Seminar: Carlos Sierra

We present here a model-based estimate of the time it takes carbon atoms to travel through the terrestrial biosphere, since the time they are taken up through photosynthesis until they are released back to the atmosphere through autotrophic and heterotrophic respiration.

Using a simple model, we explored the consequences of increasing productivity versus increasing respiration rates on the transit time distribution of carbon. We found that while temperature-induced respiration rates increase the transit time because older carbon is respired, increases in productivity induced by elevated atmospheric CO2 causes a decline in transit times because more young carbon is available to supply increased metabolism. The combined effect of increases in temperature and atmospheric CO2 results in a decrease in transit times, with the productivity effect dominating over the respiration effect. Using an ensemble of simulation trajectories from the Carbon Data Model Framework (CARDAMOM), we obtained time-dependent transit time distributions incorporating 20th century global change. In these historical simulations, the transit time of carbon declined over the 20th century, suggesting an increased productivity effect that augmented the amount of respired young carbon. The mean transit time showed an increase during the first part of the 20th century, very likely as a result of emissions of older carbon due to temperature-induced respiration and fires, but after the 1980s mean transit time declined. The median transit time showed a consistent decline during the entire simulation period, with changes in the seasonal cycle of the age of respired carbon. The temporal change in the relation between median and mean transit time suggests that the transit time distribution of carbon for the entire terrestrial biosphere is becoming more asymmetric, with more carbon transiting faster through tropical and temperate regions, and more old carbon being respired from high latitude regions.

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