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

==== 3.4.3.5 Regional and ecosystem-specific risks ====

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A large number of threatened systems, including mountain ecosystems, highly biodiverse tropical wet and dry forests, deserts, freshwater systems and dune systems, were assessed in AR5. These include Mediterranean areas in Europe, Siberian, tropical and desert ecosystems in Asia, Australian rainforests, the Fynbos and succulent Karoo areas of South Africa, and wetlands in Ethiopia, Malawi, Zambia and Zimbabwe. In all these systems, it has been shown that impacts accrue with greater warming, and thus impacts at 2°C are expected to be greater than those at 1.5°C ( ''medium confidence'' ).

The High Arctic region, with tundra-dominated landscapes, has warmed more than the global average over the last century (Section 3.3; Settele et al., 2014) <sup>[[#fn:r520|520]]</sup> . The Arctic tundra biome is experiencing increasing fire disturbance and permafrost degradation (Bring et al., 2016; DeBeer et al., 2016; Jiang et al., 2016; Yang et al., 2016) <sup>[[#fn:r521|521]]</sup> . Both of these processes facilitate the establishment of woody species in tundra areas. Arctic terrestrial ecosystems are being disrupted by delays in winter onset and mild winters associated with global warming ( ''high confidence'' ) (Cooper, 2014) <sup>[[#fn:r522|522]]</sup> . Observational constraints suggest that stabilization at 1.5°C of warming would avoid the thawing of approximately 1.5 to 2.5 million km <sup>2</sup> of permafrost ( ''medium confidence'' ) compared with stabilization at 2°C (Chadburn et al., 2017) <sup>[[#fn:r523|523]]</sup> , but the time scale for release of thawed carbon as CO <sub>2</sub> or CH <sub>4</sub> should be many centuries (Burke et al., 2017) <sup>[[#fn:r524|524]]</sup> . In northern Eurasia, the growing season length is projected to increase by about 3–12 days at 1.5°C and 6–16 days at 2°C of warming ( ''medium confidence'' ) (Zhou et al., 2018) <sup>[[#fn:r525|525]]</sup> . Aalto et al. (2017) <sup>[[#fn:r526|526]]</sup> predicted a 72% reduction in cryogenic land surface processes in northern Europe for RCP2.6 in 2040–2069 (corresponding to a global warming of approximately 1.6°C), with only slightly larger losses for RCP4.5 (2°C of global warming).

Projected impacts on forests as climate change occurs include increases in the intensity of storms, wildfires and pest outbreaks (Settele et al., 2014) <sup>[[#fn:r527|527]]</sup> , potentially leading to forest dieback ( ''medium confidence'' ). Warmer and drier conditions in particular facilitate fire, drought and insect disturbances, while warmer and wetter conditions increase disturbances from wind and pathogens (Seidl et al., 2017) <sup>[[#fn:r528|528]]</sup> . Particularly vulnerable regions are Central and South America, Mediterranean Basin, South Africa, South Australia where the drought risk will increase (see Figure 3.12). Including disturbances in simulations may influence productivity changes in European forests in response to climate change (Reyer et al., 2017b) <sup>[[#fn:r529|529]]</sup> . There is additional evidence for the attribution of increased forest fire frequency in North America to anthropogenic climate change during 1984–2015, via the mechanism of increasing fuel aridity almost doubling the western USA forest fire area compared to what would have been expected in the absence of climate change (Abatzoglou and Williams, 2016) <sup>[[#fn:r530|530]]</sup> . This projection is in line with expected fire risks, which indicate that fire frequency could increase over 37.8% of the global land area during 2010–2039 (Moritz et al., 2012) <sup>[[#fn:r531|531]]</sup> , corresponding to a global warming level of approximately 1.2°C, compared with over 61.9% of the global land area in 2070–2099, corresponding to a warming of approximately 3.5°C <sup>[[#fn:9|9]]</sup> .The values in Table 26-1 in a recent paper by Romero-Lankao et al. (2014) <sup>[[#fn:r532|532]]</sup> also indicate significantly lower wildfire risks in North America for near-term warming (2030–2040, considered a proxy for 1.5°C of warming) than at 2°C ( ''high confidence'' ).

The Amazon tropical forest has been shown to be close to its climatic limits (Hutyra et al., 2005) <sup>[[#fn:r533|533]]</sup> , but this threshold may move under elevated CO <sub>2</sub> (Good et al., 2011) <sup>[[#fn:r534|534]]</sup> . Future changes in rainfall, especially dry season length, will determine responses of the Amazon forest (Good et al., 2013) <sup>[[#fn:r535|535]]</sup> . The forest may be especially vulnerable to combined pressure from multiple stressors, namely changes in climate and continued anthropogenic disturbance (Borma et al., 2013; Nobre et al., 2016) <sup>[[#fn:r536|536]]</sup> . Modelling (Huntingford et al., 2013) <sup>[[#fn:r537|537]]</sup> and observational constraints (Cox et al., 2013) <sup>[[#fn:r538|538]]</sup> suggest that large-scale forest dieback is less ''likely'' than suggested under early coupled modelling studies (Cox et al., 2000; Jones et al., 2009) <sup>[[#fn:r539|539]]</sup> . Nobre et al. (2016) <sup>[[#fn:r540|540]]</sup> estimated a climatic threshold of 4°C of warming and a deforestation threshold of 40%.

