Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations

Abstract

Abstract. Nine different global models with detailed aerosol modules have independently produced instantaneous direct radiative forcing due to anthropogenic aerosols. The anthropogenic impact is derived from the difference of two model simulations with prescribed aerosol emissions, one for present-day and one for pre-industrial conditions. The difference in the solar energy budget at the top of the atmosphere (ToA) yields a new harmonized estimate for the aerosol direct radiative forcing (RF) under all-sky conditions. On a global annual basis RF is −0.22 Wm −2 , ranging from +0.04 to −0.41 Wm −2 , with a standard deviation of ±0.16 Wm −2 . Anthropogenic nitrate and dust are not included in this estimate. No model shows a significant positive all-sky RF. The corresponding clear-sky RF is −0.68 Wm −2 . The cloud-sky RF was derived based on all-sky and clear-sky RF and modelled cloud cover. It was significantly different from zero and ranged between −0.16 and +0.34 Wm −2 . A sensitivity analysis shows that the total aerosol RF is influenced by considerable diversity in simulated residence times, mass extinction coefficients and most importantly forcing efficiencies (forcing per unit optical depth). The clear-sky forcing efficiency (forcing per unit optical depth) has diversity comparable to that for the all-sky/ clear-sky forcing ratio. While the diversity in clear-sky forcing efficiency is impacted by factors Correspondence to: M. Schulz (michael.schulz@cea.fr) such as aerosol absorption, size, and surface albedo, we can show that the all-sky/clear-sky forcing ratio is important because all-sky forcing estimates require proper representation of cloud fields and the correct relative altitude placement between absorbing aerosol and clouds. The analysis of the sulphate RF shows that long sulphate residence times are compensated by low mass extinction coefficients and vice versa. This is explained by more sulphate particle humidity growth and thus higher extinction in those models where short-lived sulphate is present at lower altitude and vice versa. Solar atmospheric forcing within the atmospheric column is estimated at +0.82±0.17 Wm −2 . The local annual average maxima of atmospheric forcing exceed +5 Wm −2 confirming the regional character of aerosol impacts on climate. The annual average surface forcing is −1.02±0.23 Wm −2 . With the current uncertainties in the modelling of the radiative forcing due to the direct aerosol effect we show here that an estimate from one model is not sufficient but a combination of several model estimates is necessary to provide a mean and to explore the uncertainty.

Spatially Refined Aerosol Direct Radiative Forcing Efficiencies

“…The DRF efficiencies come from a single modeling framework, and thus do not reflect uncertainty in the underlying model and emissions, 58−61 the former aspect alone can have a large impact on aerosol DRF even under unified emissions. 62 Application of yearly average RCP emissions does not account for key seasonalities, such as the biomass burning season in boreal areas being out-of-phase with seasons of peak albedo. Ignoring the potential for organic aerosol to form semiabsorptive (i.e., brown) particles, in addition to secondary formation of organic aerosol, is a further simplification.…”

Aerosol and ozone changes as forcing for climate evolution between 1850 and 2100

“…In the meantime, multi-model experiments were performed with global climate-chemistry models (CCMs) and chemistry transport models (CTMs) to better assess the past and future evolution of the atmospheric composition under anthropogenic emission evolution as well as climate change: PHOTOCOMP for tropospheric chemistry Stevenson et al 2006;Dentener et al 2006a), CCMVal for stratospheric ozone (Eyring et al 2005(Eyring et al , 2006(Eyring et al , 2007SPARC 2010) and AeroCom for aerosols Schulz et al 2006). Such climate-chemistry models solely consider atmospheric circulation and include a more or less detailed representation of atmospheric chemistry.…”

Occurrence of pristine aerosol environments on a polluted planet

“…We use the term pristine to refer to such regions, meaning aerosol in an original (i.e., preindustrial) state, although the term can often be used misleadingly to imply extremely low aerosol concentrations. Anthropogenic emissions in 1750 were not zero (22), and our reference year is therefore not truly "prehuman" (12), but is appropriate for defining the properties and behavior of aerosols in a reference state that is used for radiative forcing calculations (23). We focus our analysis on cloud condensation nuclei (CCN) because of the high sensitivity of cloud radiative forcing to the aerosol load (24).…”