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The terrestrial net biome production (NBP) is considered as one of the major drivers of interannual variation in atmospheric CO2 levels. However, the determinants of variability in NBP under the background climate (i.e., preindustrial conditions) remain poorly understood, especially on decadal-to-centennial timescales. We analyzed 1,000-year simulations spanning 850-1,849 from the Community Earth System Model (CESM) and found that the variability in NBP and heterotrophic respiration (RH) were largely driven by fluctuations in the net primary production (NPP) and carbon turnover rates in response to climate variability. On interannual to multidecadal timescales, variability in NBP was dominated by variation in NPP, while variability in RH was driven by variation in turnover rates. However, on centennial timescales (100-1,000 years), the RH variability became more tightly coupled to that of NPP. The NBP variability on centennial timescales was low, due to the near cancellation of NPP and NPP-driven RH changes arising from climate internal variability and external forcings: preindustrial greenhouse gases, volcanic eruptions, land use changes, orbital change, and solar activity. Factorial experiments showed that globally on centennial timescales, the forcing of changes in greenhouse gas concentrations were the largest contributor (51%) to variations in both NPP and RH, followed by volcanic eruptions impacting NPP (25%) and RH (31%). Our analysis of the carbon-cycle suggests that geoengineering solutions by injection of stratospheric aerosols might be ineffective on longer timescales.

Plain Language Summary We used an earth system model to simulate the climate over last 1,000-year climate, including how carbon is cycled to and from ecosystems on land. These simulations allowed us to study how internal factors (e.g., turnover rate in soil carbon) and external drivers (e.g., volcanoes and land-use change) affect how much carbon is removed from the atmosphere (net biome production or NBP) on timescales of decades to centuries. We found that across the globe the variability in NBP on timescales of 2-100 years was mainly driven by carbon input through net primary production (NPP), while variation in the amount of carbon released by soil organisms other than plants (heterotrophic respiration [RH]) was dominated by variations of turnover rates. On centennial timescales, the variability in NBP was very small, as changes related to NPP were offset by NPP-driven RH changes. Additional simulations, modifying one factor at a time, demonstrated that the GHGs had the largest influence (followed by volcanic eruptions) on variations in global NPP and RH. Our findings suggest that certain strategies of mitigating climate change may be ineffective on longer timescales, and more earth system models including both biophysical and biogeochemical feedbacks are required to fully assess impacts of such interventions.