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

==== 2.6.1.2 Land management in forests ====

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The mitigation potential for reducing and/or halting deforestation and degradation ranges from 0.4–5.8 GtCO <sub>2</sub> yr <sup>–1</sup> ( ''high confidence'' ) (Griscom et al. 2017 <sup>[[#fn:r1533|1533]]</sup> ; Hawken 2017 <sup>[[#fn:r1534|1534]]</sup> ; Busch and Engelmann 2017 <sup>[[#fn:r1535|1535]]</sup> ; Baccini et al. 2017 <sup>[[#fn:r1536|1536]]</sup> ; Zarin et al. 2016 <sup>[[#fn:r1537|1537]]</sup> ; Federici et al. 2015 <sup>[[#fn:r1538|1538]]</sup> ; Carter et al. 2015 <sup>[[#fn:r1539|1539]]</sup> ; Houghton et al. 2015 <sup>[[#fn:r1540|1540]]</sup> ; Smith et al. 2013a <sup>[[#fn:r1541|1541]]</sup> ; Houghton and Nassikas 2018 <sup>[[#fn:r1542|1542]]</sup> ). The higher figure represents a complete halting of land use conversion in forests and peatlands (i.e., assuming recent rates of carbon loss are saved each year). Separate estimates of degradation only range from 1.0–2.18 GtCO <sub>2</sub> yr <sup>–1</sup> . Reduced deforestation and forest degradation include conservation of existing carbon pools in vegetation and soil through protection in reserves, controlling disturbances such as fire and pest outbreaks, and changing management practices. Differences in estimates stem from varying land cover definitions, the time periods assessed and the carbon pools included (most higher estimates include belowground, dead wood, litter, soil and peat carbon). When deforestation and degradation are halted, it may take many decades to fully recover the biomass initially present in native ecosystems (Meli et al. 2017 <sup>[[#fn:r1543|1543]]</sup> ) (Section 4.8.3).

Afforestation/reforestation (A/R) and forest restoration can increase carbon sequestration in both vegetation and soils by 0.5–10.1 GtCO <sub>2</sub> yr <sup>–1</sup> ( ''medium confidence'' ) (Fuss et al. 2018 <sup>[[#fn:r1544|1544]]</sup> ; Griscom et al. 2017 <sup>[[#fn:r1545|1545]]</sup> ; Hawken 2017 <sup>[[#fn:r1546|1546]]</sup> ; Kreidenweis et al. 2016 <sup>[[#fn:r1547|1547]]</sup> ; Li et al. 2016 <sup>[[#fn:r1548|1548]]</sup> ; Huang et al. 2017 <sup>[[#fn:r1549|1549]]</sup> ; Sonntag et al. 2016 <sup>[[#fn:r1550|1550]]</sup> ; Lenton 2014 <sup>[[#fn:r1551|1551]]</sup> ; McLaren 2012 <sup>[[#fn:r1552|1552]]</sup> ; Lenton 2010 <sup>[[#fn:r1553|1553]]</sup> ; Erb et al. 2018 <sup>[[#fn:r1554|1554]]</sup> ; Dooley and Kartha 2018 <sup>[[#fn:r1555|1555]]</sup> ; Yan et al. 2017 <sup>[[#fn:r1556|1556]]</sup> ; Houghton et al. 2015 <sup>[[#fn:r1557|1557]]</sup> ; Houghton and Nassikas 2018 <sup>[[#fn:r1558|1558]]</sup> ). Afforestation is the conversion to forest of land that historically has not contained forests. Reforestation is the conversion to forest of land that has previously contained forests but that has been converted to some other use. Forest restoration refers to practices aimed at regaining ecological integrity in a deforested or degraded forest landscape. The lower estimate represents the lowest range from an ESM (Yan et al. 2017 <sup>[[#fn:r1559|1559]]</sup> ) and of sustainable global negative emissions potential (Fuss et al. 2018 <sup>[[#fn:r1560|1560]]</sup> ), and the higher estimate reforests all areas where forests are the native cover type, constrained by food security and biodiversity considerations (Griscom et al. 2017 <sup>[[#fn:r1561|1561]]</sup> ). It takes time for full carbon removal to be achieved as the forest grows. Removal occurs at faster rates in young- to medium-aged forests and declines thereafter such that older forest stands have smaller carbon removals but larger stocks, with net uptake of carbon slowing as forests reach maturity (Yao et al. 2018 <sup>[[#fn:r1562|1562]]</sup> ; Poorter et al. 2016 <sup>[[#fn:r1563|1563]]</sup> ; Tang et al. 2014 <sup>[[#fn:r1564|1564]]</sup> ). The land intensity of afforestation and reforestation has been estimated at 0.0029 km <sup>2</sup> tC <sup>–1</sup> yr <sup>–1</sup> (Smith et al. 2016a <sup>[[#fn:r1565|1565]]</sup> ). Boysen et al. (2017) <sup>[[#fn:r1566|1566]]</sup> estimated that to sequester about 100 GtC by 2100 would require 13 Mkm <sup>2</sup> of abandoned cropland and pastures (Section 4.8.3).

