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

==== 2.5.2.2 Impacts of changes in land management ====

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There have been little changes in net cropland area over the past 50 years (at the global scale) compared to continuous changes in land management (Erb et al. 2017 <sup>[[#fn:r1183|1183]]</sup> ). Similarly, in Europe, change in forest management has resulted in a very significant anthropogenic land change. Management affects water, energy and GHG fluxes exchanged between the land and the atmosphere, and thus affects temperature and rainfall, sometimes to the same extent as changes in land cover do (as discussed in Luyssaert et al. (2014) <sup>[[#fn:r1184|1184]]</sup> ).

The effects of irrigation, which is a practice that has been substantially studied, including one attempt to manage solar radiation via increases in cropland albedo (geoengineering the land) are assessed, along with a discussion of recent findings on the effects of forest management on local climate, although there is not enough literature yet on this topic to carry out a thorough assessment. The effects of urbanisation on climate are assessed in a specific cross-chapter box within this chapter (Cross-Chapter Box 4 in this chapter).

There are a number of other practices that exist whose importance for climate mitigation has been examined (some are reported in Section 2.6 and Chapter 6). There is, however, not enough literature available for assessing their biophysical effect on climate. Few papers are generally found per agricultural practice, for example, Jeong et al. (2014b) <sup>[[#fn:r1185|1185]]</sup> for double cropping, Bagley et al. (2017) <sup>[[#fn:r1186|1186]]</sup> for the timing of the growing season and Erb et al. (2017) <sup>[[#fn:r1187|1187]]</sup> for a review of 10 management practices.

Similarly, there are very few studies that have examined how choosing species varieties and harvesting strategies in forest management impacts on climate through biophysical effects, and how those effects compare to the consequences of the chosen strategies on the net CO <sub>2</sub> sink of the managed forest. The modelling studies highlight the existence of competing effects, for example, between the capacity of certain species to store more carbon than others (thus inducing cooling) while at the same time reducing the total evapotranspiration loss and absorbing more solar radiation via lower albedo (thus inducing warming) (Naudts et al. 2016a <sup>[[#fn:r1188|1188]]</sup> ; Luyssaert et al. 2018 <sup>[[#fn:r1189|1189]]</sup> ).

''Irrigation''

There is substantial literature on the effects of irrigation on local, regional and global climate as this is a major land management issue. There is very ''high confidence'' that irrigation increases total evapotranspiration, increases the total amount of water vapour in the atmosphere and decreases mean surface daytime temperature within the irrigated area and during the time of irrigation (Bonfils and Lobell 2007 <sup>[[#fn:r1190|1190]]</sup> ; Alter et al. 2015 <sup>[[#fn:r1191|1191]]</sup> ; Chen and Jeong 2018 <sup>[[#fn:r1192|1192]]</sup> ; Christy et al. 2006 <sup>[[#fn:r1193|1193]]</sup> ; Im and Eltahir 2014 <sup>[[#fn:r1194|1194]]</sup> ; Im et al. 2014 <sup>[[#fn:r1195|1195]]</sup> ; Mueller et al. 2015 <sup>[[#fn:r1196|1196]]</sup> ). Decreases in maximum daytime temperature can locally be as large as –3°C to –8°C (Cook et al. 2015 <sup>[[#fn:r1197|1197]]</sup> ; Han and Yang 2013 <sup>[[#fn:r1198|1198]]</sup> ; Huber et al. 2014 <sup>[[#fn:r1199|1199]]</sup> ; Alter et al. 2015 <sup>[[#fn:r1200|1200]]</sup> ; Im et al. 2014 <sup>[[#fn:r1201|1201]]</sup> ). Estimates of the contribution of irrigation to past historical trends in ambient air temperature vary between –0.07°C and –0.014°C/decade in northern China (Han and Yang 2013 <sup>[[#fn:r1202|1202]]</sup> ; Chen and Jeong 2018 <sup>[[#fn:r1203|1203]]</sup> ) while being quite larger in California, USA (–0.14°C to –0.25°C/decade) (Bonfils and Lobell 2007 <sup>[[#fn:r1204|1204]]</sup> ). Surface cooling results from increased energy being taken up from the land via larger evapotranspiration rates. In addition, there is growing evidence from modelling studies that such cooling can locally mitigate the effect of heatwaves (Thiery et al. 2017 <sup>[[#fn:r1205|1205]]</sup> ; Mueller et al. 2015 <sup>[[#fn:r1206|1206]]</sup> ).

There is ''no agreement'' on changes in night-time temperatures, as discussed in Chen and Jeong (2018) <sup>[[#fn:r1207|1207]]</sup> who summarised the findings from observations in many regions of the world (India, China, North America and eastern Africa) (Figure 2.18). Where night-time warming is found (Chen and Jeong 2018 <sup>[[#fn:r1208|1208]]</sup> ; Christy et al. 2006 <sup>[[#fn:r1209|1209]]</sup> ), two explanations are put forward, (i) an increase in incoming longwave radiation in response to increased atmospheric water vapour content (greenhouse effect), and (ii) an increased storage of heat in the soil during daytime. Because of the larger heat capacity of moister soil, heat is then released to the atmosphere at night.

