Editing IPCC:AR6/SRCCL/Chapter-2 (section)

==== 2.4.2.2 Effects of past climate change on carbonaceous aerosols emissions and feedbacks ====

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Annual global emission estimates of BC range from 7.2–7.5 Tg yr <sup>–1</sup> (using bottom-up inventories) (Bond et al. 2013 <sup>[[#fn:r878|878]]</sup> ; Klimont et al. 2017 <sup>[[#fn:r879|879]]</sup> ) up to 17.8 ± 5.6 Tg yr <sup>–1</sup> (using a fully coupled climate-aerosol-urban model constrained by aerosol measurements) (Cohen and Wang 2014 <sup>[[#fn:r880|880]]</sup> ), with considerably higher BC emissions for Eastern Europe, southern East Asia, and Southeast Asia, mostly due to higher anthropogenic BC emissions estimates. A significant source of BC, the net trend in global burned area from 2000–2012 was a modest decrease of 4.3 Mha yr <sup>–1</sup> (–1.2% yr <sup>–1</sup> ).

Carbonaceous aerosols are important in urban areas as well as pristine continental regions, since they can be responsible for 50–85% of PM2.5 (Contini et al. 2018 <sup>[[#fn:r881|881]]</sup> ; Klimont et al. 2017 <sup>[[#fn:r882|882]]</sup> ). In boreal and tropical forests, carbonaceous aerosols originate from BVOC oxidation (Section 2.4.3). The largest global source of BC aerosols is open burning of forests, savannah and agricultural lands with emissions of about 2700 Gg yr <sup>–1</sup> in the year 2000 (Bond et al. 2013 <sup>[[#fn:r883|883]]</sup> ).

ESMs most likely underestimate globally averaged EC emissions (Bond et al. 2013 <sup>[[#fn:r884|884]]</sup> ; Cohen and Wang 2014 <sup>[[#fn:r885|885]]</sup> ), although recent emission inventories have included an upwards adjustment in these numbers (Hoesly et al. 2018 <sup>[[#fn:r886|886]]</sup> ). Vertical EC profiles have also been shown to be poorly constrained (Samset et al. 2014 <sup>[[#fn:r887|887]]</sup> ), with a general tendency of too much EC at high altitudes. Models differ strongly in the magnitude and importance of the coating-enhancement of ambient EC absorption (Boucher et al. 2016 <sup>[[#fn:r888|888]]</sup> ; Gustafsson and Ramanathan 2016 <sup>[[#fn:r889|889]]</sup> ) in their estimated lifetime of these particles, as well as in dry and wet removal efficiency ( ''limited evidence, medium agreement'' ) (Mahmood et al. 2016 <sup>[[#fn:r890|890]]</sup> ).

The equilibrium in emissions and concentrations between the scattering properties of organic aerosol versus the absorption component of BC is a key ingredient in the future climatic projections of aerosol effects ( ''limited evidence, high agreement'' ). The uncertainties in net climate forcing from BC-rich sources are substantial, largely due to lack of knowledge about cloud interactions with both BC and co-emitted OC. A strong positive forcing of about 1.1 wm <sup>–2</sup> was calculated by Bond et al. (2013), but this forcing is balanced by a negative forcing of –1.45 wm <sup>–2</sup> , and shows clearly a need to work on the co-emission issue for carbonaceous aerosols. The forcing will also depend on the aerosol-cloud interactions, where carbonaceous aerosol can be coated and change their CCN capability. It is difficult to estimate the changes in any of these components in a future climate, but this will strongly influence the radiative forcing ( ''high confidence'' ) (Contini et al. 2018 <sup>[[#fn:r891|891]]</sup> ; Boucher et al. 2013 <sup>[[#fn:r892|892]]</sup> ; Bond et al. 2013 <sup>[[#fn:r893|893]]</sup> ).

De Coninck et al. (2018) <sup>[[#fn:r894|894]]</sup> reported studies estimating a lower global temperature effect from BC mitigation (e.g., Samset et al. 2014 <sup>[[#fn:r895|895]]</sup> ; Boucher et al. 2016 <sup>[[#fn:r896|896]]</sup> ), although commonly used models do not capture properly observed effects of BC and co-emissions on climate (e.g., Bond et al. 2013 <sup>[[#fn:r897|897]]</sup> ). Regionally, the warming effects can be substantially larger, for example, in the Arctic (Sand et al. 2015 <sup>[[#fn:r898|898]]</sup> ) and high mountain regions near industrialised areas or areas with heavy biomass-burning impacts ( ''high confidence'' ) (Ming et al. 2013 <sup>[[#fn:r899|899]]</sup> ).

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