Forest management has the potential to mitigate 0.4–2.1 GtCO <sub>2</sub> -eq yr <sup>–1</sup> ( ''medium confidence'' ) (Sasaki et al. 2016 <sup>[[#fn:r1567|1567]]</sup> ; Griscom et al. 2017 <sup>[[#fn:r1568|1568]]</sup> ; Sasaki et al. 2012 <sup>[[#fn:r1569|1569]]</sup> ). Forest management can alter productivity, turnover rates, harvest rates carbon in soil and carbon in wood products (Erb et al. 2017 <sup>[[#fn:r1570|1570]]</sup> ; Campioli et al. 2015 <sup>[[#fn:r1571|1571]]</sup> ; Birdsey and Pan 2015 <sup>[[#fn:r1572|1572]]</sup> ; Erb et al. 2016 <sup>[[#fn:r1573|1573]]</sup> ; Noormets et al. 2015 <sup>[[#fn:r1574|1574]]</sup> ; Wäldchen et al. 2013 <sup>[[#fn:r1575|1575]]</sup> ; Malhi et al. 2015 <sup>[[#fn:r1576|1576]]</sup> ; Quesada et al. 2018 <sup>[[#fn:r1577|1577]]</sup> ; Nabuurs et al. 2017 <sup>[[#fn:r1578|1578]]</sup> ; Bosello et al. 2009 <sup>[[#fn:r1579|1579]]</sup> ) (Section 4.8.4). Fertilisation may enhance productivity but would increase N <sub>2</sub> O emissions. Preserving and enhancing carbon stocks in forests has immediate climate benefits but the sink can saturate and is vulnerable to future climate change (Seidl et al. 2017 <sup>[[#fn:r1580|1580]]</sup> ). Wood can be harvested and used for bioenergy substituting for fossil fuels (with or without carbon capture and storage) (Section 2.6.1.5), for long- lived products such as timber (see below), to be buried as biochar (Section 2.6.1.1) or for use in the wider bioeconomy, enabling areas of land to be used continuously for mitigation. This leads to initial carbon loss and lower carbon stocks but with each harvest cycle, the carbon loss (debt) can be paid back and after a parity time, result in net savings (Laganière et al. 2017 <sup>[[#fn:r1581|1581]]</sup> ; Bernier and Paré 2013 <sup>[[#fn:r1582|1582]]</sup> ; Mitchell et al. 2012 <sup>[[#fn:r1583|1583]]</sup> ; Haberl et al. 2012 <sup>[[#fn:r1584|1584]]</sup> ; Haberl 2013 <sup>[[#fn:r1585|1585]]</sup> ; Ter-Mikaelian et al. 2015 <sup>[[#fn:r1586|1586]]</sup> ; Macintosh et al. 2015 <sup>[[#fn:r1587|1587]]</sup> ). The trade-off between maximising forest carbon stocks and maximising substitution is highly dependent on the counterfactual assumption (no-use vs extrapolation of current management), initial forest conditions and site-specific contexts (such as regrowth rates and the displacement factors and efficiency of substitution), and relative differences in emissions released during extraction, transport and processing of the biomass- or fossil- based resources, as well as assumptions about emission associated with the product or energy source that is substituted (Grassi et al. 2018b <sup>[[#fn:r1588|1588]]</sup> ; Nabuurs et al. 2017 <sup>[[#fn:r1589|1589]]</sup> ; Pingoud et al. 2018 <sup>[[#fn:r1590|1590]]</sup> ; Smyth et al. 2017a <sup>[[#fn:r1591|1591]]</sup> ; Luyssaert et al. 2018 <sup>[[#fn:r1592|1592]]</sup> ; Valade et al. 2017 <sup>[[#fn:r1593|1593]]</sup> ; York 2012 <sup>[[#fn:r1594|1594]]</sup> ; Ter-Mikaelian et al. 2014 <sup>[[#fn:r1595|1595]]</sup> ; Naudts et al. 2016b <sup>[[#fn:r1596|1596]]</sup> ; Mitchell et al. 2012 <sup>[[#fn:r1597|1597]]</sup> ; Haberl et al. 2012 <sup>[[#fn:r1598|1598]]</sup> ; Macintosh et al. 2015 <sup>[[#fn:r1599|1599]]</sup> ; Laganière et al. 2017 <sup>[[#fn:r1600|1600]]</sup> ; Haberl 2013 <sup>[[#fn:r1601|1601]]</sup> ). This leads to uncertainty about optimum mitigation strategies in managed forests, while high carbon ecosystems such as primary forests would have large initial carbon losses and long pay-back times, and thus protection of stocks would be more optimal (Lemprière et al. 2013 <sup>[[#fn:r1602|1602]]</sup> ; Kurz et al. 2016 <sup>[[#fn:r1603|1603]]</sup> ; Keith et al. 2014 <sup>[[#fn:r1604|1604]]</sup> ) (Section 4.8.4).