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<span id="figure-2.18"></span>
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'''Figure 2.18'''

<span id="global-map-of-areas-equipped-for-irrigation-colours-expressed-as-a-percentage-of-total-area-or-irrigation-fraction.-source-siebert-et-al.-2013.-numbered-boxes-show-regions-where-irrigation-causes-cooling-down-arrow-of-surface-mean-tmean-maximum-tmax-or-minimum-tmin-temperature-or-else-no-significant-effect-right-arrow-or-where-the-effect-is"></span>
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'''Global map of areas equipped for irrigation (colours), expressed as a percentage of total area, or irrigation fraction. Source: Siebert et al. (2013). Numbered boxes show regions where irrigation causes cooling (down arrow) of surface mean (Tmean), maximum (Tmax) or minimum (Tmin) temperature, or else no significant effect (right arrow) or where the effect is […]'''
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[[File:8a2d4154a588ac50461beb56b94e25c9 Figure-2.18-724x1024.jpg]]

Global map of areas equipped for irrigation (colours), expressed as a percentage of total area, or irrigation fraction. Source: Siebert et al. (2013) <sup>[[#fn:r1210|1210]]</sup> . Numbered boxes show regions where irrigation causes cooling (down arrow) of surface mean (Tmean), maximum (Tmax) or minimum (Tmin) temperature, or else no significant effect (right arrow) or where the effect is uncertain (question mark), based on observational studies as reviewed in Chen and Jeong (2018) <sup>[[#fn:r1211|1211]]</sup> . Tmax refers to the warmest daily temperature while Tmin to the coldest one, which generally occurs at night (Alter et al. 2015 <sup>[[#fn:r1212|1212]]</sup> ; Han and Yang 2013 <sup>[[#fn:r1213|1213]]</sup> ; Roy et al. 2007 <sup>[[#fn:r1214|1214]]</sup> ; Shi et al. 2013 <sup>[[#fn:r1215|1215]]</sup> ; Bonfils and Lobell 2007 <sup>[[#fn:r1216|1216]]</sup> ; Lobell et al. 2008 <sup>[[#fn:r1217|1217]]</sup> ; Lobell and Bonfils 2008 <sup>[[#fn:r1218|1218]]</sup> ; Christy et al. 2006 <sup>[[#fn:r1219|1219]]</sup> ; Mahmood et al. 2006 <sup>[[#fn:r1220|1220]]</sup> ; Mueller et al. 2015 <sup>[[#fn:r1221|1221]]</sup> ).

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There is ''robust evidence'' from modelling studies that implementing irrigation enhances rainfall, although there is very ''low confidence'' on where this increase occurs. When irrigation occurs in Sahelian Africa during the monsoon period, rainfall is decreased over irrigated areas ( ''high agreement'' ), increased in the southwest if the crops are located in western Africa (Alter et al. 2015 <sup>[[#fn:r1222|1222]]</sup> ) and increased in the east/northeast when crops are located further east in Sudan (Im and Eltahir 2014 <sup>[[#fn:r1223|1223]]</sup> ; Im et al. 2014 <sup>[[#fn:r1224|1224]]</sup> ) The cooler irrigated surfaces in the Sahel, because of their greater evapotranspiration, inhibit convection and create an anomalous descending motion over crops that suppresses rainfall but influences the circulation of monsoon winds. Irrigation in India occurs prior to the start of the monsoon season and the resulting land cooling decreases the land-sea temperature contrast. This can delay the onset of the Indian monsoon and decrease its intensity (Niyogi et al. 2010 <sup>[[#fn:r1225|1225]]</sup> ; Guimberteau et al. 2012 <sup>[[#fn:r1226|1226]]</sup> ). Results from a modelling study by De Vrese et al. (2016) <sup>[[#fn:r1227|1227]]</sup> suggest that part of the excess rainfall triggered by Indian irrigation falls westward, in the horn of Africa. The theory behind those local and downwind changes in rainfall support the findings from the models, but we do not yet have sufficient literature to robustly assess the magnitude and exact location of the expected changes driven by irrigation.

