8. Emergent constraints on carbon cycle feedbacks

8.1. Overview

Figures from Wenzel et al. (2014) are reproduced with namelist_wenz14jgr.xml. Variables relevant for the carbon cycle-climate feedback such as near surface air temperature (tas), net biosphere productivity (nbp) and (fgco2) are analyzed for coupled (1pctCO2; here the carbon cycle is fully coupled to the climate response) and uncoupled (esmFixCLim1; here the carbon cycle is uncoupled to the climate response) simulations. The standard namelist includes a comparison of cumulated nbp from coupled and uncoupled simulations and includes a set of routines to diagnose the long-term carbon cycle-climate feedback parameter (GammaLT) from an ensemble of CMIP5 models. Also included in the namelist is a comparison of the interannual variability of nbp and fgco2 for historical simulations used to diagnose the observable sensitivity of CO2 to tropical temperature changes (GammaIAV). As a key figure of this namelist, the diagnosed values from the models GammaLT vs. GammaIAV are compared in a scatter plot constituting an emergent constraint.

8.2. Available namelists and diagnostics

Namelists are stored in nml/

  • namelist_wenzel14jgr.xml

Diagnostics are stored in diag_scripts/

  • tsline.ncl: time line plots of annual means for spatial averages
  • carbon_corr_2var.ncl: scatter plot of annual mean anomalies of two different variables; diagnosing and saving GammaIAV
  • carbon_constraint.ncl: scatter plot of GammaLT vs. GammaIAV + line plot of probability density functions; diagnosing GammaLT

8.3. User settings

User setting files (cfg files) are stored in nml/cfg_carbon/

  1. tsline (cfg_carbon_line.ncl, cfg_carbon_line_h.ncl)

    Required diag_script_info attributes

    • styleset: “CMIP5”
    • ts_minlat: minimum latitude for area-averaging
    • ts_maxlat: maximum latitude for area-averaging
    • ts_minlon: minimum longitude for area-averaging
    • ts_maxlon: maximum longitude for area-averaging
    • multi_model_mean: True for multi-model mean calculation
    • ts_maxyear: last year (time range)
    • ts_minyear: first year (time range)
    • time_avg: “yearly” (currently, only yearly is available)
    • ts_anomaly: calculates anomalies with respect to the first 10-year average (anom, noanom)
    • area_opper: type of area operation (average, sum)
  2. ts_line (cfg_carbon_line_h.ncl)

    Required diag_script_info attributes

    • ts_detrend: detrend time series (detr, nodetr)
  3. create_co2_flux.ncl, carbon_corr_2var.ncl

    Required diag_script_info attributes

    • styleset: “CMIP5”
    • con_latrange: 2-element array of latitudes for time series plots if 2D or 3D
    • con_lonrange: 2-element array of pressure levels for time series plots if 2D or 3D
    • con_units: label string for units, e.g., “GtC/K”
    • pcBGC: label for CMIP5 experiment (for creating filename mask), e.g., “esmFixClim1”
    • reg_models: models to be included in emergent constraint (n-element array or (/”all”/)); note: has to be consistent with the list of models specified in the namelist!

8.4. Variables

  • tas (atmos, monthly mean, longitude latitude time)
  • co2 (atmos, monthly mean, plev longitude latitude time)
  • nbp (land, monthly mean, longitude latitude time)
  • fgco2 (ocean, monthly mean, longitude latitude time)

8.5. Observations and reformat scripts

Note: (1) obs4mips data can be used directly without any preprocessing; (2) see headers of reformat scripts for non-obs4mips data for download instructions.

  • GCP - Global Carbon Budget including land (nbp) and ocean (fgco2) carbon fluxes

Reformat script: reformat_scripts/obs/reformat_obs_GCP.ncl

  • NCEP - National Centers for Environmental Prediction reanalysis data for near surface temperature

Reformat script: reformat_scripts/obs/reformat_obs_NCEP.ncl

8.6. References

  • Cox, P. M., D. B. Pearson, B. B. Booth, P. Friedlingstein, C. C. Huntingford, C. D. B. Jones, and C. M. Luke, 2013, Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability, Nature, 494(7437), 341–344. doi: 10.1038/nature11882
  • Wenzel, S., P. M. Cox, V. Eyring, and P. Friedlingstein, 2014, Emergent Constraints on Climate Carbon Cycle Feedbacks in the CMIP5 Earth System Models, JGR Biogeoscience, 119(5), doi: 2013JG002591.

8.7. Example plots

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Figure 8.1 Time series of tropical (30°S - 30°N) mean near surface temperature (tas) change between year 30 and year 110 for the CMIP5 models simulated with prescribed CO2 (1%/yr CO2 increase) coupled simulation (1pctCO2).

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Figure 8.2 Correlations between the interannual variability of global co2flux (nbp+fgco2) and tropical temperature for the individual CMIP5 models using esmHistorical simulations, and for observations.

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Figure 8.3 Carbon cycle-climate feedback of tropical land carbon vs. the sensitivity of co2flux to interannual temperature variability in the tropics (30°S-30°N). The red line shows the linear best fit of the regression together with the prediction error (orange shading) and the gray shading shows the observed range.

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Figure 8.4 Probability Density Functions for the pure CMIP5 ensemble (black dashed) and after applying the observed constraint to the models (red solid)