Editing IPCC:AR6/SRCCL/Chapter-1 (section)

==== 1.1.2.1 1.1.2.1 Land ecosystems and climate change ====

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Land ecosystems play a key role in the climate system, due to their large carbon pools and carbon exchange fluxes with the atmosphere (Ciais et al. 2013b <sup>[[#fn:r60|60]]</sup> ). Land use, the total of arrangements, activities and inputs applied to a parcel of land (such as agriculture, grazing, timber extraction, conservation or city dwelling; see Glossary), and land management (sum of land-use practices that take place within broader land-use categories; see Glossary) considerably alter terrestrial ecosystems and play a key role in the global climate system. An estimated one-quarter of total anthropogenic GHG emissions arise mainly from deforestation, ruminant livestock and fertiliser application (Smith et al. 2014 <sup>[[#fn:r61|61]]</sup> ; Tubiello et al. 2015 <sup>[[#fn:r62|62]]</sup> ; Le Quere et al. 2018 <sup>[[#fn:r63|63]]</sup> ; Ciais et al. 2013a <sup>[[#fn:r64|64]]</sup> ), and especially methane (CH <sub>4</sub> ) and nitrous oxide (N <sub>2</sub> O) emissions from agriculture have been rapidly increasing over the last decades (Hoesly et al. 2018 <sup>[[#fn:r65|65]]</sup> ; Tian et al. 2019 <sup>[[#fn:r66|66]]</sup> ) (Figure 1.1 and Sections 2.3.2–2.3.3).

Globally, land also serves as a large CO <sub>2</sub> sink, which was estimated for the period 2008–2017 to be nearly 30% of total anthropogenic emissions (Le Quere et al. 2015 <sup>[[#fn:r67|67]]</sup> ; Canadell and Schulze 2014 <sup>[[#fn:r68|68]]</sup> ; Ciais et al. 2013a <sup>[[#fn:r69|69]]</sup> ; Zhu et al. 2016 <sup>[[#fn:r70|70]]</sup> ) (Section 2.3.1). This sink has been attributed to increasing atmospheric CO <sub>2</sub> concentration, a prolonged growing season in cool environments, or forest regrowth (Le Quéré et al. 2013 <sup>[[#fn:r71|71]]</sup> ; Pugh et al. 2019 <sup>[[#fn:r72|72]]</sup> ; Le Quéré et al. 2018 <sup>[[#fn:r73|73]]</sup> ; Ciais et al. 2013a <sup>[[#fn:r74|74]]</sup> ; Zhu et al. 2016 <sup>[[#fn:r75|75]]</sup> ). Whether or not this sink will persist into the future is one of the largest uncertainties in carbon cycle and climate modelling (Ciais et al. 2013a <sup>[[#fn:r76|76]]</sup> ; Bloom et al. 2016 <sup>[[#fn:r77|77]]</sup> ; Friend et al. 2014 <sup>[[#fn:r78|78]]</sup> ; Le Quere et al. 2018 <sup>[[#fn:r79|79]]</sup> ). In addition, changes in vegetation cover caused by land use (such as conversion of forest to cropland or grassland, and vice versa) can result in regional cooling or warming through altered energy and momentum transfer between ecosystems and the atmosphere. Regional impacts can be substantial, but whether the effect leads to warming or cooling depends on the local context (Lee et al. 2011 <sup>[[#fn:r80|80]]</sup> ; Zhang et al. 2014 <sup>[[#fn:r81|81]]</sup> ; Alkama and Cescatti 2016 <sup>[[#fn:r82|82]]</sup> ) (Section 2.6). Due to the current magnitude of GHG emissions and CO <sub>2</sub> carbon dioxide removal in land ecosystems, there is ''high confidence'' that GHG reduction measures in agriculture, livestock management and forestry would have substantial climate change mitigation potential, with co-benefits for biodiversity and ecosystem services (Smith and Gregory 2013 <sup>[[#fn:r84|84]]</sup> ; Smith et al. 2014 <sup>[[#fn:r85|85]]</sup> ; Griscom et al. 2017 <sup>[[#fn:r86|86]]</sup> ) (Sections 2.6 and 6.3).

The mean temperature over land for the period 2006–2015 was 1.53°C higher than for the period 1850–1900, and 0.66°C larger than the equivalent global mean temperature change (Section 2.2). Climate change affects land ecosystems in various ways (Section 7.2). Growing seasons and natural biome boundaries shift in response to warming or changes in precipitation (Gonzalez et al. 2010 <sup>[[#fn:r87|87]]</sup> ; Wärlind et al. 2014 <sup>[[#fn:r88|88]]</sup> ; Davies-Barnard et al. 2015 <sup>[[#fn:r89|89]]</sup> ; Nakamura et al. 2017 <sup>[[#fn:r90|90]]</sup> ). Atmospheric CO <sub>2</sub> increases have been attributed to underlie, at least partially, observed woody plant cover increase in grasslands and savannahs (Donohue et al. 2013 <sup>[[#fn:r91|91]]</sup> ). Climate change-induced shifts in habitats, together with warmer temperatures, cause pressure on plants and animals (Pimm et al. 2014 <sup>[[#fn:r92|92]]</sup> ; Urban et al. 2016 <sup>[[#fn:r93|93]]</sup> ). National cereal crop losses of nearly 10% have been estimated for the period 1964–2007 as a consequence of heat and drought weather extremes (Deryng et al. 2014 <sup>[[#fn:r94|94]]</sup> ; Lesk et al. 2016 <sup>[[#fn:r95|95]]</sup> ). Climate change is expected to reduce yields in areas that are already under heat and water stress (Schlenker and Lobell 2010 <sup>[[#fn:r96|96]]</sup> ; Lobell et al. 2011 <sup>[[#fn:r97|97]]</sup> , 2012 <sup>[[#fn:r98|98]]</sup> ; Challinor et al. 2014 <sup>[[#fn:r99|99]]</sup> ) (Section 5.2.2). At the same time, warmer temperatures can increase productivity in cooler regions (Moore and Lobell 2015 <sup>[[#fn:r100|100]]</sup> ) and might open opportunities for crop area expansion, but any overall benefits might be counterbalanced by reduced suitability in warmer regions (Pugh et al. 2016 <sup>[[#fn:r101|101]]</sup> ; Di Paola et al. 2018 <sup>[[#fn:r102|102]]</sup> ). Increasing atmospheric CO <sub>2</sub> is expected to increase productivity and water use efficiency in crops and in forests (Muller et al. 2015 <sup>[[#fn:r103|103]]</sup> ; Nakamura et al. 2017 <sup>[[#fn:r104|104]]</sup> ; Kimball 2016 <sup>[[#fn:r105|105]]</sup> ). The increasing number of extreme weather events linked to climate change is also expected to result in forest losses; heat waves and droughts foster wildfires (Seidl et al. 2017 <sup>[[#fn:r106|106]]</sup> ; Fasullo et al. 2018 <sup>[[#fn:r107|107]]</sup> ) (Cross-Chapter Box 3 in Chapter 2). Episodes of observed enhanced tree mortality across many world regions have been attributed to heat and drought stress (Allen et al. 2010 <sup>[[#fn:r108|108]]</sup> ; Anderegg et al. 2012 <sup>[[#fn:r109|109]]</sup> ), whilst weather extremes also impact local infrastructure and hence transportation and trade in land-related goods (Schweikert et al. 2014 <sup>[[#fn:r110|110]]</sup> ; Chappin and van der Lei 2014 <sup>[[#fn:r111|111]]</sup> ). Thus, adaptation is a key challenge to reduce adverse impacts on land systems (Section 1.3.6).

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