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

=== 3.3.3 Consequences and Impacts ===

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==== 3.3.3.1 Sea Level ====

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Chapter 4 assesses the sea level impacts from observed and projected changes in ice sheets (Section 3.3.1) and polar glaciers (Section 3.3.2), including uncertainties related to marine ice sheets (Cross-Chapter Box 8 in Chapter 3).

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==== 3.3.3.2 Physical Oceanography ====

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The major large-scale impacts of freshwater release from Greenland on ocean circulation relate to the potential modulation/inhibition of the formation of water masses that represent the headwaters of the Atlantic Meridional Overturning Circulation. The timescales and likelihood of such effects are assessed separately in Chapter 6 (Section 6.7). Freshwater release also affects local circulation within fjords through two principle mechanisms; subglacial release from tidewater glaciers enhances buoyancy driven circulation, whereas runoff from land-terminating glaciers contributes to surface layer freshening and estuarine circulation (Straneo and Cenedese, 2015 <sup>[[#fn:r1229|1229]]</sup> ). There is ''limited evidence'' that freshening occurred between 2003 and 2015 in North East Greenland fjords and coastal waters (Sejr et al., 2017 <sup>[[#fn:r1230|1230]]</sup> ).

For Antarctica, freshwater input to the ocean from the ice sheet is divided approximately equally between melting of calved icebergs and of ice shelves ''in situ'' (Depoorter et al., 2013 <sup>[[#fn:r1231|1231]]</sup> ; Rignot et al., 2014 <sup>[[#fn:r1232|1232]]</sup> ). There is ''high confidence'' that the input of ice shelf meltwater has increased in the Amundsen and Bellingshausen Seas since the 1990s, but ''low confidence'' in trends in other sectors (Paolo et al., 2015 <sup>[[#fn:r1233|1233]]</sup> ).

Freshwater injected from the AIS affect water mass circulation and transformation, though sea ice dominates upper ocean properties away from the Antarctic ice shelves (Abernathey et al., 2016 <sup>[[#fn:r1234|1234]]</sup> ; Haumann et al., 2016 <sup>[[#fn:r1235|1235]]</sup> ). Over the ice shelf regions, where dense waters sink and flood the global ocean abyss, the role of glacial freshwater input is clearer. From 1980 to 2012, the salinity of Antarctic Bottom Water reduced by an amount equivalent to 73 ± 26 Gt y –1 of freshwater added, around half the estimated increase in freshwater input by Antarctic glacial discharge up to that time (Purkey and Johnson, 2013 <sup>[[#fn:r1236|1236]]</sup> ). In some places, notably the Indian-Australian sector, Antarctic Bottom Water freshening may be accelerating (Menezes et al., 2017 <sup>[[#fn:r1237|1237]]</sup> ). There is ''medium confidence'' in an overall freshening trend and ''low confidence'' that this is accelerating, given the sparsity of information and significant interannual variability in Antarctic Bottom Water properties at other export locations (Meijers et al., 2016 <sup>[[#fn:r1238|1238]]</sup> ).

For the Southern Ocean, there is ''limited evidence'' for stratification changes in the post-AR5 period, and ''low confidence'' in how stratification changes are affecting sea ice and basal ice shelf melt. An increase in stratification caused by release of freshwater from the AIS was invoked as a mechanism to suppress vertical heat flux and permit an increase in sea ice extent (Bintanja et al., 2013 <sup>[[#fn:r1239|1239]]</sup> ; Bronselaer et al., 2018 <sup>[[#fn:r1240|1240]]</sup> ; Purich et al., 2018 <sup>[[#fn:r1241|1241]]</sup> ), though some studies conclude that glacial freshwater input is insufficient to cause a significant sea ice expansion (Swart and Fyfe, 2013 <sup>[[#fn:r1242|1242]]</sup> ; Pauling et al., 2017 <sup>[[#fn:r1243|1243]]</sup> ) (Section 3.2.1.1). In contrast, where warm water intrusions drive melting within ice shelf cavities, a significant entrained heat flux to the surface can exist and increase stratification and potentially reduce sea ice extent (Jourdain et al., 2017 <sup>[[#fn:r1244|1244]]</sup> ; Merino et al., 2018 <sup>[[#fn:r1245|1245]]</sup> ). It has been argued that freshening from glacial melt can enhance basal melting of ice shelves by reducing dense water production and modulating oceanic heat flow into ice shelf cavities (Silvano et al., 2018 <sup>[[#fn:r1246|1246]]</sup> ).

