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

=== 2.5.3 Amplifying/dampening climate changes via land responses ===

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Section 2.1 and Box 2.1 illustrate the various ways through which land can affect the atmosphere and thereby climate and weather. Section 2.2 illustrates the many impacts that climate changes have on the functioning of land ecosystems. Section 2.3 discusses the effects that future climatic conditions have on the capacity of the land to absorb anthropogenic CO <sub>2</sub> , which then controls the sign of the feedback to the initial global warming. Sections 2.5.1 and 2.5.2 show the effects of changes in anthropogenic land cover or land management on climate variables or processes. Therefore, land has the potential to dampen or amplify the GHG-induced global climate warming, or can be used as a tool to mitigate regional climatic consequences of global warming such as extreme weather events, in addition to increasing the capacity of land to absorb CO <sub>2</sub> (Figure 2.20).

Land-to-climate feedbacks are difficult to assess with global or regional climate models, as both types of models generally omit a large number of processes. Among these are (i) the response of vegetation to climate change in terms of growth, productivity, and geographical distribution, (ii) the dynamics of major disturbances such as fires, (iii) the nutrients dynamics, and (iv) the dynamics and effects of short-lived chemical tracers such as biogenic volatile organic compounds (Section 2.4). Therefore, only those processes that are fully accounted for in climate models are considered here.

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'''Figure 2.20'''

<span id="schematics-of-the-various-ways-land-has-been-shown-in-the-literature-to-either-amplify-or-dampen-the-initial-ghg-induced-climatic-change.-brown-arrows-and-boxes-represent-the-global-scale-and-blue-arrows-and-boxes-represent-the-regionallocal-level.-grey-arrows-and-boxes-refer-to-what-we-consider-herein-as-imposed-changes-that-is"></span>
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'''Schematics of the various ways land has been shown in the literature to either amplify or dampen the initial GHG-induced climatic change. Brown arrows and boxes represent the global scale and blue arrows and boxes represent the regional/local level. Grey arrows and boxes refer to what we consider herein as imposed changes – that is, […]'''
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[[File:2ef84d76969a00271ecd285a3ded3639 Figure-2.20-1024x507.jpg]]

Schematics of the various ways land has been shown in the literature to either amplify or dampen the initial GHG-induced climatic change. Brown arrows and boxes represent the global scale and blue arrows and boxes represent the regional/local level. Grey arrows and boxes refer to what we consider herein as imposed changes – that is, the initial atmospheric GHG content as well as anthropogenic land cover change and land management. Dampening feedbacks are represented with dashed lines, amplifying ones with solid lines and feedbacks where the direction may be variable are represented using dotted lines. The feedbacks initiated by changes in snow and permafrost areas in boreal regions are discussed in Section 2.5.3.2, the ones initiated by changes in ecosystem distribution are discussed in Sections 2.5.3.1, 2.5.1 and 2.5.2, and the feedbacks related to changes in the land functioning are discussed in Sections 2.5.3.3 and 2.5.1, as well as in Sections 2.3and 2.5 (for changes in net CO <sub>2</sub> fluxes). References supporting this figure can be found in each of those sections.

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==== 2.5.3.1 Effects of changes in land cover and productivity resulting from global warming ====

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In boreal regions, the combined northward migration of the treeline and increased growing season length in response to increased temperatures in those regions (Section 2.2) will have positive feedbacks both on global and regional annual warming ( ''high confidence'' ) (Garnaud and Sushama 2015 <sup>[[#fn:r1251|1251]]</sup> ; Jeong et al. 2014a <sup>[[#fn:r1252|1252]]</sup> ; O’ishi and Abe-Ouchi 2009 <sup>[[#fn:r1253|1253]]</sup> ; Port et al. 2012 <sup>[[#fn:r1254|1254]]</sup> ; Strengers et al. 2010 <sup>[[#fn:r1255|1255]]</sup> ). The warming resulting from the decreased surface albedo remains the dominant signal in all modelling studies at the annual timescale and during the snow season, while cooling is obtained during the growing season (Section 2.5.2.1 and Figure 2.21, right panel).

