1 Nature 2014 Vol: 505(7481):37-42. DOI: 10.1038/nature12829

Spread in model climate sensitivity traced to atmospheric convective mixing

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
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. 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 ), 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. 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. 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. 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.
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References
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