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==== 3.3.3.3 Biogeochemistry ====

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Both polar ice sheets have the potential to release dissolved and sediment-bound nutrients and organic carbon directly to the surface ocean via subglacial and surface meltwater, icebergs, melting of the base of ice shelves (Shadwick et al., 2013 <sup>[[#fn:r1247|1247]]</sup> ; Wadham et al., 2013 <sup>[[#fn:r1248|1248]]</sup> ; Hood et al., 2015 <sup>[[#fn:r1249|1249]]</sup> ; Herraiz-Borreguero et al., 2016 <sup>[[#fn:r1250|1250]]</sup> ; Raiswell et al., 2016 <sup>[[#fn:r1251|1251]]</sup> ; Yager et al., 2016; Hodson et al., 2017), in addition to indirectly stimulating nutrient input via upwelling associated with subglacial meltwater plumes (Meire et al., 2016b; Cape et al., 2018 <sup>[[#fn:r1272|1272]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1253|1253]]</sup> ; Kanna et al., 2018 <sup>[[#fn:r1254|1254]]</sup> ) (Figure 3.9). These nutrient additions stimulate primary production in the surrounding ocean waters in some regions ( ''medium confidence'' ) (Gerringa et al., 2012 <sup>[[#fn:r1255|1255]]</sup> ; Death et al., 2014 <sup>[[#fn:r1256|1256]]</sup> ; Duprat et al., 2016 <sup>[[#fn:r1257|1257]]</sup> ; Arrigo et al., 2017b <sup>[[#fn:r1258|1258]]</sup> ). There is also some evidence to support melting ice sheets as source of contaminants (AMAP, 2015 <sup>[[#fn:r1259|1259]]</sup> ).

In Greenland, direct measurements suggest that meltwater is a significant source of bioavailable silica and iron (Bhatia et al., 2013 <sup>[[#fn:r1260|1260]]</sup> ; Hawkings et al., 2014 <sup>[[#fn:r1261|1261]]</sup> ; Meire et al., 2016a <sup>[[#fn:r1262|1262]]</sup> ; Hawkings et al., 2017 <sup>[[#fn:r1263|1263]]</sup> ) but may be less important for the supply of bioavailable forms of dissolved nitrogen or phosphorous (Hawkings et al., 2016 <sup>[[#fn:r1264|1264]]</sup> ; Wadham et al., 2016 <sup>[[#fn:r1265|1265]]</sup> ), which often limit the integrated primary production during summer in fjords (Meire et al., 2016a <sup>[[#fn:r1266|1266]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1267|1267]]</sup> ). The offshore export of iron, however, has been linked to primary productivity in surface ocean waters in the Labrador Sea (Arrigo et al., 2017b <sup>[[#fn:r1268|1268]]</sup> ) ( ''limited evidence, high agreement'' ).

