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Nature | Article

日本語要約

Spread in model climate sensitivity traced to atmospheric convective mixing

Journal name:
Nature
Volume:
505,
Pages:
37–42
Date published:
DOI:
doi:10.1038/nature12829
Received
Accepted
Published online

Abstract

Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.

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Figures

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  1. Multimodel-mean local stratification parameter s.
    Figure 1: Multimodel-mean local stratification parameter s.

    The index S is the mean of s within the regions outlined in white. Multimodel averages of s are shown separately for low-sensitivity (ECS < 3.0 °C) (a) and high-sensitivity (ECS > 3.5 °C) (b) models, among coupled models with known ECS. The white dots inside the S-averaging region show the locations of radiosonde stations used to help estimate S observationally. A few coastal regions that are off-scale appear white.

  2. Basis for the index S of small-scale lower-tropospheric mixing and its relationship to the warming response.
    Figure 2: Basis for the index S of small-scale lower-tropospheric mixing and its relationship to the warming response.

    a, ΔT700–850 versus ΔR700–850, each averaged over a tropical region of mean ascent (see Fig. 1), from all 48 coupled models. For reference, a saturated-adiabatic value of ΔT is shown by dotted line at −7.2 K, and a dry-adiabatic value (not shown) would be about −16 K. Error bars are 2σ ranges. b, Change in small-scale moisture source Msmall below 850 hPa in the tropics upon +4 K ocean warming, versus S computed from the control run, in eight atmosphere models and one CMIP3 model. Symbol colour indicates modelling centre or centre where atmosphere model was originally developed and symbol shape indicates model generation.

  3. The structure of monthly-mean tropospheric ascent reveals large-scale lower-tropospheric mixing in observations and models.
    Figure 3: The structure of monthly-mean tropospheric ascent reveals large-scale lower-tropospheric mixing in observations and models.

    Upward pressure velocity ω in one month (September) from the MERRA reanalysis (a), the IPSL-CM5A model (b) and the IPSL-CM5B model (c) with values at 850 hPa shown in red and those at 500 hPa shown in green plus blue. Bright red implies ascent that is weighted toward the lower troposphere with mid-tropospheric divergence (see colour scale), white implies deep ascent, and dark colours imply descent. In a, black lines outline the region in which the index D of large-scale lower-tropospheric mixing is computed. The Pacific and Atlantic Intertropical Convergence Zone regions are consistently red in the reanalyses and models, whereas isolated red patches in other areas tend to vary with time.

  4. Estimated water vapour source MLT,[thinsp]large due to large-scale lower-tropospheric mixing and its response to warming.
    Figure 4: Estimated water vapour source MLT, large due to large-scale lower-tropospheric mixing and its response to warming.

    See Methods for calculation details. Data are from ten atmosphere models, averaged from 30° S to 30° N over oceans, with the average of the four models having the largest D shown in magenta and the average of the four models with the smallest D shown in blue. Dashes show results in +4 K climate. Changes at +4 K are nearly identical whether or not land areas are included.

  5. Relation of lower-tropospheric mixing indices to ECS.
    Figure 5: Relation of lower-tropospheric mixing indices to ECS.

    ECS versus S (a), D (b) and LTMI = S + D (c) from the 43 coupled models with known ECS. Linear correlation coefficients r are given in each panel (r = 0.70 in c is the correlation to the total system feedback). Error bars shown near panel axes indicate 2σ ranges of the direct radiosonde estimate (a) and the S value from radiosondes added to the D value from each of the two reanalyses (c). ERAi and MERRA are the two monthly reanalysis products.

  6. Illustration of atmospheric overturning circulations.
    Extended Data Fig. 1: Illustration of atmospheric overturning circulations.

    Deep overturning strongly coupled to the hydrological cycle and atmospheric energy budget is shown by solid lines; lower-tropospheric mixing is shown by dashed lines. The MILC feedback results from the increasing relative role of lower-tropospheric mixing in exporting humidity from the boundary layer as the climate warms, thus depleting the layer of water vapour needed to sustain low cloud cover.

  7. Small-scale moisture source Msmall.
    Extended Data Fig. 2: Small-scale moisture source Msmall.

    Vertical profile averaged over all tropical oceans, for two selected climate models (see legend) with very different warming responses, in present-day (solid) and +4 K (dashed) climates.

