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    Interdisciplinary research


    Millennium time-scale experiments on climate-carbon cycle with doubled CO2 concentration

    Hajima T, Yamamoto A, Kawamiya M, Su X, Watanabe M, Ohgaito R, Tatebe H

    global warming, earth system models, millennium time-scales, carbon cycle feedbacks, transient climate response to cumulative carbon emission, anthropogenic emission

    Change in temperature (ΔT) (top) and changes in cumulative land (middle) and ocean (bottom) carbon (C) uptake in the millennium time-scale experiments. Left (a–c) panels present the simulation results from MIROC-ESM; FULL2×CO2 (black), BGC2×CO2 (blue) and RAD2×CO2 (red), where atmospheric CO2 concentrations are abruptly doubled and held constant over 2000 years. Right (d–f) panels present the results from MIROC-ES2L with the results from FULL2×CO2 over 1000 years. The control experiment of each model is shown by the gray line. All simulation results were detrended by the control run of each model, by fitting spline curves over 1000/2000 years.

    Earth system models (ESMs) are commonly used for simulating the climate–carbon (C) cycle and for projecting future global warming. While ESMs are most often applied to century-long climate simulations, millennium-long simulations, which have been conducted by other types of models but not by ESM because of the computational cost, can provide basic fundamental properties of climate–C cycle models and will be required for estimating the carbon dioxide (CO2) concentration and subsequent climate stabilization in the future. This study used two ESMs (the Model for Interdisciplinary Research on Climate, the Earth system model version (MIROC-ESM) and the MIROC Earth system version 2 for long-term simulation (MIROC-ES2L)) to investigate millennium-scale climate and C cycle adjustment to external forcing. The CO2 concentration was doubled abruptly at the beginning of the model simulations and kept at that level for the next 1000 or 2000 years; these model simulations were compared with transient simulations where the CO2 was increased at the rate of 1% year−1 for up to 140 years (1pctCO2). Model simulations to separate and evaluate the C cycle feedbacks were also performed. Unlike the 1pctCO2 experiment, the change in temperature–cumulative anthropogenic C emission (ΔT–CE) relationship was non-linear over the millennium time-scales; there were differences in this nonlinearity between the two ESMs. The differences in ΔT–CE among existing models suggest large uncertainty in the ΔT and CE in the millennium-long climate-C simulations. Ocean C and heat transport were found to be disconnected over millennium time-scales, leading to longer time-scale of ocean C accumulation than heat uptake. Although the experimental design used here was highly idealized, this long-lasting C uptake by the ocean should be considered as part of the stabilization of CO2 concentration and global warming. Future studies should perform millennium time-scale simulations using a hierarchy of models to clarify climate-C cycle processes and to understand the long-term response of the Earth system to anthropogenic perturbations.