Editing IPCC:AR6/SR15/Chapter-3 (section)

== 3.5.5.2 Tundra ==

<div id="section-3-5-5-2-block-1"></div>

Tree growth in tundra-dominated landscapes is strongly constrained by the number of days with mean air temperature above 0°C. A potential tipping point exists where the number of days below 0°C decreases to the extent that the tree fraction increases significantly. Tundra-dominated landscapes have warmed more than the global average over the last century (Settele et al., 2014) <sup>[[#fn:r1230|1230]]</sup> , with associated increases in fires and permafrost degradation (Bring et al., 2016; DeBeer et al., 2016; Jiang et al., 2016; Yang et al., 2016) <sup>[[#fn:r1231|1231]]</sup> . These processes facilitate conditions for woody species establishment in tundra areas, and for the eventual transition of the tundra to boreal forest. The number of investigations into how the tree fraction may respond in the Arctic to different degrees of global warming is limited, and studies generally indicate that substantial increases will ''likely'' occur gradually (e.g., Lenton et al., 2008) <sup>[[#fn:r1232|1232]]</sup> . Abrupt changes are only plausible at levels of warming significantly higher than 2°C ( ''low confidence'' ) and would occur in conjunction with a collapse in permafrost (Drijfhout et al., 2015) <sup>[[#fn:r1233|1233]]</sup> .

<div id="section-3-5-5-3"></div>

<span id="permafrost"></span>