In the tropics, climate change will cause both greening and browning (Section 2.2). Where global warming provokes a decrease in rainfall, the induced decrease in biomass production leads to increased local warming ( ''high confidence'' ) (Port et al. 2012 <sup>[[#fn:r1256|1256]]</sup> ; Wu et al. 2016 <sup>[[#fn:r1257|1257]]</sup> ; Yu et al. 2016 <sup>[[#fn:r1258|1258]]</sup> ). The reverse is true where warming generates increases in rainfall and thus greening. As an example, Port e tal. (2012) <sup>[[#fn:r1259|1259]]</sup> simulated decreases in tree cover and shortened growing season in the Amazon, despite the CO <sub>2</sub> fertilisation effects, in response to both future tropical warming and reduced precipitation (Figure 2.21, left panel). This browning of the land decreases both evapotranspiration and atmospheric humidity. The warming driven by the drop in evapotranspiration is enhanced via a decrease in cloudiness, increasing solar radiation, and is dampened by reduced water vapour greenhouse radiation.

There is ''very low confidence'' on how feedbacks affect rainfall in the tropics where vegetation changes may occur, as the sign of the change in precipitation depends on where the greening occurs and on the season (as discussed in Section 2.5.2). There is, however, ''high confidence'' that increased vegetation growth in the southern Sahel increases African monsoon rains (Yu et al. 2016 <sup>[[#fn:r1260|1260]]</sup> ; Port et al. 2012 <sup>[[#fn:r1261|1261]]</sup> ; Wu et al. 2016 <sup>[[#fn:r1263|1263]]</sup> ). Confidence on the direction of such feedbacks is also based on a significant number of paleoclimate studies that analysed how vegetation dynamics helped maintain a northward position of the African monsoon during the Holocene time period (9–6 kyr BP) (de Noblet-Ducoudré et al. 2000 <sup>[[#fn:r1264|1264]]</sup> ; Rachmayani et al. 2015 <sup>[[#fn:r1265|1265]]</sup> ).

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'''Figure 2.21'''

<span id="schematic-illustration-of-the-processes-through-which-the-effects-of-global-warming-in-a-the-amazon-blue-arrows-and-boxes-and-b-boreal-regions-grey-arrows-and-boxes-feedback-on-the-regional-climate-change.-in-boreal-regions-the-sign-of-the-feedbacks-depends-on-the-season-although-annually-global-warming-is-further-enhanced-in-those"></span>
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'''Schematic illustration of the processes through which the effects of global warming in (a) the Amazon (blue arrows and boxes), and (b) boreal regions (grey arrows and boxes) feedback on the regional climate change. In boreal regions, the sign of the feedbacks depends on the season, although annually global warming is further enhanced in those […]'''
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[[File:d22ae8a9968926035f16a50fcbc1f7d9 Figure-2.21-1024x691.jpg]]

Schematic illustration of the processes through which the effects of global warming in (a) the Amazon (blue arrows and boxes), and (b) boreal regions (grey arrows and boxes) feedback on the regional climate change. In boreal regions, the sign of the feedbacks depends on the season, although annually global warming is further enhanced in those regions. Dashed lines illustrate negative feedbacks, while solid lines indicate positive feedbacks. References supporting this figure can be found in the text.

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==== 2.5.3.2 Feedbacks to climate from high-latitude land-surface changes ====

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In high latitudes, snow albedo and permafrost carbon feedbacks are the most well-known and most important surface-related climate feedbacks because of their large-scale impacts.

