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

==== 2.2.5.2 Impacts of heat extremes and drought on land ====

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There is ''high confidence'' that heat extremes such as unusually hot nights, extremely high daytime temperatures, heatwaves and drought are damaging to crop production (Chapter 5). Extreme heat events impact a wide variety of tree functions including reduced photosynthesis, increased photooxidative stress, leaves abscise, a decreased growth rate of remaining leaves and decreased growth of the whole tree (Teskey et al. 2015 <sup>[[#fn:r281|281]]</sup> ). Although trees are more resilient to heat stress than grasslands (Teuling et al. 2010 <sup>[[#fn:r282|282]]</sup> ), it has been observed that different types of forest (e.g., needleleaf vs broadleaf) respond differently to drought and heatwaves (Babst et al. 2012 <sup>[[#fn:r283|283]]</sup> ). For example, in the Turkish Anatolian forests net primary productivity (NPP) generally decreased during drought and heatwave events between 2000 and 2010 but in a few other regions, NPP of needle leaf forests increased (Erşahin et al. 2016 <sup>[[#fn:r284|284]]</sup> ). However, forests may become less resilient to heat stress in future due to the long recovery period required to replace lost biomass and the projected increased frequency of heat and drought events (Frank et al. 2015a <sup>[[#fn:r285|285]]</sup> ; McDowell and Allen 2015 <sup>[[#fn:r286|286]]</sup> ; Johnstone et al. 2016 <sup>[[#fn:r287|287]]</sup> ; Stevens-Rumann et al. 2018 <sup>[[#fn:r288|288]]</sup> ). Additionally, widespread regional tree mortality may be triggered directly by drought and heat stress (including warm winters) and exacerbated by insect outbreak and fire (Neuvonen et al. 1999 <sup>[[#fn:r289|289]]</sup> ; Breshears et al. 2005 <sup>[[#fn:r290|290]]</sup> ; Berg et al. 2006 <sup>[[#fn:r291|291]]</sup> ; Soja et al. 2007 <sup>[[#fn:r292|292]]</sup> ; Kurz et al. 2008 <sup>[[#fn:r293|293]]</sup> ; Allen et al. 2010 <sup>[[#fn:r294|294]]</sup> ).

Gross primary production (GPP) and soil respiration form the first and second largest carbon fluxes from terrestrial ecosystems to the atmosphere in the global carbon cycle (Beer et al. 2010 <sup>[[#fn:r295|295]]</sup> ; Bond- Lamberty and Thomson 2010 <sup>[[#fn:r296|296]]</sup> ). Heat extremes impact the carbon cycle through altering these and change ecosystem-atmosphere CO <sub>2</sub> fluxes and the ecosystem carbon balance. Compound heat and drought events result in a stronger carbon sink reduction compared to single-factor extremes as GPP is strongly reduced and ecosystem respiration less so (Reichstein et al. 2013 <sup>[[#fn:r297|297]]</sup> ; Von Buttlar et al. 2018 <sup>[[#fn:r298|298]]</sup> ). In forest biomes, however, GPP may increase temporarily as a result of increased insolation and photosynthetic activity as was seen during the 2015–2016 ENSO related drought over Amazonia (Zhu et al. 2018 <sup>[[#fn:r299|299]]</sup> ). Longer extreme events (heatwave or drought or both) result in a greater reduction in carbon sequestration and may also reverse long-term carbon sinks (Ciais et al. 2005 <sup>[[#fn:r300|300]]</sup> ; Phillips et al. 2009 <sup>[[#fn:r301|301]]</sup> ; Wolf et al. 2016b <sup>[[#fn:r302|302]]</sup> ; Ummenhofer and Meehl 2017 <sup>[[#fn:r303|303]]</sup> ; Von Buttlar et al. 2018 <sup>[[#fn:r304|304]]</sup> ; Reichstein et al. 2013 <sup>[[#fn:r305|305]]</sup> ). Furthermore, extreme heat events may impact the carbon cycle beyond the lifetime of the event. These lagged effects can slow down or accelerate the carbon cycle: it will slow down if reduced vegetation productivity and/or widespread mortality after an extreme drought are not compensated by regeneration, or speed up if productive tree and shrub seedlings cause rapid regrowth after windthrow or fire (Frank et al. 2015a <sup>[[#fn:r306|306]]</sup> ). Although some ecosystems may demonstrate resilience to a single heat climate stressor like drought (e.g., forests), compound effects of, for example, deforestation, fire and drought, potentially can result in changes to regional precipitation patterns and river discharge, losses of carbon storage and a transition to a disturbance-dominated regime (Davidson et al. 2012 <sup>[[#fn:r307|307]]</sup> ). Additionally, adaptation to seasonal drought may be overwhelmed by multi-year drought and their legacy effects (Brando et al. 2008 <sup>[[#fn:r308|308]]</sup> ; da Costa et al. 2010 <sup>[[#fn:r309|309]]</sup> ).

Under medium- and high-emission scenarios, global warming will exacerbate heat stress, thereby amplifying deficits in soil moisture and runoff despite uncertain precipitation changes (Ficklin and Novick 2017 <sup>[[#fn:r310|310]]</sup> ; Berg and Sheffield 2018 <sup>[[#fn:r311|311]]</sup> ; Cook et al. 2018 <sup>[[#fn:r312|312]]</sup> ; Dai et al. 2018 <sup>[[#fn:r313|313]]</sup> ; Engelbrecht et al. 2015 <sup>[[#fn:r314|314]]</sup> ; Ramarao et al. 2015 <sup>[[#fn:r315|315]]</sup> ; Grillakis 2019 <sup>[[#fn:r316|316]]</sup> ). This will  increase the rate of drying causing drought to set in quicker, become more intense and widespread, last longer and could result in an increased global aridity (Dai 2011 <sup>[[#fn:r317|317]]</sup> ; Prudhomme et al. 2014 <sup>[[#fn:r318|318]]</sup> ).

The projected changes in the frequency and intensity of extreme temperatures and drought is expected to result in decreased carbon sequestration by ecosystems and degradation of ecosystems health and loss of resilience (Trumbore et al. 2015 <sup>[[#fn:r319|319]]</sup> ). Also affected are many aspects of land functioning and type including agricultural productivity (Lesk et al. 2016 <sup>[[#fn:r320|320]]</sup> ), hydrology (Mosley 2015 <sup>[[#fn:r321|321]]</sup> ; Van Loon and Laaha 2015 <sup>[[#fn:r322|322]]</sup> ), vegetation productivity and distribution (Xu et al. 2011 <sup>[[#fn:r323|323]]</sup> ; Zhou et al. 2014 <sup>[[#fn:r324|324]]</sup> ), carbon fluxes and stocks, and other biogeochemical cycles (Frank et al. 2015b <sup>[[#fn:r325|325]]</sup> ; Doughty et al. 2015 <sup>[[#fn:r326|326]]</sup> ; Schlesinger et al. 2016 <sup>[[#fn:r327|327]]</sup> ). Carbon stocks are particularly vulnerable to extreme events due to their large carbon pools and fluxes, potentially large lagged impacts and long recovery times to regain lost stocks (Frank et al. 2015a <sup>[[#fn:r328|328]]</sup> ) (Section 2.2).

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