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

==== 2.4.2.5 Observed Complex Phenological and Range Shift Responses ====

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Early meta-analyses tested the straightforward hypotheses that warming should shift timing earlier and ranges polewards. Once these trends had been established ( [[#IPCC--2014b|IPCC, 2014b]] ; [[#Parmesan--2015|Parmesan and Hanley, 2015]] ), exceptions to them became a focus of study. For example, in northern regions of the Northern Hemisphere, the spring flowering of some plants was delayed instead of being advanced as to be expected with warming ( [[#Cook--2012a|Cook et al., 2012a]] ; [[#Cook--2012b|Cook et al., 2012b]] ; [[#Legave--2015|Legave et al., 2015]] ). These turned out to be species requiring vernalisation (winter chilling) to speed their development in spring ( [[#Ettinger--2020|Ettinger et al., 2020]] ). For these plants, phenological changes result from the combined effects of advancement caused by spring warming and retardation caused by winter warming. Incorporating this level of complexity into analyses revealed that a greater proportion of species was responding to climate change than estimated according to the simple expectation that warming would always cause advancement (92% responding versus 72% from earlier analyses) ( [[#Cook--2012b|Cook et al., 2012b]] ).

Animal species can show vernalisation equivalent to that in plants ( [[#Stålhandske--2017|Stålhandske et al., 2017]] ). However, a semi-global meta-analysis of terrestrial animals failed to detect the delaying effects of warming winters ( [[#Cohen--2018|Cohen et al., 2018]] ). The same animal-based meta-analysis contrasted phenological changes in temperate-zone animals, which are principally explained by changes in temperature, with those at lower latitudes, which tend to follow changes in precipitation ( [[#Cohen--2018|Cohen et al., 2018]] ).

Vitasse et al. (2018), working with alpine trees, found that phenological delay with increasing elevation had declined from 34 days/1000 m in 1960 to 22 days/1000 m in 2016, greatly reducing the differences in timing between trees growing at different elevations. This reduction was greatest after warmer winters, suggesting that winter warming is a principal cause of the overall trend.

[[#Lian--2020|Lian et al. (2020)]] observed that earlier spring leaf-out in the Northern Hemisphere is causing increases in evapotranspiration that are not fully compensated by increased precipitation. The consequence is a greater soil moisture deficit in summer, expected to exacerbate impacts of heat waves as well as drought stress. In Arctic freshwater ecosystems, [[#Heim--2015|Heim et al. (2015)]] demonstrated the importance of seasonal cues for fish migration, which can be impacted by climate change due to reduced stream connectivity and fragmentation, earlier peak flows and increased evapotranspiration.

Precipitation has also been implicated in exceptions to the rule that ranges should be shifting to higher elevations. In dry climates, increases in precipitation accompanying climate warming can facilitate downslope range shifts ( [[#Tingley--2012|Tingley et al., 2012]] ).

Multiple responses can co-occur. [[#Hällfors--2021|Hällfors et al. (2021)]] , in a study of 289 Lepidoptera in Finland, found that, with warming, 45% had either shifted their range northward or advanced their flight season. The 15% of species that did both (shifting northward by 113.1 km and advancing their flight period by 2.7 days per decade, on average, over a 20-year period) had the largest population increases, and the 40% of species that showed no response had the largest population declines.

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