In response to ongoing and projected decrease in seasonal snow cover (Derksen and Brown 2012 <sup>[[#fn:r1266|1266]]</sup> ; Brutel-Vuilmet et al. 2013 <sup>[[#fn:r1267|1267]]</sup> ) warming is, and will continue to be, enhanced in boreal regions ( ''high confidence'' ) (Brutel-Vuilmet et al. 2013 <sup>[[#fn:r1268|1268]]</sup> ; Perket et al. 2014 <sup>[[#fn:r1269|1269]]</sup> ; Thackeray and Fletcher 2015 <sup>[[#fn:r1270|1270]]</sup> ; Mudryk et al. 2017 <sup>[[#fn:r1271|1271]]</sup> ). One reason for this is the large reflectivity (albedo) the snow exerts on shortwave radiative forcing: the all- sky global land snow shortwave radiative effect is evaluated to be around –2.5 ± 0.5 W m <sup>–2</sup> (Flanner et al. 2011 <sup>[[#fn:r1272|1272]]</sup> ; Singh et al. 2015 <sup>[[#fn:r1273|1273]]</sup> ). In the southern hemisphere, perennial snow on the Antarctic is the dominant contribution, while in the northern hemisphere, this is essentially attributable to seasonal snow, with a smaller contribution  from snow on glaciated areas. Another reason is the sensitivity of snow cover to temperature: Mudryk et al. (2017) <sup>[[#fn:r1274|1274]]</sup> recently showed that, in the high latitudes, climate models tend to correctly represent this sensitivity, while in mid-latitude and alpine regions, the simulated snow cover sensitivity to temperature variations tends to be biased low. In total, the global snow albedo feedback is about 0.1 W m <sup>–2</sup> K <sup>–1</sup> , which amounts to about 7% of the strength of the globally dominant water vapour feedback (e.g., Thackeray and Fletcher (2015) <sup>[[#fn:r1275|1275]]</sup> . While climate models do represent this feedback, a persistent spread in the modelled feedback strength has been noticed (Qu and Hall 2014 <sup>[[#fn:r1276|1276]]</sup> ) and, on average, the simulated snow albedo feedback strength tends to be somewhat weaker than in reality ( ''medium confidence'' ) (Flanner et al. 2011 <sup>[[#fn:r1277|1277]]</sup> ; Thackeray and Fletcher 2015 <sup>[[#fn:r1278|1278]]</sup> ). Various reasons for the spread and biases of the simulated snow albedo feedback have been identified, notably inadequate representations of vegetation masking snow in forested areas (Loranty et al. 2014 <sup>[[#fn:r1279|1279]]</sup> ; Wang et al. 2016c <sup>[[#fn:r1280|1280]]</sup> ; Thackeray and Fletcher 2015 <sup>[[#fn:r1281|1281]]</sup> ).

The second most important potential feedback from land to climate relates to permafrost decay. There is ''high confidence'' that, following permafrost decay from a warming climate, the resulting emissions of CO <sub>2</sub> and/or CH <sub>4</sub> (caused by the decomposition of organic matter in previously frozen soil) will produce additional GHG- induced warming. There is, however, substantial uncertainty on the magnitude of this feedback, although recent years have seen large progress in its quantification. Lack of agreement results from several critical factors that carry large uncertainties. The most important are (i) the size of the permafrost carbon pool, (ii) its decomposability, (iii) the magnitude, timing and pathway of future high-latitude climate change, and (iv) the correct identification and model representation of the processes at play (Schuur et al. 2015 <sup>[[#fn:r1282|1282]]</sup> ). The most recent comprehensive estimates establish a total soil organic carbon storage in permafrost of about 1500 ± 200 PgC (Hugelius et al. 2014 <sup>[[#fn:r1283|1283]]</sup> , 2013 <sup>[[#fn:r1284|1284]]</sup> ; Olefeldt et al. 2016 <sup>[[#fn:r1285|1285]]</sup> ), which is about 300 Pg C lower than previous estimates ( ''low confidence'' ). Important progress has been made in recent years at incorporating permafrost-related processes in complex ESMs (e.g., McGuire et al. (2018) <sup>[[#fn:r1286|1286]]</sup> ), but representations of some critical processes such as thermokarst formation are still in their infancy (Schuur et al. 2015) <sup>[[#fn:r1287|1287]]</sup> . Recent model-based estimates of future permafrost carbon release (Koven et al. 2015 <sup>[[#fn:r1288|1288]]</sup> ; McGuire et al. 2018 <sup>[[#fn:r1289|1289]]</sup> ) have converged on an important insight. Their results suggest that substantial net carbon release of the coupled vegetation-permafrost system will probably not occur before about 2100 because carbon uptake by increased vegetation growth will initially compensate for GHG releases from permafrost ( ''limited evidence, high agreement'' ).

