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

==== 3.4.3.1 Global Climate Feedbacks ====

<div id="section-3-4-3-1global-climate-feedbacks-block-1"></div>

<span id="carbon-cycle"></span>
===== 3.4.3.1.1 Carbon cycle =====

Climate warming is expected to change the storage of carbon in vegetation and soils in northern regions, and net carbon transferred to the atmosphere as CO 2 and methane acts as a feedback to accelerate global climate change. There is ''high confidence'' that the northern region acted as a net carbon sink as carbon accumulated in terrestrial ecosystems over the Holocene (Loisel et al., 2014 <sup>[[#fn:r1575|1575]]</sup> ; Lindgren et al., 2018 <sup>[[#fn:r1576|1576]]</sup> ). There is ''medium evidence'' with ''low agreement'' whether changing climate in the modern period has shifted these ecosystems into net carbon sources. Syntheses of ecosystem CO 2 fluxes have alternately showed tundra ecosystems as carbon sinks or neutral averaged across the circumpolar region for the 1990s and 2000s (McGuire et al., 2012 <sup>[[#fn:r1577|1577]]</sup> ), or carbon sources over the same time period (Belshe et al., 2013 <sup>[[#fn:r1578|1578]]</sup> ). Both syntheses agree that the summer growing season is a period of net carbon uptake into terrestrial ecosystems ( ''high confidence'' ), and this uptake appears to be increasing as a function of vegetation density/biomass (Ueyama et al., 2013 <sup>[[#fn:r1579|1579]]</sup> ). The discrepancy between these syntheses may be a result of CO 2 release rates during the non-summer season that are now thought to be higher than previously estimated ( ''high confidence'' ) (Webb et al., 2016 <sup>[[#fn:r1580|1580]]</sup> ) or the separation of upland and wetland ecosystem types, which was done in one synthesis but not the other. Moisture status is a primary control over ecosystem carbon sink/source strength with wetlands more often than not still acting as annual net carbon sinks even while methane is emitted (Lund et al., 2010 <sup>[[#fn:r1581|1581]]</sup> ). Recent aircraft measurements of atmospheric CO 2 concentrations over Alaska showed that tundra regions of Alaska were a consistent net CO 2 source to the atmosphere, whereas boreal forest regions were either neutral or net CO 2 sinks for the period 2012–2014 (Commane et al., 2017 <sup>[[#fn:r1582|1582]]</sup> ). That study region as a whole was estimated to be a net carbon source of 25 ± 14 Tg CO 2 -C yr -1 averaged over the land area of both biomes for the entire study period. For comparison to projected global emissions, this would be equivalent to a net source of 0.3 Pg CO 2 -C yr -1 assuming the Alaska study region (1.6 x 10 6 km 2 ) could be scaled to the entire northern circumpolar permafrost region soil area (17.8 x 10 6 km 2 ).

The permafrost soil carbon pool is climate sensitive and an order of magnitude larger than carbon stored in plant biomass (Schuur et al., 2018 <sup>[[#fn:r1583|1583]]</sup> ) ( ''very high confidence'' ). Initial estimates were converging on a range of cumulative emissions from soils to the atmosphere by 2100, but recent studies have actually widened that range somewhat (Figure 3.11) ( ''medium confidence'' ). Expert assessment and laboratory soil incubation studies suggest that substantial quantities of C (tens to hundreds Pg C) could potentially be transferred from the permafrost carbon pool into the atmosphere under RCP8.5 (Schuur et al., 2013 <sup>[[#fn:r1584|1584]]</sup> ; Schädel et al., 2014 <sup>[[#fn:4|4]]</sup> ) . Global dynamical models supported these findings, showing potential carbon release from the permafrost zone ranging from 37–174 Pg C by 2100 under high emission climate warming trajectories, with an average across models of 92 ± 17 Pg C (mean ± SE) (Zhuang et al., 2006 <sup>[[#fn:r1585|1585]]</sup> ; Koven et al., 2011 <sup>[[#fn:r1586|1586]]</sup> ; Schaefer et al., 2011 <sup>[[#fn:r1587|1587]]</sup> ; MacDougall et al., 2012 <sup>[[#fn:r1588|1588]]</sup> ; Burke et al., 2013 <sup>[[#fn:r1589|1589]]</sup> ; Schaphoff et al., 2013 <sup>[[#fn:r1590|1590]]</sup> ; Schneider von Deimling et al., 2015 <sup>[[#fn:r1591|1591]]</sup> ). This range is generally consistent with several newer data-driven modelling approaches that estimated that soil carbon releases by 2100 (for RCP8.5) will be 57 Pg C (Koven et al., 2015 <sup>[[#fn:r1592|1592]]</sup> ) and 87 Pg C (Schneider von Deimling et al., 2015 <sup>[[#fn:r1593|1593]]</sup> ), as well as an updated estimate of 102 Pg C from one of the previous models (MacDougall and Knutti, 2016 <sup>[[#fn:r1594|1594]]</sup> ). However, the latest model runs performed with either structural enhancements to better represent permafrost carbon dynamics (Burke et al., 2017a <sup>[[#fn:r1595|1595]]</sup> ), or common environmental input data (McGuire et al., 2016 <sup>[[#fn:r1596|1596]]</sup> ) show similar soil carbon losses, but also indicate the potential for stimulated plant growth (nutrients, temperature/growing season length, CO 2 fertilisation) to offset some (Kleinen and Brovkin, 2018 <sup>[[#fn:r1597|1597]]</sup> ) or all of these losses, at least during this century, by sequestering new carbon into plant biomass and increasing carbon inputs into the surface soil (McGuire et al., 2018 <sup>[[#fn:r1598|1598]]</sup> ). These future carbon emission levels would be a significant fraction of those projected from fossil fuels with implications for allowable carbon budgets that are consistent with limiting global warming, but will also depend on how vegetation responds ( ''high confidence'' ). Furthermore, there is ''high confidence'' that climate scenarios that involve mitigation (e.g., RCP4.5) will help to dampen the response of carbon emissions from the Arctic and boreal regions.

