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

==== 3.4.3.2 Ecosystems and their Services ====

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===== 3.4.3.2.1 Vegetation =====

Changes in tundra vegetation can have important ecosystem effects, in particular on hydrology, carbon and nutrient cycling and surface energy balance, which together impact permafrost (e.g., Myers-Smith and Hik, 2013; Frost and Epstein, 2014 <sup>[[#fn:r1638|1638]]</sup> ; Nauta et al., 2014 <sup>[[#fn:r1639|1639]]</sup> ). Aside from physical impacts, changing vegetation influences the diversity and abundance of herbivores (e.g., Fauchald et al., 2017b; Horstkotte et al., 2017 <sup>[[#fn:r1640|1640]]</sup> ) in the Arctic. The overall trend for tundra vegetation across the 36–year satellite period (1982–2017) shows increasing above ground biomass (greening) throughout a majority of the circumpolar Arctic ( ''high confidence'' ) (Xu et al., 2013a <sup>[[#fn:r1641|1641]]</sup> ; Ju and Masek, 2016 <sup>[[#fn:r1642|1642]]</sup> ; Bhatt et al., 2017 <sup>[[#fn:r1643|1643]]</sup> ). Increasing greenness has been in some cases linked with shifts in plant species dominance away from graminoids (grasses and sedges) towards shrubs ( ''high confidence'' ) (Myers-Smith et al., 2015 <sup>[[#fn:r1644|1644]]</sup> ). Within the overall trend of increases (greening), some tundra areas show declines (browning) (Bhatt et al., 2017).

The spatial variation in greening and browning trends in tundra are also not consistent over time (decadal scale) and can vary across landform/ecosystem types (Lara et al., 2018 <sup>[[#fn:r1645|1645]]</sup> ), suggesting interactions between the changing environment and the biological components of the system that control these trends. There is ''high confidence'' that increases in summer, spring and winter temperatures lead to tundra greening, as well as increases in growing season length (e.g., Vickers et al., 2016; Myers-Smith and Hik, 2018 <sup>[[#fn:r1646|1646]]</sup> ) that are in part linked to reductions in Arctic Ocean sea ice cover (Bhatt et al., 2017 <sup>[[#fn:r1647|1647]]</sup> ; Macias-Fauria et al., 2017 <sup>[[#fn:r1648|1648]]</sup> ). Other factors that stimulate tundra greening include increases in snow water equivalent and soil moisture (Westergaard-Nielsen et al., 2017 <sup>[[#fn:r1649|1649]]</sup> ), increases in active layer thickness (via nutrient availability or changes in moisture), changes in herbivore activity, and to a lesser degree, human use of the land (e.g., Salmon et al., 2016; Horstkotte et al., 2017; Martin et al., 2017; Yu et al., 2017). Research on tundra browning is more limited but suggests causal mechanisms that include changes in winter climate—specifically reductions in snow cover due to winter warming events that expose tundra to subsequent freezing and desiccation—, insect and pathogen outbreaks, increased herbivore grazing and ground ice melting and subsidence that increases surface water (Phoenix and Bjerke, 2016; Bjerke et al., 2017) ( ''medium confidence'' ).

Projections of tundra vegetation distribution across the Arctic by 2050 in response to changing environmental conditions suggest that the areal extent of most tundra types will decrease by at least 50% (Pearson et al., 2013) ( ''medium confidence'' ). Woody shrubs and trees are projected to expand to cover 24–52% of the current tundra region by 2050, or 12–33% if tree dispersal is restricted. Adding to this, the expansion of fire into tundra that has not experienced large-scale disturbance for centuries causes large reductions in soil carbon stocks (Mack et al., 2011), shifts in vegetation composition and productivity (Bret-Harte et al., 2013), and can lead to widespread permafrost degradation (Jones et al., 2015a) at faster rates than would occur by changing environmental conditions alone. In tundra regions, graminoid (grasses and sedges) tundra is projected to be replaced by more flammable shrub tundra in future climate scenarios, and tree migration into tundra could further increase fuel loading (Pastick et al., 2017) ( ''medium confidence'' ).

