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

==== 2.4.2.1 Observed Range Shifts Driven by Climate Change ====

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Poleward and upward range shifts were already attributable to climate warming with ''high confidence'' in AR5. Publication of observed range shifts in accord with climate change have accelerated since AR5 and strengthened attribution. Ongoing latitudinal and elevational range shifts driven by regional climate trends are now well-established globally across many groups of organisms, and attributable to climate change with ''very high confidence'' due to very high consistency across a now very large body of species and studies and an in-depth understanding of mechanisms underlying physiological and ecological responses to climate drivers (Table 2.2; Table 2.3, Table SM2.1) ( [[#Pöyry--2009|Pöyry et al., 2009]] ; [[#Chen--2011|Chen et al., 2011]] ; [[#Grewe--2013|Grewe et al., 2013]] ; [[#Gibson-Reinemer--2015|Gibson-Reinemer and Rahel, 2015]] ; [[#MacLean--2017|MacLean and Beissinger, 2017]] ; [[#Pacifici--2017|Pacifici et al., 2017]] ; [[#Anderegg--2019|Anderegg et al., 2019]] ). Range shifts stem from local population extinctions along warm-range boundaries ( [[#Anderegg--2019|Anderegg et al., 2019]] ) as well as from the colonisation of new regions at cold-range boundaries ( [[#Ralston--2017|Ralston et al., 2017]] ).

Many studies since AR4 have tended not to be designed as attribution studies, particularly recent large-scale, multi-species meta-analyses. That is to say, all the data available was included in such studies (from both undisturbed and highly degraded lands and including very short-term data sets of &lt;20 years), with little attempt to design the studies to differentiate the effects of climate change from those of other potential confounding variables. These studies tended to find greater lag and a lower proportion of species changing in the directions expected from climate change, with the authors concluding that LULCC, particularly habitat loss and fragmentation, was impeding wild species from effectively tracking climate change ( [[#Lenoir--2015|Lenoir and Svenning, 2015]] ; [[#Rumpf--2019|Rumpf et al., 2019]] ; [[#Lenoir--2020|Lenoir et al., 2020]] ).

Attribution is strong for species and species-interactions for which there is a robust mechanistic understanding of the role of climate on biological processes ''(high confidence)'' . Unprecedented outbreaks of spruce beetles occurring from Alaska to Utah in the 1990s were attributed to warm weather that, in Alaska, facilitated a halving of the insect’s life cycle from two years to one ( [[#Logan--2003|Logan et al., 2003]] ). Milder winters and warmer growing seasons were likewise implicated in poleward expansions and increasing outbreaks of several forest pests ( [[#Weed--2013|Weed et al., 2013]] ), leading to the current prediction that 41% of major insect pest species will further increase their damage as climate warms, and only 4% will reduce their impacts, while the rest will show mixed responses ( [[#Lehmann--2020|Lehmann et al., 2020]] ).

During their range shifts, forest pests remain climate-sensitive. For example, the distribution of the western spruce budworm is limited at its warm range edges by the adverse effects of mild winters on overwinter survival, and at its cool range by the ability to arrive at a cold-resistant stage before winter arrives ( [[#Régnière--2019|Régnière and Nealis, 2019]] ). We might therefore expect tree mortality from insect outbreaks to be most severe at sites climatically less suitable for the plants, where plants would be under more stress. However, ( [[#Jaime--2019|Jaime et al., 2019]] ), using separate species distribution models (SDMs) (MaxEnt) for the insects and plants, found that observed mortality of Scots pine from bark beetles was highest at sites that were most climatically suitable for the trees as well as for the insects. In a study of tree mortality in California, bark beetles selectively killed highly stressed fir trees, but killed pines according to their size irrespective of stress status ( [[#Stephenson--2019|Stephenson et al., 2019]] ).

Range shifts in a poleward and upward direction, following expected trajectories according to local and regional climate trends, are strongly occurring in freshwater fish populations in North America ( [[#Lynch--2016b|Lynch et al., 2016b]] ), Europe ( [[#Comte--2013|Comte and Grenouillet, 2013]] ; [[#Gozlan--2019|Gozlan et al., 2019]] ) and Central Asia ( [[#Gozlan--2019|Gozlan et al., 2019]] ) ''(medium evidence, high agreement)'' . Cold-water fish, such as coregonids and smelt have been negatively affected at the equatorial borders of their distributions ( [[#Jeppesen--2012|Jeppesen et al., 2012]] ). Upward elevational range shifts in rivers and streams have been observed. Systematic shifts towards higher elevation and upstream were found for 32 stream-fish species in France following regional variation in climate change ( [[#Comte--2013|Comte and Grenouillet, 2013]] ). Bull trout ( ''Salvelinus confluentus'' ) in Idaho (USA), were estimated to have lost 11–20% (8–16% per decade) of the headwater stream lengths necessary for cold-water spawning and early juvenile rearing, with the largest losses occurring in the coldest habitats ( [[#Isaak--2010|Isaak et al., 2010]] ). Range contractions of the same species have been found in the Rocky Mountains watershed ( [[#Eby--2014|Eby et al., 2014]] ). Likewise, the distribution of the stonefly ''Zapada glacier'' , endemic to the alpine streams of the Glacier National Park in Montana (USA), has been reduced over several decades by an upstream retreat to higher, cooler sites as water temperatures have increased and glacial masses have decreased ( [[#Giersch--2015|Giersch et al., 2015]] ).

The melting of glaciers has led to a change in water discharge associated with community turnover in glacier-fed streams ( [[#Cauvy-Fraunié--2019|Cauvy-Fraunié and Dangles, 2019]] ). For instance, glacier-obligate macro-invertebrates have started disappearing when glacial cover drops below approximately 50% ( ''robust evidence'' , ''high agreement'' ), reviewed in ( [[#Hotaling--2017|Hotaling et al., 2017]] ). For freshwater invertebrates, no meaningful trends have been detected in geographic extent or population size for most species ( [[#Gozlan--2019|Gozlan et al., 2019]] ).

An invasive freshwater cyanobacterium in lakes, ''Cylindrospermopsis raciborskii'' , originating from the tropics, has spread to temperate zones over the last few decades due to the climate change-induced earlier increase of water temperature in spring ( [[#Wiedner--2007|Wiedner et al., 2007]] ), aided by a competitive advantage in eutrophic systems ( [[#Ekvall--2013|Ekvall et al., 2013]] ; [[#Urrutia-Cordero--2016|Urrutia-Cordero et al., 2016]] ).

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