Global mitigation potential from increasing the demand of wood products to replace construction materials range from 0.25–1 GtCO <sub>2</sub> -eq yr <sup>–1</sup> ( ''medium confidence'' ) (McLaren 2012 <sup>[[#fn:r1605|1605]]</sup> ; Miner 2010 <sup>[[#fn:r1606|1606]]</sup> ), the uncertainty is determined in part by consideration of the factors described above, and is sensitive to the displacement factor, or the substitution benefit in CO <sub>2</sub> , when wood is used instead of another material, which may vary in the future as other sectors reduce emissions (and may also vary due to market factors) (Sathre and O’Connor 2010 <sup>[[#fn:r1607|1607]]</sup> ; Nabuurs et al. 2018 <sup>[[#fn:r1608|1608]]</sup> ; Iordan et al. 2018 <sup>[[#fn:r1609|1609]]</sup> ; Braun et al. 2016 <sup>[[#fn:r1610|1610]]</sup> ; Gustavsson et al. 2017 <sup>[[#fn:r1611|1611]]</sup> ; Peñaloza et al. 2018 <sup>[[#fn:r1612|1612]]</sup> ; Soimakallio et al. 2016 <sup>[[#fn:r1613|1613]]</sup> ; Grassi et al. 2018b <sup>[[#fn:r1614|1614]]</sup> ). Using harvested carbon in long-lived products (e.g., for construction) can represent a store that can sometimes be from decades to over a century, while the wood can also substitute for intensive building materials, avoiding emissions from the production of concrete and steel (Sathre and O’Connor 2010 <sup>[[#fn:r1615|1615]]</sup> ; Smyth et al. 2017b <sup>[[#fn:r1616|1616]]</sup> ; Nabuurs et al. 2007 <sup>[[#fn:r1617|1617]]</sup> ; Lemprière et al. 2013 <sup>[[#fn:r1618|1618]]</sup> ). The harvest of carbon and storage in products affects the net carbon balance of the forest sector, with the aim of sustainable forest management strategies being to optimise carbon stocks and use harvested products to generate sustained mitigation benefits (Nabuurs et al. 2007 <sup>[[#fn:r1619|1619]]</sup> ).

Biophysical effects of forest response options are variable depending on the location and scale of activity (Section 2.6). Reduced deforestation or afforestation in the tropics contributes to climate mitigation through both biogeochemical and biophysical effects. It also maintains rainfall recycling to some extent. In contrast, in higher latitude boreal areas, observational and modelling studies show that afforestation and reforestation lead to local and global warming effects, particularly in snow covered regions in the winter as the albedo is lower for forests than bare snow (Bathiany et al. 2010 <sup>[[#fn:r1620|1620]]</sup> ; Dass et al. 2013 <sup>[[#fn:r1621|1621]]</sup> ; Devaraju et al. 2018 <sup>[[#fn:r1622|1622]]</sup> ; Ganopolski et al. 2001 <sup>[[#fn:r1623|1623]]</sup> ; Snyder et al. 2004 <sup>[[#fn:r1624|1624]]</sup> ; West et al. 2011 <sup>[[#fn:r1625|1625]]</sup> ; Arora and Montenegro 2011 <sup>[[#fn:r1626|1626]]</sup> ) (Section 2.6). Management, for example, thinning practices in forestry, could increase the albedo in regions where albedo decreases with age. The length of rotation cycles in forestry affects tree height and thus roughness, and through the removal of leaf mass harvest reduces evapotranspiration (Erb et al. 2017 <sup>[[#fn:r1627|1627]]</sup> ), which could lead to increased fire susceptibility in the tropics. In temperate and boreal sites, biophysical forest management effects on surface temperature were shown to be of similar magnitude than changes in land cover (Luyssaert et al. 2014 <sup>[[#fn:r1628|1628]]</sup> ). These biophysical effects could be of a magnitude to overcompensate biogeochemical effects, for example, the sink strength of regrowing forests after past depletions (Luyssaert et al. 2018 <sup>[[#fn:r1629|1629]]</sup> ; Naudts et al. 2016b <sup>[[#fn:r1630|1630]]</sup> ), but many parameters and assumptions on counterfactual influence the account (Anderson et al. 2011 <sup>[[#fn:r1631|1631]]</sup> ; Li et al. 2015b <sup>[[#fn:r1632|1632]]</sup> ; Bright et al. 2015 <sup>[[#fn:r1633|1633]]</sup> ).

Forest cover also affects climate through reactive gases and aerosols, with ''limited evidence and medium agreement'' that the decrease in the emissions of BVOC resulting from the historical conversion of forests to cropland has resulted in a positive radiative forcing through direct and indirect aerosol effects. A negative radiative forcing through reduction in the atmospheric lifetime of CH <sub>4</sub> has increased and decreased ozone concentrations in different regions (Section 2.4).

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