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

=== 2.2.7 Land Use and Land Cover ===

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The AR5 assessed that land use change ''very likely'' increased the Earth’s albedo with a radiative forcing of –0.15 (± 0.10) W m <sup>–2</sup> . AR5 also assessed that a net cooling of the surface, accounting for processes that are not limited to the albedo, was ''about as likely as not'' . The SRCCL concluded with ''medium confidence'' that the biophysical effects of land cover change (mainly increased albedo) had a cooling effect on surface temperatures. The SRCCL also concluded with ''very high confidence'' that the biogeochemical effects of land cover change (i.e., GHG emissions) resulted in a mean annual surface warming.

Much of the global land surface has been modified or managed to some extent by human activities during the Holocene. Reconstructions based on pollen data indicate that natural vegetation probably covered most of the Earth’s ice-free terrestrial surface until roughly the mid-Holocene ( [[#Marquer--2017|Marquer et al., 2017]] ; [[#Harrison--2020|Harrison et al., 2020]] ; F. [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ). Reconstructions based on pollen, archaeological, and historical data indicate deforestation at the regional scale since at least 6 ka ( [[#Marquer--2017|Marquer et al., 2017]] ; [[#Stephens--2019|Stephens et al., 2019]] ; [[#Harrison--2020|Harrison et al., 2020]] ; F. [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ). From a global perspective, land-use forcing datasets ( [[#Lawrence--2016|Lawrence et al., 2016]] ) estimate that changes in land use (and related deforestation) were small on the global scale until the mid-19th century and accelerated markedly thereafter, with larger uncertainties prior to industrialization ( [[#Kaplan--2017|Kaplan et al., 2017]] ). Since the early 1980s, about 60% of all land cover changes have been associated with direct human activities, with spatial patterns emphasizing the regional character of land use and land management, including tropical deforestation, temperate afforestation, cropland intensification, and increased urbanization ( [[#Song--2018|Song et al., 2018]] ; [[#Zeng--2018|Zeng et al., 2018]] ). At present, nearly three-quarters of the ice-free terrestrial surface is under some form of human use ( [[#Venter--2016|Venter et al., 2016]] ; [[#Erb--2017|Erb et al., 2017]] ), particularly in agriculture and forest management.

The impact of historical land-cover change on global climate is assessed with model simulations that consider multiple climate and biophysical processes (e.g., changes in albedo, evapotranspiration, and roughness) and/or biogeochemical processes (e.g., changes in atmospheric composition such as carbon release from deforestation). The dominant biophysical response to land cover changes is albedo, which is estimated (using a MODIS albedo product and a historical land-use harmonization product) to have increased gradually prior to the mid-19th century and then strongly through the mid-20th century, with a slightly slower rise thereafter ( [[#Ghimire--2014|Ghimire et al., 2014]] ). Recent radiative forcing estimates arising from biophysical processes generally fall at the lower end of the AR5 assessed range. For instance, based on historical simulations from 13 CMIP6 models, C.J. [[#Smith--2020|]] [[#Smith--2020|Smith et al. (2020)]] estimated that the ERF from surface albedo changes (including snow cover and leaf area) was –0.08 [–0.22 to +0.06] W m <sup>–2</sup> since 1850. Similarly, based on simulations from 13 CMIP5 models, [[#Lejeune--2020|Lejeune et al. (2020)]] estimated the radiative forcing from transitions between trees, crops, and grasslands was –0.11 [–0.16 to +0.04] W m <sup>–2</sup> since 1860. [[#Andrews--2017|Andrews et al. (2017)]] identified an ERF of –0.40 W m <sup>–2</sup> since 1860, ascribing much of the effect to increases in albedo (including the unmasking of underlying snow cover); notably, however, the analysis was based on a single model with a known tendency to overestimate the ERF ( [[#Collins--2011|Collins et al., 2011]] ). [[#Ward--2014|Ward et al. (2014)]] examined the combined effects of biophysical and biogeochemical processes, obtaining an RF of 0.9 ± 0.5 W m <sup>–2</sup> since 1850 that was driven primarily by increases in land-use related GHG emissions from deforestation and agriculture ( [[#Ward--2015|Ward and Mahowald, 2015]] ). According to a large suite of historical simulations, the biophysical effects of changes in land cover (i.e., increased surface albedo and decreased turbulent heat fluxes) led to a net global cooling of 0.10°C ± 0.14°C at the surface (SRCCL). Available model simulations suggest that biophysical and biogeochemical effects jointly may have contributed to a small global warming of 0.078°C ± 0.093°C at the surface over about the past two centuries (SRCCL), with a potentially even larger warming contribution over the Holocene as a whole ( [[#He--2014|He et al., 2014]] ).

In summary, biophysical effects from historical changes in land use have an overall negative ERF ( ''medium confidence'' ). The best-estimate ERF from the increase in global albedo is –0.15 W m <sup>–2</sup> since 1700 and –0.12 W m <sup>–2</sup> since 1850 ( ''medium confidence'' ) (Section 7.3.4.1). Biophysical effects of land-use change ''likely'' resulted in a net global cooling of about 0.1°C since 1750 ( ''medium confidence'' ) (Section 7.3.5.3).

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