Northern ecosystems contribute significantly to the global methane budget, but there is ''low confidence'' about the degree to which additional methane from northern lakes, ponds, wetland ecosystems, and the shallow Arctic Ocean shelves is currently contributing to increasing atmospheric concentrations. Analyses of atmospheric concentrations in Alaska concluded that local ecosystems surrounding the observation site have not changed in the exchange of methane from the 1980s until the present, which suggests that either the local wetland ecosystems are responding similarly to other northern wetland ecosystems, or that increasing atmospheric methane concentrations in northern observation sites is derived from methane coming from mid-latitudes (Sweeney et al., 2016 <sup>[[#fn:r1600|1600]]</sup> ). However, this contrasts with indirect integrated estimates of methane emissions from observations of expanding permafrost thaw lakes that suggest a release of an additional 1.6–5 Tg CH 4 yr –1 over the last 60 years (Walter Anthony et al., 2014 <sup>[[#fn:r1601|1601]]</sup> ). At the same time, there is ''high confidence'' that methane fluxes at the ecosystem to regional scale have been under-observed, in part due to the low  solubility of methane in water leading to ebullution (bubbling) flux to the atmosphere that is heterogeneous in time and space. Some new quantifications include: cold-season methane emissions that can be &gt;50% of the annual budget of terrestrial ecosystems (Zona et al., 2016 <sup>[[#fn:r1602|1602]]</sup> ); geological methane seeps that may be climate sensitive if permafrost currently serves as a cap preventing atmospheric release (Walter Anthony et al., 2012 <sup>[[#fn:r1603|1603]]</sup> ; Ruppel and Kessler, 2016 <sup>[[#fn:r1604|1604]]</sup> ; Kohnert et al., 2017 <sup>[[#fn:r1605|1605]]</sup> ); estimates of shallow Arctic Ocean shelf methane emissions where the range of estimates based on methane concentrations in air and water has widened with more observations and now ranges from 3 Tg CH 4 yr –1 (Thornton et al., 2016 <sup>[[#fn:r1606|1606]]</sup> ) to 17 Tg CH 4 yr –1 (Shakhova et al., 2013 <sup>[[#fn:r1607|1607]]</sup> ). Observations such as these underlie the fact that source estimates for methane made from atmospheric observations are typically lower than methane source estimates made from upscaling of ground observations (e.g., Berchet et al., 2016), and this problem has not improved, even at the global scale, over several decades of research (Saunois et al., 2016 <sup>[[#fn:r1608|1608]]</sup> ; Crill and Thornton, 2017 <sup>[[#fn:r1609|1609]]</sup> ).

