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

==== 2.4.3.2 Historical changes of BVOCs and contribution to climate change ====

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Climate warming over the past 30 years, together with the longer growing season experienced in boreal and temperate environments, have increased BVOC global emissions since the preindustrial times ( ''limited evidence, medium agreement'' ) (Peñuelas 2009 <sup>[[#fn:r951|951]]</sup> ; Sanderson et al. 2003 <sup>[[#fn:r952|952]]</sup> ; Pacifico et al. 2012 <sup>[[#fn:r953|953]]</sup> ). This was opposed by lower BVOC emissions caused by the historical conversion of natural vegetation and forests to cropland ( ''limited evidence, medium agreement'' ) (Unger 2013 <sup>[[#fn:r954|954]]</sup> , 2014a <sup>[[#fn:r955|955]]</sup> ; Fu and Liao 2014 <sup>[[#fn:r956|956]]</sup> ). The consequences of historical anthropogenic land cover change were a decrease in the global formation of SOA (–13%) (Scott et al. 2017 <sup>[[#fn:r957|957]]</sup> ) and tropospheric burden (–13%) (Heald and Geddes 2016 <sup>[[#fn:r958|958]]</sup> ). This has resulted in a positive radiative forcing (and thus warming) from 1850–2000 of 0.017 W m <sup>–2</sup> (Heald and Geddes 2016 <sup>[[#fn:r959|959]]</sup> ), 0.025 W m <sup>–2</sup> (Scott et al. 2017 <sup>[[#fn:r960|960]]</sup> ) and 0.09 W m <sup>–2</sup> (Unger 2014b <sup>[[#fn:r961|961]]</sup> ) through the direct aerosol effect. In present-day conditions, global SOA production from all sources spans between 13 and 121 Tg yr <sup>–1</sup> (Tsigaridis et al. 2014 <sup>[[#fn:r962|962]]</sup> ). The indirect aerosol effect (change in cloud condensation nuclei), resulting from land use induced changes in BVOC emissions, adds an additional positive radiative forcing of 0.008 W m <sup>–2</sup> (Scott et al. 2017 <sup>[[#fn:r963|963]]</sup> ). More studies with different model setups are needed to fully assess this indirect aerosol effect associated with land use change from the preindustrial to present. CMIP6 global emissions pathways (Hoesly et al. 2018 <sup>[[#fn:r964|964]]</sup> ; Gidden et al. 2018 <sup>[[#fn:r965|965]]</sup> ) estimates global VOCs emissions in 2015 at 230 MtVOC yr <sup>–1</sup> . They also estimated that, from 2000–2015, emissions were up from 200–230 MtVOC yr <sup>–1</sup> .

There is ( ''limited evidence, medium agreement'' ) that historical changes in BVOC emissions have also impacted on tropospheric ozone. At most surface locations where land use has changed, the NOx concentrations are sufficiently high for the decrease in BVOC emissions to lead to decreasing ozone concentrations (Scott et al. 2017 <sup>[[#fn:r966|966]]</sup> ). However, in more pristine regions (with low NOx concentrations), the imposed conversion to agriculture has increased ozone through decreased BVOC emissions and their subsequent decrease in OH (Scott et al. 2017 <sup>[[#fn:r967|967]]</sup> ; Heald and Geddes 2016 <sup>[[#fn:r968|968]]</sup> ). In parallel, the enhanced soil NOx emissions from agricultural land can increase the ozone concentrations in NOx limited regions (Heald and Geddes 2016 <sup>[[#fn:r969|969]]</sup> ).

Another impact of the historical decrease in BVOC emissions is the reduction in the atmospheric lifetime of methane ( ''limited evidence, medium agreement'' ), which results in a negative radiative forcing that ranges from –0.007 W m <sup>–2</sup> (Scott et al. 2017 <sup>[[#fn:r970|970]]</sup> ) to –0.07 W m <sup>–2</sup> (Unger 2014b <sup>[[#fn:r971|971]]</sup> ). However, knowledge of the degree that BVOC emissions impact on oxidant concentrations, in particular OH (and thus methane concentrations), is still limited and therefore these numbers are very uncertain (Heald and Spracklen 2015 <sup>[[#fn:r972|972]]</sup> ; Scott et al. 2017 <sup>[[#fn:r973|973]]</sup> ). The effect of land use change on BVOC emissions are highly heterogeneous (Rosenkranz et al. 2015 <sup>[[#fn:r974|974]]</sup> ) and though the global values of forcing described above are small, the local or regional values can be higher, and even of opposite sign, than the global values.

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