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

=== 2.5.4 Non-local and downwind effects resulting from changes in land cover ===

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Changes in land cover or land management do not just have local consequences but also affect adjacent or more remote areas. Those non-local impacts may occur in three different ways.

# Any action on land that affects photosynthesis and respiration has an impact on the atmospheric CO <sub>2</sub> content as this GHG is well mixed in the atmosphere. In turn, this change affects the downwelling longwave radiation everywhere on the planet and contributes to global climate change. This is more thoroughly discussed in Section 2.6 where various land-based mitigation solutions are examined. Local land use changes thus have the potential to affect the global climate via changes in atmospheric CO <sub>2</sub> .
# Any change in land cover or land management may impact on local surface air temperature and moisture, and thus sea-level pressure. Thermal, moisture and surface pressure gradients between the area of change and neighbouring areas are then modified and affect the amount of heat, water vapour and pollutants flowing out (downwind) of the area (e.g., Ma et al. (2013b) <sup>[[#fn:r1317|1317]]</sup> , McLeod et al. (2017) <sup>[[#fn:r1318|1318]]</sup> , Abiodun et al. (2012) <sup>[[#fn:r1319|1319]]</sup> , Keys (2012) <sup>[[#fn:r1320|1320]]</sup> ). Forests, for example, provide water vapour to the atmosphere which supports terrestrial precipitation downwind (Ellison et al. 2017 <sup>[[#fn:r1321|1321]]</sup> ; Layton and Ellison 2016 <sup>[[#fn:r1322|1322]]</sup> ; Spracklen et al. 2012 <sup>[[#fn:r1323|1323]]</sup> , 2018 <sup>[[#fn:r1324|1324]]</sup> ). Within a few days, water vapour can travel several hundred kilometres before being condensed into rain and potentially being transpired again (Makarieva et al. 2009 <sup>[[#fn:r1325|1325]]</sup> ). This cascading moisture recycling (succession of evapotranspiration, water vapour transport and condensation-rainfall) has been observed in South America (Spracklen et al. 2018 <sup>[[#fn:r1326|1326]]</sup> ; Zemp et al. 2014 <sup>[[#fn:r1327|1327]]</sup> ; Staal et al. 2018 <sup>[[#fn:r1328|1328]]</sup> ; Spracklen et al. 2012 <sup>[[#fn:r1329|1329]]</sup> ). Deforestation can thus potentially decrease rainfall downwind, while combining ‘small- scale’ forestation and irrigation, which in the semi-arid region is susceptible to boost the precipitation-recycling mechanism with better vegetation growth downwind (Ellison et al. 2017 <sup>[[#fn:r1330|1330]]</sup> ; Layton and Ellison, 2016 <sup>[[#fn:r1331|1331]]</sup> ) (Figure 2.22).
# Many studies using global climate models have reported that the climatic changes resulting from changes in land are not limited to the lower part of the atmosphere, but can reach the upper levels via changes in large-scale ascent (convection) or descent (subsidence) of air. This coupling to the upper atmosphere triggers perturbations in large-scale atmospheric transport (of heat, energy and water) and subsequent changes in temperature and rainfall in regions located quite far away from the original perturbation (Laguë and Swann 2016 <sup>[[#fn:r1332|1332]]</sup> ; Feddema et al. 2005 <sup>[[#fn:r1333|1333]]</sup> , Badger and Dirmeyer 2016 <sup>[[#fn:r1334|1334]]</sup> ; Garcia 2016 <sup>[[#fn:r1335|1335]]</sup> ; Stark 2015 <sup>[[#fn:r1336|1336]]</sup> ; Devaraju 2018 <sup>[[#fn:r1337|1337]]</sup> ; Quesada et al. 2017a <sup>[[#fn:r1338|1338]]</sup> ) (Figure 2.23).

De Vrese et al. (2016) <sup>[[#fn:r1339|1339]]</sup> for example, using a global climate model, found that irrigation in India could affect regions as remote as eastern

Africa through changes in the atmospheric transport of water vapour. At the onset of boreal spring (February to March) evapotranspiration is already large over irrigated crops and the resulting excess moisture in the atmosphere is transported southwestward by low-level winds. This results in increases in precipitation as large as 1mm d–1 in the Horn of Africa. Such a finding implies that, if irrigation is to decrease in India, rainfall can decrease in eastern Africa where the consequences of drought are already disastrous.

