Interannual and Seasonal Drivers of Carbon Cycle Variability Represented by the Community Earth System Model (CESM2)

Earth system models are intended to make long- term projections, but they can be evaluated at interannual and seasonal time scales. Although the Community Earth System Model (CESM2) showed improvements in a number of terrestrial carbon cycle benchmarks, relative to its predecessor, our analysis suggests that the interannual variability (IAV) in net terrestrial carbon fluxes did not show similar improvements. The model simulated low IAV of net ecosystem production (NEP), resulting in a weaker than observed sensitivity of the carbon cycle to climate variability. Low IAV in net fluxes likely resulted from low variability in gross primary productivity (GPP)- especially in the tropics- and a high covariation between GPP and ecosystem respiration. Although lower than observed, the IAV of NEP had significant climate sensitivities, with positive NEP anomalies associated with warmer and drier conditions in high latitudes, and with wetter and cooler conditions in mid and low latitudes. We identified two dominant modes of seasonal variability in carbon cycle flux anomalies in our fully coupled CESM2 simulations that are characterized by seasonal amplification and redistribution of ecosystem fluxes. Seasonal amplification of net and gross carbon fluxes showed climate sensitivities mirroring those of annual fluxes. Seasonal redistribution of carbon fluxes is initiated by springtime temperature anomalies, but subsequently negative feedbacks in soil moisture during the summer and fall result in net annual carbon losses from land. These modes of variability are also seen in satellite proxies of GPP, suggesting that CESM2 appropriately represents regional sensitivities of photosynthesis to climate variability on seasonal time scales.Plain Language SummaryEarth system models that are intended to make climate change projections also represent the global exchange of carbon dioxide (CO2) between the atmosphere, oceans, and land. As such, the growth rate and variability of CO2 concentrations in the atmosphere provide a robust measurement to evaluate models. We looked at the interannual variability (IAV) of terrestrial carbon fluxes and their sensitivity to variations in temperature and water that were simulated by the Community Earth System Model and compared them to observations. We found that the model underestimates the IAV of net terrestrial carbon fluxes, especially in the tropics. We also identified two modes of variability that correspond to an increase in summer land carbon uptake (amplification) and a change in the seasonal timing (redistribution) of land carbon fluxes. Notably, seasonal redistribution was initialized by warmer springs that increased early- season productivity, but subsequent water limitations in the summer and fall resulted in lower- than- average productivity over the growing season and net annual C losses to the atmosphere. Similar patterns of seasonal amplification and redistribution are seen in satellite observations of photosynthesis, suggesting that the model is realistically simulating characteristics of terrestrial ecosystems necessary for capturing carbon cycle- climate feedbacks.Key PointsSimulated net carbon fluxes show low interannual variability because of low tropical variability and high correlation in component fluxesSeasonal amplification and redistribution of fluxes characterize dominant modes of carbon cycle variability, consistent with observationsSeasonal redistribution is characterized by warm springs, dry summers, and can result in a net carbon loss from landPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/170288/1/2021GB007034-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/170288/2/gbc21163_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/170288/3/gbc21163.pd