Similar to tundra, boreal forest vegetation shows trends of both greening and browning over multiple years in different regions across the satellite record (Beck and Goetz, 2011; Ju and Masek, 2016) ( ''high confidence'' ). Here, patterns of changing vegetation are a result of direct responses to changes in climate (temperature, precipitation and seasonality) and other driving factors for vegetation (nutrients, disturbance) similar to what has been reported in tundra. While boreal forest may expand at the northern edge (Pearson et al., 2013), climate projections suggest that it could diminish at the southern edge and be replaced by lower biomass woodland/shrublands (Koven, 2013; Gauthier et al., 2015) ( ''medium confidence'' ). Furthermore, changes in fire disturbance are leading to shifts in landscape distribution of early and late successional ecosystem types, which is also a major factor in satellite trends. Fires that burn deeply into the organic soil layer can alter permafrost stability, hydrology and vegetation. Loss of the soil organic layer exposes mineral soil seedbeds (Johnstone et al., 2009), leading to recruitment of deciduous tree and shrub species that do not establish on organic soil (Johnstone et al., 2010). This recruitment has been shown to shift post-fire vegetation to alternate successional trajectories (Johnstone et al., 2010). Model projections suggest that Alaskan boreal forest soon may cross a point where recent increases in fire activity have made deciduous stands as abundant as spruce stands on the landscape (Mann et al., 2012) ( ''medium confidence'' ). This projected trend of increasing deciduous forest at the expense of evergreen forest is mirrored in Russian and Chinese boreal forests as well (Shakhova et al., 2013; Shuman et al., 2015; Wu et al., 2017) ( ''medium confidence'' ).

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'''Figure 3.11'''

<span id="figure-3.11-estimates-of-cumulative-net-soil-carbon-pool-change-for-the-northern-circumpolar-permafrost-region-by-2100-following-medium-and-high-emission-scenarios-e.g.-representative-concentration-pathway-rcp4.5-and-rcp8.5-or-equivalent.-cumulative-carbon-amounts-are-shown-in-gigatons-c-1-gt-c1-billion-metric-tonnes-with-source-negative-values-indicating-net-carbon"></span>
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'''Figure 3.11 | Estimates of cumulative net soil carbon pool change for the northern circumpolar permafrost region by 2100 following medium and high emission scenarios (e.g., Representative Concentration Pathway (RCP)4.5 and RCP8.5 or equivalent). Cumulative carbon amounts are shown in Gigatons C (1 Gt C=1 billion metric tonnes), with source (negative values) indicating net carbon […]'''
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[[File:3a8fa4a00a17dd1acc4f3014b3ee2577 IPCC-SROCC-CH_3_11.jpg]]

Figure 3.11 | Estimates of cumulative net soil carbon pool change for the northern circumpolar permafrost region by 2100 following medium and high emission scenarios (e.g., Representative Concentration Pathway (RCP)4.5 and RCP8.5 or equivalent). Cumulative carbon amounts are shown in Gigatons C (1 Gt C=1 billion metric tonnes), with source (negative values) indicating net carbon movement from soil to the atmosphere and sink (positive values) indicating the reverse. Some data-constrained models differentiated CO2 and CH4; bars show total carbon by weight, paired bars with * indicate CO2-equivalent, which takes into account the global warming potential of CH4. Ensemble mean bars refer to the model average for the Permafrost Carbon Model Intercomparison Project [5 models]. Bars that do not start at zero are in part informed by expert assessment and are shown as ranges; all other bars represent model mean estimates. Data are from 1 (Schuur et al., 2013 <sup>[[#fn:r1650|1650]]</sup> ); 2 (Schaefer et al., 2014 <sup>[[#fn:r1651|1651]]</sup> ) [8 models]; 3 (Schuur et al., 2015 <sup>[[#fn:r1652|1652]]</sup> ); 4 (Koven et al., 2015 <sup>[[#fn:r1653|1653]]</sup> ; Schneider von Deimling et al., 2015 <sup>[[#fn:r1654|1654]]</sup> ; Walter Anthony et al., 2018 <sup>[[#fn:r1655|1655]]</sup> ); 5 (MacDougall and Knutti, 2016 <sup>[[#fn:r1656|1656]]</sup> ; Burke et al., 2017a <sup>[[#fn:r1657|1657]]</sup> ; Kleinen and Brovkin, 2018 <sup>[[#fn:r1658|1658]]</sup> ); 6 (McGuire et al., 2018 <sup>[[#fn:r1659|1659]]</sup> ).

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<span id="wildlife"></span>
===== 3.4.3.2.2 Wildlife =====

Reindeer and caribou ( ''Rangifer tarandus'' ), through their numbers and ecological role as a large-bodied herbivores, are a key driver of Arctic ecology. The seasonal migrations that characterise ''Rangifer'' link the coastal tundra to the continental boreal forests for some herds, while others live year-round on the tundra. Population estimates and trends exist for most herds, and indicate that pan-Arctic migratory tundra ''Rangifer'' have declined from about 5 million in the 1990s to about 2 million in 2017 (Gunn, 2016 <sup>[[#fn:r1682|1682]]</sup> ; Fauchald et al., 2017a <sup>[[#fn:r1683|1683]]</sup> ) ( ''high confidence'' ). Numbers have recently increased for two Alaska herds and the Porcupine caribou herd straddling Yukon and Alaska is at a historic high.

