Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.2.2.3 Vegetation Emissions of Organic Compounds ====

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A wide range of BVOCs are emitted from vegetation with the dominant compounds being isoprene and monoterpenes but also including sesquiterpenes, alkenes, alcohols, aldehydes and ketones. The photooxidation of BVOC emissions plays a fundamental role in atmospheric composition by controlling the regional and global budgets of ozone and organic aerosols, and impacting the lifetime of methane and other reactive components ( [[#Arneth--2010b|Arneth et al., 2010b]] ; [[#Heald--2015|Heald and Spracklen, 2015]] ). Substantial uncertainty exists across different modelling frameworks for estimates of global total BVOC emissions and individual compound emissions ( [[#Messina--2016|Messina et al., 2016]] ). Global isoprene emissions estimates differ by a factor of two from 300–600 TgC yr <sup>–1</sup> and global monoterpene emissions estimates by a factor of five from 30–150 TgC yr <sup>–1</sup> ( [[#Messina--2016|Messina et al., 2016]] ). A main driver of the uncertainty ranges is the choice of basal emissions rates assigned to different plant functional types in the model; however, the smaller uncertainty range for isoprene than for monoterpenes is not fully understood ( [[#Arneth--2008|Arneth et al., 2008]] ). The evaluation of global BVOC emissions is challenging because of poor measurement data coverage in many regions and the lack of year-round measurements ( [[#Unger--2013|Unger et al., 2013]] ). Several observational approaches have been developed in the past few years to improve understanding of BVOC emissions, including indirect methods such as the measurement of the OH loss rate in forested environments ( [[#Yang--2016|Yang et al., 2016]] ) and application of the variability in satellite formaldehyde concentrations ( [[#Palmer--2006|Palmer et al., 2006]] ; [[#Barkley--2013|Barkley et al., 2013]] ; [[#Stavrakou--2014|Stavrakou et al., 2014]] ). Direct space-borne isoprene retrievals using infrared radiance (IR) measurements have very recently become available ( [[#Fu--2019|Fu et al., 2019]] ; [[#Wells--2020|Wells et al., 2020]] ). Collectively these approaches have identified weaknesses in the ability of the parametrizations in global models to reproduce BVOC emissions hotspots ( [[#Wells--2020|Wells et al., 2020]] ). However, none of the current observational approaches have yet been able to reduce the uncertainty ranges in global emissions estimates.

At the plant level, BVOC emissions rates and composition depend strongly on plant species with plants tending to emit either isoprene or monoterpenes but not both. Photosynthetic activity is a main driver of isoprene and monoterpene production. Therefore, radiation and temperature, along with leaf-water status, phenological state and atmospheric CO <sub>2</sub> mixing ratio, affect emissions directly (on the leaf scale) and indirectly (via plant productivity; Guenther et al. , 2012; Loreto et al. , 2014; Niinemets et al. , 2014) . CO <sub>2</sub> directly influences the isoprene-synthesis process, with inhibition under increasing atmospheric CO <sub>2</sub> ( [[#Rosenstiel--2003|Rosenstiel et al., 2003]] ; [[#Possell--2005|Possell et al., 2005]] ; [[#Wilkinson--2009|Wilkinson et al., 2009]] ). Direct CO <sub>2</sub> inhibition has been observed for some monoterpene compounds ( [[#Loreto--2001|Loreto et al., 2001]] ; [[#Llorens--2009|Llorens et al., 2009]] ). Severe/long-term water stress may reduce emissions whilst mild/short-term water stress may temporarily amplify or maintain BVOC emissions to protect plants against ongoing stress ( [[#Peñuelas--2010|Peñuelas and Staudt, 2010]] ; [[#Potosnak--2014|Potosnak et al., 2014]] ; [[#Genard-Zielinski--2018|Genard-Zielinski et al., 2018]] ). Furthermore, observations in the Amazon indicate that the chemical composition of monoterpene emissions could also change under elevated temperature conditions ( [[#Jardine--2016|Jardine et al., 2016]] ). In addition, all these processes are investigated over short time scales but the long-term response of BVOC emissions depends on how the vegetation itself responds to the altered climate state (including temperature and water stress).

Global BVOC emissions are highly sensitive to environmental changes including changes in climate, atmospheric CO <sub>2</sub> <sub>,</sub> and vegetation composition and cover changes in natural and managed lands. Recent global modelling studies agree that global isoprene emissions have declined since the pre-industrial period, driven predominantly by anthropogenic land-use and land-cover change (LULCC) with results converging on a 10–25% loss of isoprene emissions between 1850 and the present day ( [[#Lathière--2010|Lathière et al., 2010]] ; [[#Unger--2013|Unger, 2013]] , 2014; [[#Acosta%20Navarro--2014|Acosta Navarro et al., 2014]] ; [[#Heald--2016|Heald and Geddes, 2016]] ; [[#Hantson--2017|Hantson et al., 2017]] ; [[#Hollaway--2017|Hollaway et al., 2017]] ; [[#Scott--2017|Scott et al., 2017]] ). The historical evolution of monoterpene and sesquiterpene emissions is less well studied and there is no robust consensus on even the sign of the change ( [[#Acosta%20Navarro--2014|Acosta Navarro et al., 2014]] ; [[#Hantson--2017|Hantson et al., 2017]] ). Future global isoprene and monoterpene emissions depend strongly on the climate and land-use scenarios considered ( [[#Hantson--2017|Hantson et al., 2017]] ; [[#Szogs--2017|Szogs et al., 2017]] ). BVOC emissions will be sensitive to future land-based climate change mitigation strategies including afforestation and bioenergy, with impacts of bioenergy depending on the choice of crops ( [[#Szogs--2017|Szogs et al., 2017]] ).

Most CMIP6 models use overly simplistic parametrizations and project an increase in global BVOC emissions in response to warming temperatures ( [[#Turnock--2020|Turnock et al., 2020]] ). This good agreement actually reflects the lack of diversity in BVOC-emissions parametrizations in global models that do not fully account for the complex processes influencing emissions that are discussed above.

Overall, we assess that historical global isoprene emissions declined between the pre-industrial period and the present day by 10–25% ( ''low confidence'' ) but historical changes in global monoterpenes and sesquiterpenes are too uncertain to provide an assessment. Future changes in BVOCs depend strongly on the evolution of climate and land use and are strongly sensitive to land-based climate change mitigation strategies. However, the net response of BVOC emissions is uncertain due to the complexity of processes that are hard to constrain observationally and are considered with various degrees of details in models.

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