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

==== 3.4.3.2 Changes in phenology ====

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Advancement in spring phenology of 2.8 ± 0.35 days per decade has been observed in plants and animals in recent decades in most Northern Hemisphere ecosystems (between 30°N and 72°N), and these shifts have been attributed to changes in climate ( ''high confidence'' ) (Settele et al., 2014) <sup>[[#fn:r457|457]]</sup> . The rates of change are particularly high in the Arctic zone owing to the stronger local warming (Oberbauer et al., 2013) <sup>[[#fn:r458|458]]</sup> , whereas phenology in tropical forests appears to be more responsive to moisture stress (Zhou et al., 2014) <sup>[[#fn:r459|459]]</sup> . While a full review cannot be included here, trends consistent with this earlier finding continue to be detected, including in the flowering times of plants (Parmesan and Hanley, 2015) <sup>[[#fn:r460|460]]</sup> , in the dates of egg laying and migration in birds (newly reported in China; Wu and Shi, 2016) <sup>[[#fn:r461|461]]</sup> , in the emergence dates of butterflies (Roy et al., 2015) <sup>[[#fn:r462|462]]</sup> , and in the seasonal greening-up of vegetation as detected by satellites (i.e., in the normalized difference vegetation index, NDVI; Piao et al., 2015) <sup>[[#fn:r463|463]]</sup> .

The potential for decoupling species–species interactions owing to differing phenological responses to climate change is well established (Settele et al., 2014) <sup>[[#fn:r464|464]]</sup> , for example for plants and their insect pollinators (Willmer, 2012; Scaven and Rafferty, 2013) <sup>[[#fn:r465|465]]</sup> . Mid-century projections of plant and animal phenophases in the UK clearly indicate that the timing of phenological events could change more for primary consumers (6.2 days earlier on average) than for higher trophic levels (2.5–2.9 days earlier on average) (Thackeray et al., 2016) <sup>[[#fn:r466|466]]</sup> . This indicates the potential for phenological mismatch and associated risks for ecosystem functionality in the future under global warming of 2.1°C–2.7°C above pre-industrial levels. Further, differing responses could alter community structure in temperate forests (Roberts et al., 2015) <sup>[[#fn:r467|467]]</sup> . Specifically, temperate forest phenology is projected to advance by 14.3 days in the near term (2010–2039) and 24.6 days in the medium term (2040–2069), so as a first approximation the difference between 2°C and 1.5°C of global warming is about 10 days (Roberts et al., 2015) <sup>[[#fn:r468|468]]</sup> . This phenological plasticity is not always adaptive and must be interpreted cautiously (Duputié et al., 2015) <sup>[[#fn:r469|469]]</sup> , and considered in the context of accompanying changes in climate variability (e.g., increased risk of frost damage for plants or earlier emergence of insects resulting in mortality during cold spells). Another adaptive response of some plants is range expansion with increased vigour and altered herbivore resistance in their new range, analogous to invasive plants (Macel et al., 2017) <sup>[[#fn:r470|470]]</sup> .

In summary, limiting warming to 1.5°C compared with 2°C may avoid advance in spring phenology ( ''high confidence'' ) by perhaps a few days ( ''medium confidence'' ) and hence decrease the risks of loss of ecosystem functionality due to phenological mismatch between trophic levels, and also of maladaptation coming from the sensitivity of many species to increased climate variability. Nevertheless, this difference between 1.5°C and 2°C of warming might be limited for plants that are able to expand their range.

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