explicit models do a better job at capturing the magnitude of interannual variability when evaluated against CO2 observations from NOAA marine boundary layer sites. HR is an important component of this modeled variability: at interannual timescales, the magnitude of simulated CO2 variability from HR is comparable to that of CO2 simulated only from NPP across the Northern Hemisphere. At seasonal timescales, the amplitude of CO2 from HR is about 50-75% of that from NPP. CASA, the microbially implicit model, showed the smallest variations both seasonally and interannually. We found that NPP fluxes in warmer regions are more variable, while HR fluxes are more variable in colder regions. Given dominant patterns of atmospheric transport, this means that the apparent influence of NPP or HR on patterns of atmospheric CO2 is different. In general, the three testbed formulations suggest that CO2 from boreal HR is highly correlated with the observed CO2 variations, suggesting a possible path to use atmospheric CO2 observations together with productivity data to constrain hemispheric HR.

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