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

• Published in: Global biogeochemical cycles 2021-09, Vol.35 (9), p.e2021GB007034-n/a
• Main Authors: Wieder, William R., Butterfield, Zachary, Lindsay, Keith, Lombardozzi, Danica L., Keppel‐Aleks, Gretchen
• Format: Article
• Language: English
• Subjects: Abrupt/Rapid Climate Change; Air/Sea Constituent Fluxes; Air/Sea Interactions; Amplification; Anomalies; Atmosphere; Atmospheric; Atmospheric Composition and Structure; Atmospheric Effects; Atmospheric models; Atmospheric Processes; Avalanches; Benchmarks; Benefit‐cost Analysis; Biogeochemical Cycles, Processes, and Modeling; Biogeochemical Kinetics and Reaction Modeling; Biogeochemistry; Biogeosciences; Carbon cycle; Carbon Cycling; Carbon dioxide; Carbon dioxide atmospheric concentrations; Carbon dioxide concentration; Carbon dioxide exchange; Carbon uptake; CESM2; Climate and Interannual Variability; Climate change; Climate Change and Variability; Climate Dynamics; Climate feedback; Climate Impact; Climate Impacts; Climate models; Climate Variability; Climatology; Computational Geophysics; Cryosphere; Decadal Ocean Variability; Diel, Seasonal, and Annual Cycles; Disaster Risk Analysis and Assessment; Diurnal, Seasonal, and Annual Cycles; Earth; Earth system model; Earth System Modeling; Earthquake Ground Motions and Engineering Seismology; Effusive Volcanism; Evaluation; Explosive Volcanism; Fluxes; General Circulation; Geodesy and Gravity; Geological; Global Change; Global Change from Geodesy; Gravity and Isostasy; Growing season; Growth rate; Hydrological Cycles and Budgets; Hydrology; Identification; Impacts of Global Change; Informatics; Interannual variability; Land/Atmosphere Interactions; Latitude; Marine Geology and Geophysics; Mass Balance; Modeling; Modes; Mud Volcanism; Natural Hazards; Numerical Modeling; Numerical Solutions; Ocean influence of Earth rotation; Ocean Monitoring with Geodetic Techniques; Ocean/Atmosphere Interactions; Ocean/Earth/atmosphere/hydrosphere/cryosphere interactions; Oceanic; Oceanography: Biological and Chemical; Oceanography: General; Oceanography: Physical; Oceans; Paleoceanography; Photosynthesis; Physical Modeling; Policy Sciences; Primary production; Productivity; Radio Oceanography; Radio Science; Regional Climate Change; Regional Modeling; Risk; Satellite observation; Satellites; Sea Level Change; Sea Level: Variations and Mean; Seasonal variability; Seasonal variation; Seasonal variations; Seasons; Seismology; Sensitivity; Simulation; Soil moisture; Soil temperature; Solid Earth; Summer; Surface Waves and Tides; Temperature anomalies; Temperature variations; terrestrial biogeochemistry; Terrestrial ecosystems; Terrestrial environments; Theoretical Modeling; Time; Tropical environments; Tsunamis and Storm Surges; Uptake; Volcanic Effects; Volcanic Hazards and Risks; Volcano Monitoring; Volcano Seismology; Volcano/Climate Interactions; Volcanology; Water Cycles; Water springs
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2021GB007034

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 Summary Earth 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 car