Editing IPCC:AR6/SRCCL/Chapter-4 (section)

=== 4.6.2 Physical impacts ===

<div id="section-4-6-2-physical-impacts-block-1"></div>

Among the physical effects of land degradation, surface albedo changes are those with the most evident impact on the net global radiative balance and net climate warming/cooling. Degradation processes affecting wild and semi-natural ecosystems, such as fire regime changes, woody encroachment, logging and overgrazing, can trigger strong albedo changes before significant biogeochemical shifts take place. In most cases these two types of effects have opposite signs in terms of net radiative forcing, making their joint assessment critical for understanding climate feedbacks (Bright et al. 2015 <sup>[[#fn:r821|821]]</sup> ).

In the case of forest degradation or deforestation, the albedo impacts are highly dependent on the latitudinal/climatic belt to which they belong. In boreal forests, the removal or degradation of the tree cover increases albedo (net cooling effect) ( ''medium evidence, high agreement'' ) as the reflective snow cover becomes exposed, which can exceed the net radiative effect of the associated carbon release to the atmosphere (Davin et al. 2010 <sup>[[#fn:r822|822]]</sup> ; Pinty et al. 2011 <sup>[[#fn:r823|823]]</sup> ). On the other hand, progressive greening of boreal and temperate forests has contributed to net albedo declines ( ''medium agreement, medium evidence'' ) (Planque et al. 2017 <sup>[[#fn:r824|824]]</sup> ; Li et al. 2018a <sup>[[#fn:r825|825]]</sup> ). In the northern treeless vegetation belt (tundra), shrub encroachment leads to the opposite effect as the emergence of plant structures above the snow cover level reduce winter-time albedo (Sturm 2005 <sup>[[#fn:r826|826]]</sup> ).

The extent to which albedo shifts can compensate for carbon storage shifts at the global level has not been estimated. A significant but partial compensation takes place in temperate and subtropical dry ecosystems in which radiation levels are higher and carbon stocks smaller compared to their more humid counterparts ( ''medium agreement, medium evidence'' ). In cleared dry woodlands, half of the net global warming effect of net carbon release has been compensated by albedo increase (Houspanossian et al. 2013 <sup>[[#fn:r827|827]]</sup> ), whereas in afforested dry rangelands, albedo declines cancelled one-fifth of the net carbon sequestration (Rotenberg and Yakir 2010 <sup>[[#fn:r828|828]]</sup> ). Other important cases in which albedo effects impose a partial compensation of carbon exchanges are the vegetation shifts associated with wildfires, as shown for the savannahs, shrublands and grasslands of Sub-Saharan Africa (Dintwe et al. 2017 <sup>[[#fn:r829|829]]</sup> ). Besides the net global effects discussed above, albedo shifts can play a significant role in local climate ( ''high agreement, medium evidence'' ), as exemplified by the effect of no-till agriculture reducing local heat extremes in European landscapes (Davin et al. 2014 <sup>[[#fn:r830|830]]</sup> ) and the effects of woody encroachment causing precipitation rises in the North American Great Plains (Ge and Zou 2013 <sup>[[#fn:r831|831]]</sup> ). Modelling efforts that integrate ground data from deforested areas worldwide accounting for both physical and biogeochemical effects, indicate that massive global deforestation would have a net warming impact (Lawrence and Vandecar 2015 <sup>[[#fn:r832|832]]</sup> ) at both local and global levels with highlight non-linear effects of forest loss on climate variables.

Beyond the albedo effects presented above, other physical impacts of land degradation on the atmosphere can contribute to global and regional climate change. Of particular relevance, globally and continentally, are the net cooling effects of dust emissions ( ''low agreement, medium evidence'' ) (Lau and Kim (2007) <sup>[[#fn:r833|833]]</sup> , but see also Huang et al. (2014) <sup>[[#fn:r834|834]]</sup> ). Anthropogenic emission of mineral particles from degrading land appear to have a similar radiative impact than all other anthropogenic aerosols (Sokolik and Toon 1996 <sup>[[#fn:r835|835]]</sup> ). Dust emissions may explain regional climate anomalies through reinforcing feedbacks, as suggested for the amplification of the intensity, extent and duration of the low precipitation anomaly of the North American Dust Bowl in the 1930s (Cook et al. 2009 <sup>[[#fn:r836|836]]</sup> ). Another source of physical effects on climate are surface roughness changes which, by affecting atmospheric drag, can alter cloud formation and precipitation (low agreement, low evidence), as suggested by modelling studies showing how the massive deployment of solar panels in the Sahara could increase rainfall in the Sahel (Li et al. 2018c <sup>[[#fn:r837|837]]</sup> ), or how woody encroachment in the Arctic tundra could reduce cloudiness and raise temperature (Cho et al. 2018 <sup>[[#fn:r838|838]]</sup> ). The complex physical effects of deforestation, as explored through modelling, converge into general net regional precipitation declines, tropical temperature increases and boreal temperature declines, while net global effects are less certain (Perugini et al. 2017 <sup>[[#fn:r839|839]]</sup> ). Integrating all the physical effects of land degradation and its recovery or reversal is still a challenge, yet modelling attempts suggest that, over the last three decades, the slow but persistent net global greening caused by the average increase of leaf area in the land has caused a net cooling of the Earth, mainly through the rise in evapotranspiration (Zeng et al. 2017 <sup>[[#fn:r840|840]]</sup> ) ( ''low confidence'' ).

<span id="impacts-of-climate-related-land-degradation-on-poverty-and-livelihoods"></span>