= Solution
<Carbon dioxide> is relatively well mixed, its abundance is measured accurately, and its spectroscopy and large-scale radiative effect are well constrained. <Aerosols> are short lived and spatially heterogeneous; uncertain emissions, removal, size, composition, <water> uptake and height change both scattering and absorption. Their <aerosol-cloud interaction> effects and subsequent <cloud> adjustments add uncertainty, and the unobserved preindustrial <aerosol> baseline complicates anthropogenic attribution. Thus total <aerosol> forcing combines poorly constrained physical pathways and a less certain counterfactual, rather than simply an uncertain molecular absorption coefficient.<The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity by Piers Forster and others>[IPCC AR6 WGI Chapter 7 (2021), §§7.3.2–7.3.3: greenhouse-gas and aerosol effective radiative forcing]{ref}
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