There is ''low confidence'' in understanding the complex drivers of observed ''Rangifer'' changes. Hunting and predation (the latter exacerbated by modification of the landscape for exploration and resource extraction; Dabros et al., 2018 <sup>[[#fn:r1684|1684]]</sup> ) increase in importance as populations decline. Climate strongly influences productivity: extremes in heat, drought, winter icing and snow depth reduce ''Rangifer'' survival (Mallory and Boyce, 2017 <sup>[[#fn:r1685|1685]]</sup> ). Changes in the timing of sea ice formation have direct effects on risks during ''Rangifer'' migration via inter-island movement and connection to the mainland (Poole et al., 2010 <sup>[[#fn:r1686|1686]]</sup> ). Summer warming is changing the composition of tundra plant communities, modifying the relationship between climate, forage and ''Rangifer'' (Albon et al., 2017 <sup>[[#fn:r1687|1687]]</sup> ), which also impacts other Arctic species such as musk ox ( ''Ovibos moschatus)'' (Schmidt et al., 2015 <sup>[[#fn:r1688|1688]]</sup> ). As polar trophic systems are highly connected (Schmidt et al., 2017 <sup>[[#fn:r1689|1689]]</sup> ), changes will propagate through the ecosystem with effects on other herbivores such as geese and voles, as well as predators such as wolves (Hansen et al., 2013 <sup>[[#fn:r1690|1690]]</sup> ; Klaczek et al., 2016 <sup>[[#fn:r1691|1691]]</sup> ).

In northern Fennoscandia, there are approximately 600,000 semi-domesticated reindeer. Lichen rangelands are key to sustaining reindeer carrying capacity, with variable response to climate change: enhanced summer precipitation increases lichen biomass, while an increase in winter precipitation lowers it (Kumpula et al., 2014 <sup>[[#fn:r1692|1692]]</sup> ). Fire disturbance reduces the amount of pasture available for domestic reindeer and increases predation on herding lands (Lavrillier and Gabyshev, 2017 <sup>[[#fn:r1693|1693]]</sup> ). Later ice formation on waterbodies can impact herding activities (Turunen et al., 2016 <sup>[[#fn:r1694|1694]]</sup> ). Ice formation from rain-on-snow events is associated with population changes including cases of catastrophic mass starvation (Bartsch et al., 2010 <sup>[[#fn:r1695|1695]]</sup> ; Forbes et al., 2016 <sup>[[#fn:r1696|1696]]</sup> ), but there is no evidence of trends in rain-on-snow events (Cohen et al., 2015 <sup>[[#fn:r1697|1697]]</sup> ; Dolant et al., 2017 <sup>[[#fn:r1698|1698]]</sup> ).

Management of keystone species requires an understanding of pathogens and disease in the context of climate warming, but evidence of changing patterns across northern ecosystems (spanning terrestrial, aquatic, and marine environments) is hindered by an incomplete picture of pathogen diversity and distribution (Hoberg, 2013 <sup>[[#fn:r1699|1699]]</sup> ; Jenkins et al., 2013 <sup>[[#fn:r1700|1700]]</sup> ; Cook et al., 2017 <sup>[[#fn:r1701|1701]]</sup> ). Among ungulates, it is ''virtually certain'' that the emergence of disease attributed to nematode pathogens has accelerated since 2000 in the Canadian Arctic islands and Fennoscandia (Kutz et al., 2013 <sup>[[#fn:r1702|1702]]</sup> ; Hoberg and Brooks, 2015 <sup>[[#fn:r1703|1703]]</sup> ; Laaksonen et al., 2017 <sup>[[#fn:r1704|1704]]</sup> ; Kafle et al., 2018 <sup>[[#fn:r1705|1705]]</sup> ). Discovery of the pathogenic bacterium ''Erysipelothrix rhusiopathiae'' has been linked to massive and widespread mortality among muskoxen from the Canadian Arctic Archipelago; loss of &gt;50% of the population since 2010 may be attributable to disease interacting with extreme temperature events, although unequivocal links to climate have not been established (Kutz et al., 2015 <sup>[[#fn:r1706|1706]]</sup> ; Forde et al., 2016a <sup>[[#fn:r1707|1707]]</sup> ; Forde et al., 2016b <sup>[[#fn:r1708|1708]]</sup> ). Anthrax is projected to expand northward in response to warming, and resulted in substantial mortality events for reindeer on the Yamal Peninsula of Russia in 2016 with mobilisation of bacteria possibly from a frozen reindeer carcass or melting permafrost (Walsh et al., 2018 <sup>[[#fn:r1709|1709]]</sup> ). In concert with climate forcing, pathogens are ''very likely'' responsible for increasing mortality in Arctic ungulates (muskox, caribou/reindeer) and alteration of transmission patterns in marine food chains, broadly threatening sustainability of subsistence and commercial hunting and fishing and safety of traditional foods for northern cultures at high latitudes (Jenkins et al., 2013 <sup>[[#fn:r1710|1710]]</sup> ; Kutz et al., 2014 <sup>[[#fn:r1711|1711]]</sup> ; Hoberg et al., 2017 <sup>[[#fn:r1712|1712]]</sup> ).

