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- Sherwood, S. C., et al.
(author)
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An Assessment of Earth's Climate Sensitivity Using Multiple Lines of Evidence
- 2020
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In: Reviews of geophysics. - 8755-1209 .- 1944-9208. ; 58:4
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Research review (peer-reviewed)abstract
- We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO2, characterized by an effective sensitivity S. This evidence includes feedback process understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density function (PDF) for S given all the evidence, including tests of robustness to difficult-to-quantify uncertainties and different priors. The 66% range is 2.6-3.9 K for our Baseline calculation and remains within 2.3-4.5 K under the robustness tests; corresponding 5-95% ranges are 2.3-4.7 K, bounded by 2.0-5.7 K (although such high-confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S, in particular using comprehensive models and process understanding to address limitations in the traditional forcing-feedback paradigm for interpreting past changes.
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| 2. |
- Bellouin, N., et al.
(author)
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Bounding Global Aerosol Radiative Forcing of Climate Change
- 2020
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In: Reviews of geophysics. - 8755-1209 .- 1944-9208. ; 58:1
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Research review (peer-reviewed)abstract
- Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of -1.6 to -0.6Wm(-2), or -2.0 to -0.4Wm(-2) with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds. Plain Language Summary Human activities emit into the atmosphere small liquid and solid particles called aerosols. Those aerosols change the energy budget of the Earth and trigger climate changes, by scattering and absorbing solar and terrestrial radiation and playing important roles in the formation of cloud droplets and ice crystals. But because aerosols are much more varied in their chemical composition and much more heterogeneous in their spatial and temporal distributions than greenhouse gases, their perturbation to the energy budget, called radiative forcing, is much more uncertain. This review uses traceable and arguable lines of evidence, supported by aerosol studies published over the past 40 years, to quantify that uncertainty. It finds that there are two chances out of three that aerosols from human activities have increased scattering and absorption of solar radiation by 14% to 29% and cloud droplet number concentration by 5 to 17% in the period 2005-2015 compared to the year 1850. Those increases exert a radiative forcing that offsets between a fifth and a half of the radiative forcing by greenhouse gases. The degree to which human activities affect natural aerosol levels, and the response of clouds, and especially ice clouds, to aerosol perturbations remain particularly uncertain.
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| 3. |
- Jönsson, Aiden, 1991-, et al.
(author)
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A recipe for simulating the observed interhemispheric albedo symmetry and constraining cloud radiative feedbacks
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Other publication (other academic/artistic)abstract
- Earth’s albedo has remained symmetric between the northern and southern hemispheres over the satellite record, a feature that climate models have difficulty capturing. We investigate causes of these biases using a perturbed parameter ensemble of atmospheric simulations to probe the sensitivity of the albedo symmetry to cloud properties and the processes that control them. We find that the most significant parameters to simulated albedo symmetry impact precipitation, turbulent dissipation, and sea salt aerosol emissions. Constraining shortwave cloud feedbacks using the observed albedo symmetry leads to a range of +0.61±0.24 W m-2 K-1 (66% confidence). These are stronger than the model’s control settings due to greater loss of subtropical low clouds and weaker negative cloud phase feedback. Comparing the constrained and control parameter settings shows a preference towards settings that would reduce the control simulation’s biases, indicating that the constraint can select for representations that capture the observed cloud cover.
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