In many of the dynamical model projections previously discussed, methane release is not explicitly represented because fluxes are small even though higher global warming potential of methane makes these emissions relatively more important than on a mass basis alone. Global models that do include methane show that emissions may already (from 2000 to 2012) be increasing at a rate of 1.2 Tg CH 4 yr –1 in the northern region as a direct response to temperature (Riley et al., 2011 <sup>[[#fn:r1610|1610]]</sup> ; Gao et al., 2013 <sup>[[#fn:r1611|1611]]</sup> ; Poulter et al., 2017 <sup>[[#fn:r1612|1612]]</sup> ). A model intercomparison study forecast northern methane emissions to increase from 18 Tg CH 4 yr –1 to 42 Tg CH 4 yr –1 under RCP8.5 by 2100 largely as a result of an increase in wetland extent (Zhang et al., 2017 <sup>[[#fn:r1613|1613]]</sup> ). However, projected methane emissions are sensitive to changes in surface hydrology (Lawrence et al., 2015 <sup>[[#fn:r1614|1614]]</sup> ) and a suite of models that were thought to perform well in high-latitude ecosystems showed a general soil drying trend even as the overall water cycle intensified (McGuire et al., 2018 <sup>[[#fn:r1615|1615]]</sup> ). Furthermore, most models described above do not include many of the abrupt thaw processes that can result in lake expansion, wetland formation, and massive erosion and exposure to decomposition of previously frozen carbon-rich permafrost, leading to ''medium confidence'' in future model projections of methane. Recent studies that addressed some of these landscape controls over future emissions projected increases in methane above the current levels on the order 10–60 Tg CH 4 yr -1 under RCP8.5 by 2100 (Schuur et al., 2013 <sup>[[#fn:r1616|1616]]</sup> ; Koven et al., 2015 <sup>[[#fn:r1617|1617]]</sup> ; Lawrence et al., 2015 <sup>[[#fn:r1618|1618]]</sup> ; Schneider von Deimling et al., 2015 <sup>[[#fn:r1619|1619]]</sup> ; Walter Anthony et al., 2018 <sup>[[#fn:r1620|1620]]</sup> ). These additional methane fluxes are projected to cause 40–70% of total permafrost-affected radiative forcing in this century even though methane emissions are much less than CO 2 by mass (Schneider von Deimling et al., 2015 <sup>[[#fn:r1621|1621]]</sup> ; Walter Anthony et al., 2018 <sup>[[#fn:r1622|1622]]</sup> ). As with total carbon emissions, there is ''high confidence'' that mitigation of anthropogenic methane sources could help to dampen the impact of increased methane emissions from the Arctic and boreal regions (Christensen et al., 2019 <sup>[[#fn:r1623|1623]]</sup> ).

<div id="section-3-4-3-1global-climate-feedbacks-block-2"></div>

<span id="energy-budget"></span>
===== 3.4.3.1.2 Energy budget =====

Warming induced reductions in the duration and extent of Arctic spring snow cover (Section 3.4.1.1) lower albedo because snow-free land reflects much less solar radiation than snow. The corresponding increase in net radiation absorption at the surface provides a positive feedback to global temperatures (Flanner et al., 2011 <sup>[[#fn:r1624|1624]]</sup> ; Qu and Hall, 2014 <sup>[[#fn:r1625|1625]]</sup> ; Thackeray and Fletcher, 2016 <sup>[[#fn:r1626|1626]]</sup> ) ( ''high confidence'' ). Estimates of increases in global net solar energy flux due to snow cover loss range from 0.10–0.22 W m –2 (± 50%; ''medium confidence'' ) depending on dataset and time period (Flanner et al., 2011 <sup>[[#fn:r1627|1627]]</sup> ; Chen et al., 2015 <sup>[[#fn:r1628|1628]]</sup> ; Singh et al., 2015 <sup>[[#fn:r1629|1629]]</sup> ; Chen et al., 2016b <sup>[[#fn:r1630|1630]]</sup> ). Sources of uncertainty include the range in observed spring snow cover extent trends (Hori et al., 2017 <sup>[[#fn:r1631|1631]]</sup> ) and the influence of clouds on shortwave feedbacks (Sedlar, 2018 <sup>[[#fn:r1632|1632]]</sup> ; Sledd and L’Ecuyer, 2019 <sup>[[#fn:r1633|1633]]</sup> ). Terrestrial snow changes also affect the longwave energy budget via altered surface emissivity (Huang et al., 2018 <sup>[[#fn:r1634|1634]]</sup> ). Climate model simulations show that changes in snow cover dominate land surface related positive feedbacks to atmospheric heating (Euskirchen et al., 2016 <sup>[[#fn:r1635|1635]]</sup> ), but regional variations in surface albedo are also influenced by vegetation (Loranty et al., 2014 <sup>[[#fn:r1636|1636]]</sup> ). There is evidence for positive sensitivity of surface temperatures to increased northern hemisphere boreal and tundra leaf area index, which contributes a positive feedback to warming (Forzieri et al., 2017 <sup>[[#fn:r1637|1637]]</sup> ).

<div id="section-3-4-3-2ecosystems-and-their-services"></div>

<span id="ecosystems-and-their-services"></span>