I am reposting Zelinka et al (2020) and am posting O'Connor et al (2020) [which indicates that present day effective radiative forcing, ERF, is greater than the sum of the individual GHG ERFs due to nonlinear interactions] in order to point out that if (with continued anthropogenic forcing) the majority of Earth Systems contributing to climate sensitivity are nonlinear, then consensus climate science will be slower to acknowledge such a situation for reasons including:
1. As models are not equal, one would expect nonlinear Earth Systems to increase the spread of projected ECS values as is the case for the 1.8 – 5.6K range of ECS values from CMIP6 reported by Zelinka et al (2020); which is a higher range than that reported by either CMIP5 or AR5.
2. As AR6 will blend the climate sensitivity values reported by CMIP6 with values of climate sensitivity calculated from both recorded observations and from paleodata; this will have the effect of underreporting nonlinear behavior as recorded observations are limited and paleodata is noisy.
3. Decision makers will defer taking effective action to limit anthropogenic radiative forcing until consensus climate science reduces their uncertainty w.r.t. projected climate sensitivity; which will work to activate more nonlinearity in more Earth Systems (the majority of which have positive feedbacks); which will serve to result in higher realized values of climate sensitivity this century.
Mark D. Zelinka et al. (03 January 2020), "Causes of Higher Climate Sensitivity in CMIP6 Models", Geophysical Research Letters,
https://doi.org/10.1029/2019GL085782https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL085782
AbstractEquilibrium climate sensitivity, the global surface temperature response to CO2 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5–4.5 K. Global climate models (GCMs), which attempt to represent all relevant physical processes, provide the most direct means of estimating climate sensitivity via CO2 quadrupling experiments. Here we show that the closely related effective climate sensitivity has increased substantially in Coupled Model Intercomparison Project phase 6 (CMIP6), with values spanning 1.8–5.6 K across 27 GCMs and exceeding 4.5 K in 10 of them. This (statistically insignificant) increase is primarily due to stronger positive cloud feedbacks from decreasing extratropical low cloud coverage and albedo. Both of these are tied to the physical representation of clouds which in CMIP6 models lead to weaker responses of extratropical low cloud cover and water content to unforced variations in surface temperature. Establishing the plausibility of these higher sensitivity models is imperative given their implied societal ramifications.
Plain Language SummaryThe severity of climate change is closely related to how much the Earth warms in response to greenhouse gas increases. Here we find that the temperature response to an abrupt quadrupling of atmospheric carbon dioxide has increased substantially in the latest generation of global climate models. This is primarily because low cloud water content and coverage decrease more strongly with global warming, causing enhanced planetary absorption of sunlight—an amplifying feedback that ultimately results in more warming. Differences in the physical representation of clouds in models drive this enhanced sensitivity relative to the previous generation of models. It is crucial to establish whether the latest models, which presumably represent the climate system better than their predecessors, are also providing a more realistic picture of future climate warming.
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O'Connor, F. M., Abraham, N. L., Dalvi, M., Folberth, G., Griffiths, P., Hardacre, C., Johnson, B. T., Kahana, R., Keeble, J., Kim, B., Morgenstern, O., Mulcahy, J. P., Richardson, M. G., Robertson, E., Seo, J., Shim, S., Teixeira, J. C., Turnock, S., Williams, J., Wiltshire, A., and Zeng, G.: Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1, Atmos. Chem. Phys. Discuss.,
https://doi.org/10.5194/acp-2019-1152, in review, 2020.
https://www.atmos-chem-phys-discuss.net/acp-2019-1152/Abstract. Quantifying forcings from anthropogenic perturbations to the Earth System (ES) is important for understanding changes in climate since the pre-industrial period. In this paper, we quantify and analyse a wide range of present-day (PD) anthropogenic climate forcings with the UK's Earth System Model (ESM), UKESM1, following the protocols defined by the Radiative Forcing Model Intercomparison Project (RFMIP) and the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). In particular, by quantifying effective radiative forcings (ERFs) that include rapid adjustments within a full ESM, it enables the role of various climate-chemistry-aerosol-cloud feedbacks to be quantified.
Global mean ERFs are 1.83, 0.13, −0.33, and 0.93 W m−2 at the PD (Year 2014) relative to the pre-industrial (PI; Year 1850) for carbon dioxide, nitrous oxide, ozone-depleting substances, and methane, respectively. The PD total greenhouse gas ERF is 2.89 W m−2, larger than the sum of the individual GHG ERFs.
UKESM1 has an aerosol forcing of −1.13 W m−2. A relatively strong negative forcing from aerosol-cloud interactions and a small negative instantaneous forcing from aerosol-radiation interactions are partially offset by a substantial forcing from black carbon absorption. Internal mixing and chemical interactions mean that neither the forcing from aerosol-radiation interactions nor aerosol-cloud interactions are linear, making the total aerosol ERF less than the sum of the individual speciated aerosol ERFs.
Tropospheric ozone precursors, in addition to exerting a positive forcing due to ozone, lead to oxidant changes which in turn cause an indirect aerosol ERF, altering the sign of the net ERF from nitrogen oxide emissions. Together, aerosol and tropospheric ozone precursors (near-term climate forcers, NTCFs) exert a global mean ERF of −1.12 W m−2, mainly due to changes in the cloud radiative effect. There is also a negative PD ERF from land use (−0.32 W m−2). It is outside the range of previous estimates, and is most likely due to too strong an albedo response. In combination, the net anthropogenic ERF is potentially biased low (1.61 W m−2) relative to other estimates, due to the inclusion of non-linear feedbacks and ES interactions.
By including feedbacks between greenhouse gases, stratospheric and tropospheric ozone, aerosols, and clouds, some of which act non-linearly, this work demonstrates the importance of ES interactions when quantifying climate forcing. It also suggests that rapid adjustments need to include chemical as well as physical adjustments to fully account for complex ES interactions.