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

==== 2.4.3.3 Future changes of BVOCs ====

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Studies suggest that increasing temperature will change BVOC emissions through change in species composition and rate of BVOC production. A further 2°C–3°C rise in the mean global temperature could increase BVOC global emissions by an additional 30–45% (Peñuelas and Llusià 2003 <sup>[[#fn:r975|975]]</sup> ). In two modelling studies, the impact on climate from rising BVOC emissions was found to become even larger with decreasing anthropogenic aerosol emissions (Kulmala et al. 2013 <sup>[[#fn:r976|976]]</sup> ; Sporre et al. 2019 <sup>[[#fn:r977|977]]</sup> ). A negative feedback on temperature, arising from the BVOC-induced increase in the first indirect aerosol effect, has been estimated by two studies to be in the order of –0.01 W m <sup>–2</sup> K (Scott et al. 2018b <sup>[[#fn:r978|978]]</sup> ; Paasonen et al. 2013 <sup>[[#fn:r979|979]]</sup> ). Enhanced aerosol scattering from increasing BVOC emissions has been estimated to contribute to a global gain in BVOC emissions of 7% (Rap et al. 2018 <sup>[[#fn:r980|980]]</sup> ). In a warming planet, BVOC emissions are expected to increase but magnitude of this increase is unknown and will depend on future land use change, in addition to climate ( ''limited evidence, medium agreement'' ).

There is a very limited number of studies investigating the climate impacts of BVOCs using future land use scenarios (Ashworth et al. 2012 <sup>[[#fn:r981|981]]</sup> ; Pacifico et al. 2012 <sup>[[#fn:r982|982]]</sup> ). Scott et al. (2018a) <sup>[[#fn:r983|983]]</sup> found that a future deforestation according to the land use scenario in RCP8.5 leads to a 4% decrease in BVOC emissions at the end of the century. This resulted in a direct aerosol forcing of +0.006 W m <sup>–2</sup> (decreased reflection by particles in the atmosphere) and a first indirect aerosol forcing of –0.001 W m <sup>–2</sup> (change in the amount of CCN). Studies not including future land use scenarios but investigating the climate feedbacks leading to increasing future BVOC emissions, have found a direct aerosol effect of –0.06 W m <sup>–2</sup> (Sporre et al. 2019 <sup>[[#fn:r984|984]]</sup> ) and an indirect aerosol effect of –0.45 W m <sup>–2</sup> (Makkonen et al. 2012 <sup>[[#fn:r985|985]]</sup> ; Sporre et al. 2019 <sup>[[#fn:r2134|2134]]</sup> ). The stronger aerosol effects from the feedback compared to the land use are, at least partly, explained by a much larger change in the BVOC emissions.

A positive climate feedback could happen in a future scenario with increasing BVOC emissions, where higher ozone and methane concentrations could lead to an enhanced warming which could further increase BVOC emissions (Arneth et al. 2010 <sup>[[#fn:r986|986]]</sup> ). This possible feedback is mediated by NOx levels. One recent study including dynamic vegetation, land use change, CO <sub>2</sub> and climate change found no increase and even a slight decrease in global BVOC emissions at the end of the century (Hantson et al. 2017 <sup>[[#fn:r987|987]]</sup> ). There is a lack of understanding concerning the processes governing the BVOC emissions, the oxidation processes in the atmosphere, the role of the BVOC oxidation products in new particle formation and particle growth, as well as general uncertainties in aerosol–cloud interactions. There is a need for continued research into these processes, but the current knowledge indicates that changing BVOC emissions need to be taken into consideration when assessing the future climate and how land use will affect it. In summary, the magnitude and sign of net effect of BVOC emissions on the radiation budget and surface temperature is highly uncertain.

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