''Cropland albedo''

Various methods have been proposed to increase surface albedo in cropland and thus reduce local surface temperature ( ''high confidence'' ): choose ‘brighter’ crop varieties (Ridgwell et al. 2009 <sup>[[#fn:r1228|1228]]</sup> ; Crook et al. 2015 <sup>[[#fn:r1229|1229]]</sup> ; Hirsch et al. 2017 <sup>[[#fn:r1230|1230]]</sup> ; Singarayer et al. 2009 <sup>[[#fn:r1231|1231]]</sup> ; Singarayer and Davies-Barnard 2012 <sup>[[#fn:r1232|1232]]</sup> ), abandon tillage (Lobell et al. 2006 <sup>[[#fn:r1233|1233]]</sup> ; Davin et al. 2014 <sup>[[#fn:r1234|1234]]</sup> ), include cover crops into rotation in areas where soils are darker than vegetation (Carrer et al. 2018 <sup>[[#fn:r1235|1235]]</sup> ; Kaye and Quemada 2017 <sup>[[#fn:r1236|1236]]</sup> ) or use greenhouses (as in Campra et al. (2008) <sup>[[#fn:r1237|1237]]</sup> ). See Seneviratne et al. (2018) <sup>[[#fn:r1238|1238]]</sup> for a review.

Whatever the solution chosen, the induced reduction in absorbed solar radiation cools the land – more specifically during the hottest summer days ( ''low confidence'' ) (Davin et al. 2014 <sup>[[#fn:r1239|1239]]</sup> ; Wilhelm et al. 2015 <sup>[[#fn:r1240|1240]]</sup> ; Figure 2.19). Changes in temperature are essentially local and seasonal (limited to crop growth season) or sub-seasonal (when resulting from inclusion of cover crop or tillage suppression). Such management action on incoming solar radiation thus holds the potential to counteract warming in cultivated areas during crop growing season.

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'''Figure 2.19'''

<span id="change-in-summer-julyaugust-daily-maximum-temperature-ºc-resulting-from-increased-surface-albedo-in-unploughed-versus-ploughed-land-in-a-southern-and-b-northern-europe-during-the-period-19862009.-changes-are-simulated-for-different-quantiles-of-the-daily-maximum-temperature-distribution-where-q1-represents-the-coolest-1-and-q99-the-warmest-1-of-summer-days."></span>
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'''Change in summer (July–August) daily maximum temperature (ºC) resulting from increased surface albedo in unploughed versus ploughed land, in (A) southern, and (B) northern Europe, during the period 1986–2009. Changes are simulated for different quantiles of the daily maximum temperature distribution, where Q1 represents the coolest 1% and Q99 the warmest 1% of summer days. […]'''
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[[File:08d354f66f961d81b064ddf459156770 Figure-2.19-1024x398.jpg]]

Change in summer (July–August) daily maximum temperature (ºC) resulting from increased surface albedo in unploughed versus ploughed land, in (A) southern, and (B) northern Europe, during the period 1986–2009. Changes are simulated for different quantiles of the daily maximum temperature distribution, where Q1 represents the coolest 1% and Q99 the warmest 1% of summer days. Only grid cells with more than 60% of their area in cropland are included. The dashed bars represent the standard deviation calculated across all days and grid points. SE refers to southern Europe (below 45ºN) and NE to northern Europe (above 45ºN). (Davin et al., 2014)

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Introducing cover crops into a rotation can also have a warming effect in areas where vegetation has a darker albedo than soil, or in winter during snow periods if the cover crops or their residues are tall enough to overtop the snow cover (Kaye and Quemada 2017 <sup>[[#fn:r1241|1241]]</sup> ; Lombardozzi et al. 2018 <sup>[[#fn:r1242|1242]]</sup> ). In addition, evapotranspiration greater than that of bare soil during this transitional period reduces soil temperature (Ceschia et al. 2017 <sup>[[#fn:r1243|1243]]</sup> ). Such management strategy can have another substantial mitigation effect as it allows carbon to be stored in the soil and to reduce both direct and indirect N <sub>2</sub> O emissions (Basche et al. 2014 <sup>[[#fn:r1244|1244]]</sup> ; Kaye and Quemada 2017 <sup>[[#fn:r1245|1245]]</sup> ), in particular if fertilisation of the subsequent crop is reduced (Constantin et al. 2010 <sup>[[#fn:r1246|1246]]</sup> , 2011 <sup>[[#fn:r1247|1247]]</sup> ). The use of cover crops thus substantially improves the GHG budget of croplands (Kaye and Quemada 2017 <sup>[[#fn:r1248|1248]]</sup> ; Tribouillois et al. 2018 <sup>[[#fn:r1249|1249]]</sup> ). More discussion on the role of management practices for mitigation can be found in Section 2.6 and Chapter 6.

Only a handful of modelling studies have looked at effects other than changes in atmospheric temperature in response to increased cropland albedo. Seneviratne et al. (2018) <sup>[[#fn:r1250|1250]]</sup> have found significant changes in rainfall following an idealised increase in cropland albedo, especially within the Asian monsoon regions. The benefits of cooler temperature on production, resulting from increased albedo, is cancelled out by decreases in rainfall that are harmful for crop productivity. The rarity of a concomitant evaluation of albedo management impact on crop productivity prevents us from providing a robust assessment of this practice in terms of both climate mitigation and food security.

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