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<span id="freshwater"></span>
===== 3.4.3.2.3 Freshwater =====

Climate-driven changes in seasonal ice and permafrost conditions influence water quality ( ''high confidence'' ). Shortened duration of freshwater ice cover (more light absorption, increased nutrient input) is expected to result in higher primary productivity (Hodgson and Smol, 2008 <sup>[[#fn:r1713|1713]]</sup> ; Vincent et al., 2011 <sup>[[#fn:r1714|1714]]</sup> ; Griffiths et al., 2017b <sup>[[#fn:r1715|1715]]</sup> ) and may also encourage greater methane emissions from Arctic lakes (Greene et al., 2014 <sup>[[#fn:r1716|1716]]</sup> ; Tan and Zhuang, 2015 <sup>[[#fn:r1717|1717]]</sup> ). Thaw slumps, active layer detachments and peat plateau collapse affect surface water connectivity (Connon et al., 2014 <sup>[[#fn:r1718|1718]]</sup> ) and enhance sediment, particulate and solute fluxes in river and stream networks (Kokelj et al., 2013 <sup>[[#fn:r1719|1719]]</sup> ). The transfer of enhanced nutrients from land to water (driven by active layer thickening and thermokarst processes; Abbott et al., 2015 <sup>[[#fn:r1720|1720]]</sup> ; Vonk et al., 2015 <sup>[[#fn:r1721|1721]]</sup> ) has been linked to heightened autotrophic productivity in freshwater ecosystems (Wrona et al., 2016 <sup>[[#fn:r1722|1722]]</sup> ). Still, there is ''low confidence'' in the influence of permafrost changes on dissolved organic carbon, because of competing mechanisms that influence carbon export. Permafrost thaw could contribute to the mobilisation of previously frozen organic carbon (Abbott et al., 2014 <sup>[[#fn:r1723|1723]]</sup> ; Wickland et al., 2018 <sup>[[#fn:r1724|1724]]</sup> ; Walvoord et al., 2019 <sup>[[#fn:r1725|1725]]</sup> ) thereby enhancing both particulate and dissolved organic carbon export to aquatic systems. Increased delivery of this dissolved carbon from enhanced river discharge to the Arctic Ocean (Section 3.4.3.1.2) can exacerbate regionally extreme aragonite undersaturation of shelf waters (Semiletov et al., 2016 <sup>[[#fn:r1726|1726]]</sup> ) driven by ocean uptake of anthropogenic CO 2 (Section 3.2.1.2.4). Conversely, reduced dissolved organic carbon export could accompany permafrost thaw as (1) water infiltrates deeper and has longer residence times for decomposition (Striegl et al., 2005 <sup>[[#fn:r1727|1727]]</sup> ) and (2) the proportion of groundwater (typically lower in dissolved organic carbon and higher in DIC than runoff) to total streamflow increases (Walvoord and Striegl, 2007 <sup>[[#fn:r1728|1728]]</sup> ). Increased thermokarst also has the potential to impact freshwater cycling of inorganic carbon (Zolkos et al., 2018 <sup>[[#fn:r1729|1729]]</sup> ).

