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

=== 2.4.5 Conclusions on Observed Impacts ===

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The consistency of patterns of biological change with expectations from regional or global warming processes, coupled with an understanding of underlying processes and the coherence of these patterns at both regional and global scales, all form multiple lines of evidence ( [[#Parmesan--2013|Parmesan et al., 2013]] ) that it is ''very likely'' that the observed range shifts and phenological changes in individual species can be attributed to regional and global climate changes ( ''very high confidence'' ) ( [[#2.4.2|Section 2.4.2]] , Table 2.2; Table 2.3; Table SM2.1) ( [[#Parmesan--2013|Parmesan et al., 2013]] ).

Global and regional meta-analyses of diverse systems, habitats and taxonomic groupings document that approximately half of all species with long-term records have shifted their ranges poleward and/or upward in elevation and ~2/3 have advanced their timing of spring events (phenology) ''(very high confidence)'' ( [[#2.4.2|Section 2.4.2]] , Table 2.2) ( [[#Parmesan--2015|Parmesan and Hanley, 2015]] ; [[#Parmesan--2019|Parmesan, 2019]] ). Changes in abundance tend to match predictions from climate warming, with warm-adapted species significantly outperforming cold-adapted species in warming habitats ( [[#Feeley--2020|Feeley et al., 2020]] ) and the composition of local communities becoming more ‘thermophilised’, that is, experiencing an ‘increase in relative abundance of heat-loving or heat-tolerant species’ ''(high confidence)'' ( [[#2.4.2.3|Section 2.4.2.3]] ) ( [[#Cline--2013|Cline et al., 2013]] ; [[#Feeley--2020|Feeley et al., 2020]] ).

New studies since AR5, with more sophisticated analyses designed to capture complex responses, indicate that past estimates of the proportion of species impacted by recent climate change were underestimates due to unspoken assumptions that local or regional warming should lead solely to poleward/upward range shifts and advancements of spring timing ''(high confidence)'' ( [[#Duffy--2019|Duffy et al., 2019]] ). More complex analyses have documented cases of winter warming driving delayed spring timing of northern temperate species due to chilling requirements, and increased precipitation driving species’ range shifts downslope in elevation, and eastward and westward in arid regions ( ''high confidence'' ) ''.'' Further new studies have shown that phenological changes have, in some cases, successfully compensated for local climate change and reduced the extent of range shifts ( ''medium confidence'' ) ''.'' The limited number of studies of this type make it difficult to estimate the generality of these effects globally ( [[#2.4.2.5|Section 2.4.2.5]] , Table 2.2).

Responses in freshwater species are consistent with responses in terrestrial species, including poleward and upward range shifts, earlier timing of spring plankton development, earlier spawning by fish and the extension of the growing season ''(high confidence)'' . Observed changes in freshwater species are strongly related to anthropogenic climate change-driven changes in the physical environment (e.g., increased water temperature, reduced ice cover, reduced mixing in lakes, loss of oxygen and reduced river connectivity) ''(high confidence)'' . While ''evidence'' is ''robust'' for an increase in primary production in nutrient rich lakes along with warming trends ( ''high confidence'' ), increasing or declining algal formations are lake-specific and are modulated through variability in weather conditions, lake morphology, changes in salinity, stoichiometry, land use and restoration measures and food web interactions. In boreal coniferous forest, there has been an increase in terrestrial-derived DOM transported into rivers and lakes as a consequence of climate change (which has induced increases in runoff and greening of the Northern Hemisphere) as well as from changes in forestry practices. This has caused waters to become brown, resulting in an acceleration of upper-water warming and an overall cooling of deep water ( ''high confidence'' ) ''.'' Browning may accelerate primary production through the input of nutrients associated with DOM in nutrient-poor lakes and increases the growth of cyanobacteria, which cope better with low light intensity ( ''medium confidence'' ) (Sections 2.4.2.1, 2.4.2.2, 2.4.2.3, 2.4.2.4).

Field research since the AR5 has detected biome shifts at numerous sites, poleward and upslope, that are consistent with increased temperatures and altered precipitation patterns driven by climate change, and support prior studies that attributed such shifts to anthropogenic climate change ( ''high confidence'' ) ''.'' New studies help fill previous geographic and habitat gaps, for example, documenting upward shifts in the forest/alpine tundra ecotone in the Andes, Tibet and Nepal, and northward shifts in the deciduous/boreal forest ecotones in Canada. Globally, woody encroachment into open areas (grasslands, arid regions and tundra) is ''likely'' being driven by climate change and increased CO 2 , in concert with changes in grazing and fire regimes ( ''medium confidence'' ) ( [[#2.4.3|Section 2.4.3]] ).

Climate change has driven, or is contributing to, increased tree mortality directly through increased aridity and droughts and indirectly through increased wildfires and insect pests in many locations ( ''high confidence'' ) ''.'' Analyses of causal factors have attributed increasing tree mortality at sites in Africa and North America to anthropogenic climate change, and field evidence has detected tree mortality due to drought, wildfires and insect pests in temperate and tropical forests around the world ( ''high confidence'' ). Water stress, leading to plant hydraulic failure, is a principal mechanism of drought-induced tree mortality, along with the indirect effects of climate change mediated by community interactions ( ''high confidence'' ) ( [[#2.4.4.3|Section 2.4.4.3]] ).

Terrestrial ecosystems sequester and store globally critical stocks of carbon, but these stocks are at risk from deforestation and climate change ( ''high confidence'' ). Tropical deforestation and the draining and burning of peatlands produce almost all of the carbon emissions from LULCC. In the Arctic, increased temperatures have thawed permafrost at numerous sites, dried some areas and increased fires, causing net emissions of carbon from soils ( ''high confidence'' ) (Sections 2.4.4.4, 2.5.3.4).

Globally, increases in temperature, aridity and drought have increased the length of fire seasons and doubled the potentially burnable land area ( ''medium confidence'' ). Increases in the area burned have been attributed to anthropogenic climate change in North America ( ''high confidence'' ) ''.'' In parts of Africa, Asia, Australia and South America, the area burned has also increased, consistent with anthropogenic climate change. Deforestation, peat-burning, agricultural expansion or abandonment, fire suppression and inter-decadal cycles strongly influence fire occurrence. The areas with the greatest increases in the length of the fire season include the Amazon, western North America, western Asia and East Africa ( [[#2.4.4.2|Section 2.4.4.2]] ).

The changes in biodiversity and ecosystem health that we have observed, and project will continue, pose a risk of declines in human health and well-being (e.g., tourism, recreation, food, livelihoods and quality of life) ( ''medium confidence'' ) ''.'' Clear attribution of these impacts is often not possible, but inferences can be made by comparison of the observed changes in biodiversity/ecosystem health and the known services from these particular ecosystems.

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