Subglacial meltwater plumes from tidewater glaciers have emerged recently as an important indirect source of nutrients to fjords, by entraining nutrient-replete seawater (Meire et al., 2016b <sup>[[#fn:r1269|1269]]</sup> ; Meire et al., 2017 <sup>[[#fn:r1270|1270]]</sup> ; Cape et al., 2018 <sup>[[#fn:r1271|1271]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1272|1272]]</sup> ; Kanna et al., 2018 <sup>[[#fn:r1273|1273]]</sup> ) ( ''medium evidence, high agreement'' ). There is ''medium evidence'' with ''high agreement'' that these upwelled nutrient fluxes enhance primary production in fjords over a distance of up to 100 km along the trajectory of the outflowing plume (Juul-Pedersen et al., 2015 <sup>[[#fn:r1274|1274]]</sup> ; Cape et al., 2018 <sup>[[#fn:r1275|1275]]</sup> ; Kanna et al., 2018 <sup>[[#fn:r1276|1276]]</sup> ). ''' '''

In Antarctica ''',''' there is ''medium evidence'' with ''high agreement'' that enhanced input of iron from ice shelves, glacial meltwater and icebergs stimulates primary production in polynyas, coastal regions and the wider Southern Ocean (Gerringa et al., 2012 <sup>[[#fn:r1277|1277]]</sup> ; Shadwick et al., 2013 <sup>[[#fn:r1278|1278]]</sup> ; Herraiz-Borreguero et al., 2016 <sup>[[#fn:r1279|1279]]</sup> ). Satellite observations and modelling also indicate variable potential for icebergs to fertilise the Southern Ocean beyond the coastal zone (Death et al., 2014 <sup>[[#fn:r1280|1280]]</sup> ; Duprat et al., 2016 <sup>[[#fn:r1281|1281]]</sup> ; Wu and Hou, 2017 <sup>[[#fn:r1282|1282]]</sup> ).

Dissolved nutrient fluxes from ice sheets may be increasing during high melt years (Hawkings et al., 2015 <sup>[[#fn:r1283|1283]]</sup> ). The dominant sediment-bound fraction, however, may not increase with rising melt (Hawkings et al., 2015 <sup>[[#fn:r1284|1284]]</sup> ). Thus, there is ''low confidence'' overall in the magnitude of the response of direct nutrient fluxes from ice sheets to enhanced melting.

Future predictions of nutrient cycling proximal to ice sheets is made more challenging by the landward progression of marine-terminating glaciers and the collapse of ice shelves (Cook et al., 2016 <sup>[[#fn:r1285|1285]]</sup> ). This has the potential to drive major shifts in nutrient supply to coastal waters (Figure 3.9). The erosion of newly exposed glacial sediments in front of retreating land-terminating glaciers (Monien et al., 2017 <sup>[[#fn:r1286|1286]]</sup> ) and changes in the diffuse nutrient fluxes from newly exposed glacial sediments on the seafloor (Wehrmann et al., 2014 <sup>[[#fn:r1287|1287]]</sup> ) may amplify nutrient supply, whilst other nutrient sources may be cut off (e.g., icebergs, upwelling of marine water; Meire et al., 2017 <sup>[[#fn:r1288|1288]]</sup> ) ( ''low confidence'' ) ''.''

There is ''medium evidence'' with ''high agreement'' that long-term tidewater glacier retreat into shallower water or onto land, a plausible scenario for about 55% of the 243 distinct outlet glaciers in Greenland (Morlighem et al., 2017 <sup>[[#fn:r1289|1289]]</sup> ), will reduce or diminish upwelling a source of nutrients, thereby reducing summer productivity in Greenland fjord ecosystems (Meire et al., 2017 <sup>[[#fn:r1290|1290]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1291|1291]]</sup> ).

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'''Figure 3.9'''

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'''Potential shifts in nutrient fluxes with landward retreat of marine-terminating glaciers (a) at different stages (b and c).'''
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[[File:458cf80bd938340c10b5033b1a5c10c3 IPCC-SROCC-CH_3_9.jpg]]

Potential shifts in nutrient fluxes with landward retreat of marine-terminating glaciers (a) at different stages (b and c).