In many places around the world, the savanna boundary is moving into former grasslands. Woody encroachment, including increased tree cover and biomass, has increased over the past century, owing to changes in land management, rising CO <sub>2</sub> levels, and climate variability and change (often in combination) (Settele et al., 2014) <sup>[[#fn:r541|541]]</sup> . For plant species in the Mediterranean region, shifts in phenology, range contraction and health decline have been observed with precipitation decreases and temperature increases ( ''medium confidence'' ) (Settele et al., 2014) <sup>[[#fn:r542|542]]</sup> . Recent studies using independent complementary approaches have shown that there is a regional-scale threshold in the Mediterranean region between 1.5°C and 2°C of warming (Guiot and Cramer, 2016; Schleussner et al., 2016b) <sup>[[#fn:r543|543]]</sup> . Further, Guiot and Cramer (2016) <sup>[[#fn:r544|544]]</sup> concluded that biome shifts unprecedented in the last 10,000 years can only be avoided if global warming is constrained to 1.5°C ( ''medium confidence'' ) – whilst 2°C of warming will result in a decrease of 12–15% of the Mediterranean biome area. The Fynbos biome in southwestern South Africa is vulnerable to the increasing impact of fires under increasing temperatures and drier winters. It is projected to lose about 20%, 45% and 80% of its current suitable climate area under 1°C, 2°C and 3°C of global warming, respectively, compared to 1961–1990 ( ''high confidence'' ) (Engelbrecht and Engelbrecht, 2016) <sup>[[#fn:r545|545]]</sup> . In Australia, an increase in the density of trees and shrubs at the expense of grassland species is occurring across all major ecosystems and is projected to be amplified (NCCARF, 2013) <sup>[[#fn:r546|546]]</sup> . Regarding Central America, Lyra et al. (2017) <sup>[[#fn:r547|547]]</sup> showed that the tropical rainforest biomass would be reduced by about 40% under global warming of 3°C, with considerable replacement by savanna and grassland. With a global warming of close to 1.5°C in 2050, a biomass decrease of 20% is projected for tropical rainforests of Central America (Lyra et al., 2017) <sup>[[#fn:r548|548]]</sup> . If a linear response is assumed, this decrease may reach 30% ( ''medium confidence'' ).

Freshwater ecosystems are considered to be among the most threatened on the planet (Settele et al., 2014) <sup>[[#fn:r549|549]]</sup> . Although peatlands cover only about 3% of the land surface, they hold one-third of the world’s soil carbon stock (400 to 600 Pg) (Settele et al., 2014) <sup>[[#fn:r550|550]]</sup> . When drained, this carbon is released to the atmosphere. At least 15% of peatlands have drained, mostly in Europe and South east Asia, and are responsible for 5% of human derived CO <sub>2</sub> emissions (Green and Page, 2017) <sup>[[#fn:r551|551]]</sup> . Moreover, in the Congo basin (Dargie et al., 2017) <sup>[[#fn:r552|552]]</sup> and in the Amazonian basin (Draper et al., 2014) <sup>[[#fn:r553|553]]</sup> , the peatlands store the equivalent carbon as that of a tropical forest. However, stored carbon is vulnerable to land-use change and future risk of drought, for example in northeast Brazil ( ''high confidence'' ) (Figure 3.12, Section 3.3.4.2). At the global scale, these peatlands are undergoing rapid major transformations through drainage and burning in preparation for oil palm and other crops or through unintentional burning (Magrin et al., 2014) <sup>[[#fn:r554|554]]</sup> . Wetland salinization, a widespread threat to the structure and ecological functioning of inland and coastal wetlands, is occurring at a high rate and large geographic scale (Section 3.3.6; Herbert et al., 2015) <sup>[[#fn:r555|555]]</sup> . Settele et al. (2014) <sup>[[#fn:r556|556]]</sup> found that rising water temperatures are projected to lead to shifts in freshwater species distributions and worsen water quality. Some of these ecosystems respond non-linearly to changes in temperature. For example, Johnson and Poiani (2016) <sup>[[#fn:r557|557]]</sup> found that the wetland function of the Prairie Pothole region in North America is projected to decline at temperatures beyond a local warming of 2°C–3°C above present-day values (1°C local warming, corresponding to 0.6°C of global warming). If the ratio of local to global warming remains similar for these small levels of warming, this would indicate a global temperature threshold of 1.2°C–1.8°C of warming. Hence, constraining global warming to approximately 1.5°C would maintain the functioning of prairie pothole ecosystems in terms of their productivity and biodiversity, although a 20% increase of precipitation could offset 2°C of global warming ( ''high confidence'' ) (Johnson and Poiani, 2016) <sup>[[#fn:r558|558]]</sup> .

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