Anthropogenic aerosol forcing - insights from multiple estimates from aerosol-climate models with reduced complexity

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Fiedler , S , Kinne , S , Huang , W T K , Räisänen , P , O'Donnell , D , Bellouin , N , Stier , P , Merikanto , J , van Noije , T , Makkonen , R & Lohmann , U 2019 , ' Anthropogenic aerosol forcing - insights from multiple estimates from aerosol-climate models with reduced complexity ' , Atmospheric Chemistry and Physics , vol. 19 , no. 10 , pp. 6821-6841 . https://doi.org/10.5194/acp-19-6821-2019

Title: Anthropogenic aerosol forcing - insights from multiple estimates from aerosol-climate models with reduced complexity
Author: Fiedler, Stephanie; Kinne, Stefan; Huang, Wan Ting Katty; Räisänen, Petri; O'Donnell, Declan; Bellouin, Nicolas; Stier, Philip; Merikanto, Joonas; van Noije, Twan; Makkonen, Risto; Lohmann, Ulrike
Contributor: University of Helsinki, INAR Physics
Date: 2019-05-22
Language: eng
Number of pages: 21
Belongs to series: Atmospheric Chemistry and Physics
ISSN: 1680-7316
URI: http://hdl.handle.net/10138/302580
Abstract: This study assesses the change in anthropogenic aerosol forcing from the mid-1970s to the mid-2000s. Both decades had similar global-mean anthropogenic aerosol optical depths but substantially different global distributions. For both years, we quantify (i) the forcing spread due to model-internal variability and (ii) the forcing spread among models. Our assessment is based on new ensembles of atmosphere-only simulations with five state-of-the-art Earth system models. Four of these models will be used in the sixth Coupled Model Intercomparison Project (CMIP6; Eyring et al., 2016). Here, the complexity of the anthropogenic aerosol has been reduced in the participating models. In all our simulations, we prescribe the same patterns of the anthropogenic aerosol optical properties and associated effects on the cloud droplet number concentration. We calculate the instantaneous radiative forcing (RF) and the effective radiative forcing (ERF). Their difference defines the net contribution from rapid adjustments. Our simulations show a model spread in ERF from -0.4 to -0.9 W m(-2). The standard deviation in annual ERF is 0.3 W m(-2), based on 180 individual estimates from each participating model. This result implies that identifying the model spread in ERF due to systematic differences requires averaging over a sufficiently large number of years. Moreover, we find almost identical ERFs for the mid-1970s and mid-2000s for individual models, although there are major model differences in natural aerosols and clouds. The model-ensemble mean ERF is -0.54 W m(-2) for the pre-industrial era to the mid-1970s and -0.59 W m(-2) for the pre-industrial era to the mid-2000s. Our result suggests that comparing ERF changes between two observable periods rather than absolute magnitudes relative to a poorly constrained pre-industrial state might provide a better test for a model's ability to represent transient climate changes.
Subject: EARTH SYSTEM MODEL
OPTICAL-PROPERTIES
TROPOSPHERIC CHEMISTRY
RADIATIVE-TRANSFER
SIMULATION
PARAMETERIZATION
SENSITIVITY
FUTURE
UNCERTAINTIES
VARIABILITY
114 Physical sciences
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