The Signature of Atmospheric Internal Variability on the Terrestrial Carbon Cycle (Invited Presentation)

Large ensembles of simulations generated through differences in initial conditions have shown that unforced variability internal to the system is central to identifying changes in climate at timescales ranging from subseasonal to seasonal and from interannual to decadal. Ocean biogeochemists are also using multimember ensembles to identify the time of emergence of forced changes in ocean biogeochemistry and uncertainty due to internal variability. The terrestrial biosphere has received considerably less attention. This talk examines the role of internal variability to generate unforced variability in the terrestrial carbon cycle. A multimember ensemble of ten simulations with the Community Earth System Model (CESM2) and Community Land Model (CLM5) over the historical period (1850-2014) is analyzed. Internal variability generates a large spread among ensemble members in terrestrial carbon fluxes. The spread decreases with larger spatial averaging and longer temporal averaging, suggesting that identifying forced changes in the carbon cycle at individual ecological monitoring sites with a short temporal record is challenging. This conclusion is seen also in analysis of time of emergence of the forced signal in the terrestrial carbon cycle, based on a six-member ensemble of CESM simulations from CMIP5. Unforced variability in the land-atmosphere CO2 flux is large and precludes detection of change at decadal timescales; the forced response only consistently emerges after several decades in many regions of the world. However, analysis of the terrestrial carbon cycle in the CESM Decadal Prediction Large Ensemble (CESM-DPLE) suggests potential to make decadal carbon cycle predictions. Taken together, these studies show the need to extend the scientific principles and tools of climate prediction to that of Earth system prediction, including the terrestrial biosphere in a changing environment.