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==== 2.5.3.3 Feedbacks related to changes in soil moisture resulting from global warming ====

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There is medium evidence but ''high agreement'' that soil moisture conditions influence the frequency and magnitude of extremes such as drought and heatwaves. Observational evidence indicates that dry soil moisture conditions favour heatwaves, in particular in regions where evapotranspiration is limited by moisture availability (Mueller and Seneviratne 2012 <sup>[[#fn:r1290|1290]]</sup> ; Quesada et al. 2012 <sup>[[#fn:r1291|1291]]</sup> ; Miralles et al. 2018 <sup>[[#fn:r1292|1292]]</sup> ; Geirinhas et al. 2018 <sup>[[#fn:r1293|1293]]</sup> ; Miralles et al. 2014 <sup>[[#fn:r1294|1294]]</sup> ; Chiang et al. 2018 <sup>[[#fn:r1295|1295]]</sup> ; Dong and Crow 2019 <sup>[[#fn:r1296|1296]]</sup> ; Hirschi et al. 2014 <sup>[[#fn:r1297|1297]]</sup> ).

In future climate projections, soil moisture plays an important role in the projected amplification of extreme heatwaves and drought in many regions of the world ( ''medium confidence'' ) (Seneviratne et al. 2013 <sup>[[#fn:r1298|1298]]</sup> ; Vogel et al. 2017 <sup>[[#fn:r1299|1299]]</sup> ; Donat et al. 2018 <sup>[[#fn:r1300|1300]]</sup> ; Miralles et al. 2018 <sup>[[#fn:r1301|1301]]</sup> ). In addition, the areas where soil moisture affects heat extremes will not be located exactly where they are today. Changes in rainfall, temperature, and thus in evapotranspiration, will induce changes in soil moisture and therefore where temperature and latent heat flux will be negatively coupled (Seneviratne et al. 2006 <sup>[[#fn:r1302|1302]]</sup> ; Fischer et al. 2012 <sup>[[#fn:r1303|1303]]</sup> ). Quantitative estimates of the actual role of soil moisture feedbacks are, however, very uncertain due to the ''low confidence'' in projected soil moisture changes (IPCC 2013a <sup>[[#fn:r1304|1304]]</sup> ), to weaknesses in the representation of soil moisture–atmosphere interactions in climate models (Sippel et al. 2017 <sup>[[#fn:r1305|1305]]</sup> ; Ukkola et al. 2018 <sup>[[#fn:r1306|1306]]</sup> ; Donat et al. 2018 <sup>[[#fn:r1307|1307]]</sup> ; Miralles et al. 2018 <sup>[[#fn:r1308|1308]]</sup> ) and to methodological uncertainties associated with the soil moisture prescription framework commonly used to disentangle the effect of soil moisture on changes in temperature extremes (Hauser et al. 2017 <sup>[[#fn:r1309|1309]]</sup> ).

Where soil moisture is predicted to decrease in response to climate change in the subtropics and temperate latitudes, this drying could be enhanced by the existence of soil moisture feedbacks ( ''low confidence'' ) (Berg et al. 2016 <sup>[[#fn:r1310|1310]]</sup> ). The initial decrease in precipitation and increase in potential evapotranspiration and latent heat flux, in response to global climate change, leads to decreased soil moisture at those latitudes and can potentially amplify both. Such a feature is consistent with evidence that, in a warmer climate, land and atmosphere will be more strongly coupled via both the water and energy cycles (Dirmeyer et al. 2014 <sup>[[#fn:r1311|1311]]</sup> ; Guo et al. 2006 <sup>[[#fn:r1312|1312]]</sup> ). This increased sensitivity of atmospheric response to land perturbations implies that changes in land uses and cover are expected to have more impact on climate in the future than they do today.

Beyond temperature, it has been suggested that soil moisture feedbacks influence precipitation occurrence and intensity. But the importance, and even the sign of this feedback, is still largely uncertain and debated (Tuttle and Salvucci 2016 <sup>[[#fn:r1313|1313]]</sup> ; Yang et al. 2018 <sup>[[#fn:r1314|1314]]</sup> ; Froidevaux et al. 2014 <sup>[[#fn:r1315|1315]]</sup> ; Guillod et al. 2015 <sup>[[#fn:r1316|1316]]</sup> ).

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