Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.3.4.3 Light-absorbing Particles on Snow and Ice ====

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In AR5, it was assessed that the effects of light-absorbing particles (LAPs) did probably not significantly contribute to recent reductions in Arctic ice and snow ( [[#Vaughan--2013|Vaughan et al., 2013]] ). The SARF from LAPs on snow and ice was assessed to 0.04 [0.02 to 0.09] W m <sup>–2</sup> ( [[#Boucher--2013|Boucher et al., 2013]] ), a range appreciably lower than the estimates given in AR4 ( [[#Forster--2007|Forster et al., 2007]] ). This effect was assessed to be ''low confidence'' ( ''medium evidence'' , ''low agreement'' ) (Table 8.5 in [[#Myhre--2013b|Myhre et al., 2013b]] ).

Since AR5 there has been progress in the understanding of the physical state and processes in snow that govern the albedo reduction by black carbon (BC). The SROCC ( [[#IPCC--2019a|IPCC, 2019a]] ) assessed that there is ''high confidence'' that darkening of snow by deposition of BC and other light-absorbing aerosol species increases the rate of snow melt ( [[IPCC:Wg1:Chapter:Chapter-2#2.2|Section 2.2]] in [[#Hock--2019|Hock et al., 2019]] ; [[IPCC:Wg1:Chapter:Chapter-3#3.4|Section 3.4]] in [[#Meredith--2019|Meredith et al., 2019]] ). C. [[#He--2018|]] [[#He--2018|He et al. (2018)]] found that taking into account both the non-spherical shape of snow grains and internal mixing of BC in snow significantly altered the effects of BC on snow albedo. The reductions of snow albedo by dust and BC have been measured and characterized in the Arctic, the Tibetan Plateau, and mid-latitude regions subject to seasonal snowfall, including North America and northern and eastern Asia ( [[#Qian--2015|Qian et al., 2015]] ).

Since AR5, two further studies of global IRF from black carbon on snow deposition are available, with best estimates of 0.01 W m <sup>–2</sup> ( [[#Lin--2014|Lin et al., 2014]] ) and 0.045 W m <sup>–2</sup> ( [[#Namazi--2015|Namazi et al., 2015]] ). Organic carbon deposition on snow and icehas been estimated to contribute a small positive IRF of 0.001 to 0.003 W m <sup>–2</sup> ( [[#Lin--2014|Lin et al., 2014]] ). No comprehensive global assessments of mineral dust deposition on snow are available, although the effects are potentially large in relation to the total effect of LAPs on snow and ice forcing ( [[#Yasunari--2015|Yasunari et al., 2015]] ).

Most radiative forcing estimates have a regional emphasis. The regional focus makes estimating a global mean radiative forcing from aggregating different studies challenging, and the relative importance of each region is expected to change if the global pattern of emissions sources changes ( [[#Bauer--2013|Bauer et al., 2013]] ). The lower bound of the assessed range of BC on snow and ice is extended to zero to encompass [[#Lin--2014|Lin et al. (2014)]] , with the best estimate unchanged, resulting in 0.04 [0.00 to 0.09] W m <sup>–2</sup> . The efficacy of BC on snow forcing was estimated to be 2 to 4 times as large as for an equivalent CO <sub>2</sub> forcing as the effects are concentrated at high latitudes in the cryosphere ( [[#Bond--2013|Bond et al., 2013]] ). However, it is unclear how much of this effect is due to radiative adjustments leading to a higher ERF, and how much comes from a less negative feedback α due to the high-latitude nature of the forcing. To estimate the overall ERF, the IRF is doubled assuming that part of the increased efficacy is due to adjustments. This gives an overall assessed ERF of +0.08 [0.00 to 0.18] W m <sup>–2</sup> , with ''low confidence'' .

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