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==== 3.3.3.4 Ecosystems ====

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For Greenland and Svalbard, there is ''limited evidence'' with ''high agreement'' that the retreat of marine-terminating glaciers will alter food supply to higher trophic levels of marine food webs (Meire et al., 2017 <sup>[[#fn:r1292|1292]]</sup> ; Milner et al., 2017 <sup>[[#fn:r1293|1293]]</sup> ). The consequences of changes in glacial systems on marine ecosystems are often mediated via the fjordic environments that fringe the edge of the ice sheets, for example changing physical-chemical conditions have affected the benthic ecosystems of Arctic fjords (Bourgeois et al., 2016 <sup>[[#fn:r1294|1294]]</sup> ). The amplification of nutrient fluxes caused by enhanced upwelling at calving fronts (Meire et al., 2017 <sup>[[#fn:r1295|1295]]</sup> ), combined with high carbon/nutrient burial and recycling rates (Wehrmann et al., 2013 <sup>[[#fn:r1296|1296]]</sup> ; Smith et al., 2015 <sup>[[#fn:r1297|1297]]</sup> ), plays an important role in sustaining high productivity of the Arctic fjord ecosystems of Greenland and Svalbard (Lydersen et al., 2014 <sup>[[#fn:r1298|1298]]</sup> ). Glacier retreat, causing glaciers to shift from being marine-terminating to land-terminating, can reduce the productivity in coastal areas off Greenland with potentially large ecological implications, also negatively affecting production of commercially harvested fish (Meire et al., 2017 <sup>[[#fn:r1299|1299]]</sup> ). There is also evidence that marine-terminating glaciers are important feeding areas for marine mammals and seabirds at Greenland (Laidre et al., 2016 <sup>[[#fn:r1300|1300]]</sup> ) and Svalbard (Lydersen et al., 2014 <sup>[[#fn:r1301|1301]]</sup> ).

For Antarctica ''',''' there is ''high agreement'' based on ''medium evidence'' that ice shelf retreat or collapse is leading to new marine habitats and to biological colonisation (Gutt et al., 2011 <sup>[[#fn:r1302|1302]]</sup> ; Fillinger et al., 2013 <sup>[[#fn:r1303|1303]]</sup> ; Trathan et al., 2013 <sup>[[#fn:r1304|1304]]</sup> ; Hauquier et al., 2016 <sup>[[#fn:r1305|1305]]</sup> ; Ingels et al., 2018 <sup>[[#fn:r1306|1306]]</sup> ). The loss of ice shelves and retreat of coastal glaciers around the AP in the last 50 years has exposed at least 2.4 × 10 4 km 2 of new open water. These newly-revealed habitats have allowed new phytoplankton blooms to be produced resulting in new marine zooplankton and seabed communities (Gutt et al., 2011 <sup>[[#fn:r1307|1307]]</sup> ; Fillinger et al., 2013 <sup>[[#fn:r1308|1308]]</sup> ; Trathan et al., 2013 <sup>[[#fn:r1309|1309]]</sup> ; Hauquier et al., 2016 <sup>[[#fn:r1310|1310]]</sup> ) (Section 3.2.3.2.1), and have resulted in enhanced carbon uptake by coastal marine ecosystems ( ''medium confidence'' ), although quantitative estimates of biological carbon uptake are highly variable (Trathan et al., 2013 <sup>[[#fn:r1311|1311]]</sup> ; Barnes et al., 2018 <sup>[[#fn:r1312|1312]]</sup> ). Newly available habitat on coastlines may also provide breeding or haul out sites for land-based predators such as penguins and seals (Trathan et al., 2013 <sup>[[#fn:r1313|1313]]</sup> ) ( ''low confidence'' ). Fjords that have been studied in the subpolar western AP are hotspots of abundance and biodiversity of benthic macro-organisms (Grange and Smith, 2013 <sup>[[#fn:r1314|1314]]</sup> ) and there is evidence that glacier retreat in these environments can impact the structure and function of benthic communities (Moon et al., 2015 <sup>[[#fn:r1315|1315]]</sup> ; Sahade et al., 2015 <sup>[[#fn:r1316|1316]]</sup> ) ( ''low confidence'' ).

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