Enhanced subsurface water fluxes resulting from permafrost degradation has consequences for inorganic natural and anthropogenic constituents. Emerging evidence suggests large natural stores of mercury (Schuster et al., 2018 <sup>[[#fn:r1730|1730]]</sup> ; St Pierre et al., 2018 <sup>[[#fn:r1731|1731]]</sup> ) and other trace elements in permafrost (Colombo et al., 2018 <sup>[[#fn:r1732|1732]]</sup> ) may be released upon thaw, thereby having effects (largely unknown at this point) on aquatic ecosystems. In parallel, increased development activity in the Arctic is ''likely'' to lead to enhanced local sources of anthropogenic chemicals of emerging Arctic concern, including siloxanes, parabens, flame retardants, and per- and polyfluoroalkyl substances (AMAP, 2017c <sup>[[#fn:r1733|1733]]</sup> ). For legacy pollutants, there is ''high confidence'' that black carbon and persistent organic pollutants (e.g., hexachlorocyclohexanes, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls) can be transferred downstream and affect water quality (Hodson, 2014 <sup>[[#fn:r1734|1734]]</sup> ). Lakes can become sinks of these contaminants, while floodplains can be contaminated (Sharma et al., 2015).

There is ''high confidence'' that habitat loss or change due to climate change impact Arctic fishes. Thinning ice on lakes and streams changes the overwintering habitat for aquatic fauna by impacting winter water volumes and dissolved oxygen levels (Leppi et al., 2016 <sup>[[#fn:r1735|1735]]</sup> ). Surface water loss, reduced surface water connectivity among aquatic habitats, and changes to the timing and magnitude of seasonal flows (Section 3.4.1.2) result in a direct loss of spawning, feeding, or rearing habitats (Poesch et al., 2016 <sup>[[#fn:r1736|1736]]</sup> ). Changes to permafrost landscapes have reduced freshwater habitat available for fish and other aquatic biota, including aquatic invertebrates upon which the fish depend for food (Chin et al., 2016 <sup>[[#fn:r1737|1737]]</sup> ). Gullying deepens channels (Rowland et al., 2011 <sup>[[#fn:r1738|1738]]</sup> ; Liljedahl et al., 2016 <sup>[[#fn:r1739|1739]]</sup> ) that otherwise may connect lake habitats occupied by fishes. This can lead to the loss of surface water connectivity, limit fish access to key habitats, and lower fish diversity (Haynes et al., 2014 <sup>[[#fn:r1740|1740]]</sup> ; Laske et al., 2016 <sup>[[#fn:r1741|1741]]</sup> ). Small connecting stream channels, which are vulnerable to drying, provide necessary migratory pathways for fishes, allowing them to access spawning and summer rearing grounds (Heim et al., 2016 <sup>[[#fn:r1742|1742]]</sup> ; McFarland et al., 2017 <sup>[[#fn:r1743|1743]]</sup> ).

Changes to the timing, duration and magnitude of high surface flow events in early and late summer threaten Arctic fish dispersal and migration activities (Heim et al., 2016 <sup>[[#fn:r1744|1744]]</sup> ) ( ''high confidence'' ). Timing of important life history events such as spawning can become mismatched with changing stream flows (Lique et al., 2016 <sup>[[#fn:r1745|1745]]</sup> ). There is regional evidence that migration timing has shifted earlier and winter egg incubation temperature has increased for pink salmon ( ''Oncorhynchus gorbuscha'' ), directly related to warming (Taylor, 2007 <sup>[[#fn:r1746|1746]]</sup> ). While long-term, pan-Arctic data on run timing of fishes are limited, phenological shifts could create mismatches with food availability or habitat suitability in both marine and freshwater environments for anadromous species, and in freshwater environments for freshwater resident species. Changes to the Arctic growing season (Xu et al., 2013a <sup>[[#fn:r1747|1747]]</sup> ) increase the risk of drying of surface water habitats and pose a potential mismatch in seasonal availability of food in rearing habitats.

Freshwater systems across the Arctic are relatively shallow, and thus are expected to warm ( ''high confidence'' ). This may make some surface waters inhospitably warm for cold water species such as Arctic Grayling ( ''Thymallus arcticus'' ) and whitefishes ( ''Coregonus spp'' .), or may increase the risk of ''Saprolegnia'' ''fungus'' that appears to have recently spread rapidly, infecting whitefishes at much higher rates in Arctic Alaska than noted in the past (Sformo et al., 2017 <sup>[[#fn:r1748|1748]]</sup> ). High infection rates may be driven by stress or nutrient enrichment from thawing permafrost, which increases pathogen virulence with fish (Wedekind et al., 2010 <sup>[[#fn:r1749|1749]]</sup> ). Warmer water and longer growing seasons will also affect food abundance because invertebrate life histories and production are temperature and degree-day dependent (Régnière et al., 2012 <sup>[[#fn:r1750|1750]]</sup> ). Increased nutrient export from permafrost loss (Frey et al., 2007 <sup>[[#fn:r1751|1751]]</sup> ), facilitated by warmer temperatures, will ''likely'' increase food resources for consumers, but the impact on lower trophic levels within food webs is not clearly understood.

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