  8. Response of cloud fraction to warming.
    Extended Data Fig. 3: Response of cloud fraction to warming.

    Profile of average change in model cloud fractional cover at +4 K in the four atmosphere models with largest (magenta) and smallest (blue) estimated +4 K increases in planetary-boundary-layer drying, averaged from 30° S to 30° N (dashed) or 60° S to 60° N (solid). The drying estimate is obtained by adding the explicitly computed change in MLT, large to the change in Msm estimated from S via the relationship shown in Fig. 2a. The typical mean cloud fraction below 850 hPa is about 10% to 20%, and the changes shown are absolute changes in this fraction, so are of the order of 10% of the initial cloud cover.

  9. Response of large-scale lower-tropospheric mixing to warming.
    Extended Data Fig. 4: Response of large-scale lower-tropospheric mixing to warming.

    Profiles of mean vertical velocity in regions of shallow ascent, in control and +4 K climates. The similarity of dashed and solid lines indicates that mass overturning associated with these regions is roughly the same in the warmer simulations, on average.

  10. Response of small-scale, low-level drying to warming.
    Extended Data Fig. 5: Response of small-scale, low-level drying to warming.

    Change in convective moisture source Msmall below 850 hPa upon a +4 K warming in eight atmosphere models and one CMIP3 coupled model; units are W m−2, with negative values indicating stronger drying near the surface. Zero contours are shown in white (a few off-scale regions also appear white). The models used for calculating Mlarge are the eight shown here plus two for which Msmall data were unavailable: CNRM-CM5 and FGOALS-g2.

Tables

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  1. List of CMIP5 coupled models used
    Extended Data Table 1: List of CMIP5 coupled models used
  2. List of CMIP3 coupled models used
    Extended Data Table 2: List of CMIP3 coupled models used

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Author information

Affiliations

  1. Climate Change Research Centre and ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney 2052, Australia

    • Steven C. Sherwood

  2. Laboratoire de Météorologie Dynamique and Institut Pierre Simon Laplace (LMD/IPSL), CNRS, Université Pierre et Marie Curie, Paris 75252, France

    • Sandrine Bony &
    • Jean-Louis Dufresne

Contributions

S.C.S. led the study and the writing of the paper, and did the calculations of LTMI and related diagnostics. S.B. computed cloud radiative effect and assisted in interpreting results and writing the paper. J.-L.D. computed ECS and assisted in interpreting results and writing the paper.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

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Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Illustration of atmospheric overturning circulations. (128 KB)

    Deep overturning strongly coupled to the hydrological cycle and atmospheric energy budget is shown by solid lines; lower-tropospheric mixing is shown by dashed lines. The MILC feedback results from the increasing relative role of lower-tropospheric mixing in exporting humidity from the boundary layer as the climate warms, thus depleting the layer of water vapour needed to sustain low cloud cover.

  2. Extended Data Figure 2: Small-scale moisture source Msmall. (197 KB)

    Vertical profile averaged over all tropical oceans, for two selected climate models (see legend) with very different warming responses, in present-day (solid) and +4 K (dashed) climates.

  3. Extended Data Figure 3: Response of cloud fraction to warming. (268 KB)

    Profile of average change in model cloud fractional cover at +4 K in the four atmosphere models with largest (magenta) and smallest (blue) estimated +4 K increases in planetary-boundary-layer drying, averaged from 30° S to 30° N (dashed) or 60° S to 60° N (solid). The drying estimate is obtained by adding the explicitly computed change in MLT, large to the change in Msm estimated from S via the relationship shown in Fig. 2a. The typical mean cloud fraction below 850 hPa is about 10% to 20%, and the changes shown are absolute changes in this fraction, so are of the order of 10% of the initial cloud cover.

  4. Extended Data Figure 4: Response of large-scale lower-tropospheric mixing to warming. (180 KB)

    Profiles of mean vertical velocity in regions of shallow ascent, in control and +4 K climates. The similarity of dashed and solid lines indicates that mass overturning associated with these regions is roughly the same in the warmer simulations, on average.

  5. Extended Data Figure 5: Response of small-scale, low-level drying to warming. (590 KB)

    Change in convective moisture source Msmall below 850 hPa upon a +4 K warming in eight atmosphere models and one CMIP3 coupled model; units are W m−2, with negative values indicating stronger drying near the surface. Zero contours are shown in white (a few off-scale regions also appear white). The models used for calculating Mlarge are the eight shown here plus two for which Msmall data were unavailable: CNRM-CM5 and FGOALS-g2.

Extended Data Tables

  1. Extended Data Table 1: List of CMIP5 coupled models used (552 KB)
  2. Extended Data Table 2: List of CMIP3 coupled models used (584 KB)

Additional data