Changes in sea-surface temperature have also been simulated in response to large-scale vegetation changes (Cowling et al. 2009 <sup>[[#fn:r1340|1340]]</sup> ; Davin and de Noblet-Ducoudre 2010 <sup>[[#fn:r1341|1341]]</sup> ; Wang et al. 2014b <sup>[[#fn:r1342|1342]]</sup> , Notaro Liu 2007 <sup>[[#fn:r1343|1343]]</sup> ). Most of those modelling studies have been carried out with land cover changes that are extremely large and often exaggerated with respect to reality. The existence of such teleconnections can thus be biased, as discussed in Lorenz et al. (2016) <sup>[[#fn:r1344|1344]]</sup> .

In conclusion, there is ''high confidence'' that any action on land (for example, to dampen global warming effects), wherever they occur, will not only have effects on local climate but also generate atmospheric changes in neighbouring regions, and potentially as far as hundreds of kilometres downwind. More remote teleconnections, thousands of kilometres away from the initial perturbation, are impossible to observe and have only been reported by modelling studies using extreme land cover changes. There is very ''low confidence'' that detectable changes due to such long-range processes can occur.

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'''Figure 2.22'''

<span id="schematic-illustration-of-how-combined-forestation-and-irrigation-can-influence-downwind-precipitation-on-mountainous-areas-favour-vegetation-growth-and-feed-back-to-the-forested-area-via-increased-runoff.-showing-los-angeles-california-layton-and-ellison-2016.-areas-of-forest-plantations-and-irrigation-are-located-on-the-left-panel-whereas-consequent-downwind-effects-and-feedbacks-are"></span>
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'''Schematic illustration of how combined forestation and irrigation can influence downwind precipitation on mountainous areas, favour vegetation growth and feed back to the forested area via increased runoff. Showing Los Angeles, California (Layton and Ellison 2016). Areas of forest plantations and irrigation are located on the left panel, whereas consequent downwind effects and feedbacks are […]'''
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[[File:ad7e328fa9f1907a1389979ce24bba6e Figure-2.22-1024x724.jpg]]

Schematic illustration of how combined forestation and irrigation can influence downwind precipitation on mountainous areas, favour vegetation growth and feed back to the forested area via increased runoff. Showing Los Angeles, California (Layton and Ellison 2016 <sup>[[#fn:r1345|1345]]</sup> ). Areas of forest plantations and irrigation are located on the left panel, whereas consequent downwind effects and feedbacks are illustrated in the middle and right panels.

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'''Figure 2.23'''

<span id="extra-tropical-effects-on-precipitation-due-to-deforestation-in-each-of-the-three-major-tropical-regions.-increasing-circles-and-decreasing-triangles-precipitation-result-from-complete-deforestation-of-either-amazonia-red-africa-yellow-or-southeast-asia-blue-as-reviewed-by-lawrence-and-vandecar-2015.-boxes-indicate-the-area-where-tropical-forest-was-removed-in-each-region.-numbers"></span>
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'''Extra-tropical effects on precipitation due to deforestation in each of the three major tropical regions. Increasing (circles) and decreasing (triangles) precipitation result from complete deforestation of either Amazonia (red), Africa (yellow) or Southeast Asia (blue) as reviewed by Lawrence and Vandecar (2015). Boxes indicate the area where tropical forest was removed in each region. Numbers […]'''
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[[File:96cdb17f744ee3684efe61e96eecfb37 Figure-2.23-1024x482.png]]

Extra-tropical effects on precipitation due to deforestation in each of the three major tropical regions. Increasing (circles) and decreasing (triangles) precipitation result from complete deforestation of either Amazonia (red), Africa (yellow) or Southeast Asia (blue) as reviewed by Lawrence and Vandecar (2015) <sup>[[#fn:r1346|1346]]</sup> . Boxes indicate the area where tropical forest was removed in each region. Numbers refer to the study the data were derived from (Avissar and Werth 2005 <sup>[[#fn:r1347|1347]]</sup> ; Gedney and Valdes 2000 <sup>[[#fn:r1348|1348]]</sup> ; Semazzi and Song 2001 <sup>[[#fn:r1349|1349]]</sup> ; Werth 2002; Mabuchi et al. 2005 <sup>[[#fn:r1350|1350]]</sup> ; Werth 2005 <sup>[[#fn:r1351|1351]]</sup> )

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