Editing IPCC:AR6/SRCCL/Chapter-4 (section)

=== 4.9.7 Saltwater intrusion ===

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Current environmental changes, including climate change, have caused sea levels to rise worldwide, particularly in tropical and subtropical regions (Fasullo and Nerem 2018 <sup>[[#fn:r1517|1517]]</sup> ). Combined with scarcity of water in river channels, such rises have been instrumental in the intrusion of highly saline seawater inland, posing a threat to coastal areas and an emerging challenge to land managers and policymakers. Assessing the extent of salinisation due to sea water intrusion at a global scale nevertheless remains challenging. Wicke et al. (2011) <sup>[[#fn:r1518|1518]]</sup> suggest that across the world, approximately 1.1 Gha of land is affected by salt, with 14% of this categorised as forest, wetland or some other form of protected area. Seawater intrusion is generally caused by (i) increased tidal activity, storm surges, cyclones and sea storms due to changing climate, (ii) heavy groundwater extraction or land-use changes as a result of changes in precipitation, and droughts/floods, (iii) coastal erosion as a result of destruction of mangrove forests and wetlands, (iv) construction of vast irrigation canals and drainage networks leading to low river discharge in the deltaic region; and (v) sea level rise contaminating nearby freshwater aquifers as a result of subsurface intrusion (Uddameri et al. 2014 <sup>[[#fn:r1519|1519]]</sup> ).

The Indus Delta, located in the south-eastern coast of Pakistan near Karachi in the North Arabian Sea, is one of the six largest estuaries in the world, spanning an area of 600,000 ha. The Indus delta is a clear example of seawater intrusion and land degradation due to local as well as up-country climatic and environmental conditions (Rasul et al. 2012 <sup>[[#fn:r1520|1520]]</sup> ). Salinisation and waterlogging in the up-country areas including provinces of Punjab and Sindh is, however, caused by the irrigation network and over-irrigation (Qureshi 2011 <sup>[[#fn:r1521|1521]]</sup> ).

Such degradation takes the form of high soil salinity, inundation and waterlogging, erosion and freshwater contamination. The interannual variability of precipitation with flooding conditions in some years and drought conditions in others has caused variable river flows and sediment runoff below Kotri Barrage (about 200 km upstream of the Indus delta). This has affected hydrological processes in the lower reaches of the river and the delta, contributing to the degradation (Rasul et al. 2012 <sup>[[#fn:r1657|1657]]</sup> ).

Over 480,000 ha of fertile land is now affected by sea water intrusion, wherein eight coastal subdivisions of the districts of Badin and Thatta are mostly affected (Chandio et al. 2011 <sup>[[#fn:r1658|1658]]</sup> ). A very high intrusion rate of 0.179 ± 0.0315 km yr <sup>-1</sup> , based on the analysis of satellite data, was observed in the Indus delta during the 10 years between 2004 and 2015 (Kalhoro et al. 2016 <sup>[[#fn:r1522|1522]]</sup> ). The area of agricultural crops under cultivation has been declining, with economic losses of millions of USD (IUCN 2003 <sup>[[#fn:r1523|1523]]</sup> ). Crop yields have reduced due to soil salinity, in some places failing entirely. Soil salinity varies seasonally, depending largely on the river discharge: during the wet season (August 2014), salinity (0.18 mg L <sup>–1</sup> ) reached 24 km upstream, while during the dry season (May 2013), it reached 84 km upstream (Kalhoro et al. 2016 <sup>[[#fn:r1524|1524]]</sup> ). The freshwater aquifers have also been contaminated with sea water, rendering them unfit for drinking or irrigation purposes. Lack of clean drinking water and sanitation causes widespread diseases, of which diarrhoea is most common (IUCN 2003 <sup>[[#fn:r1525|1525]]</sup> ).

Lake Urmia in northwest Iran, the second-largest saltwater lake in the world and the habitat for endemic Iranian brine shrimp, ''Artemia urmiana'' , has also been affected by salty water intrusion. During a 17- year period between 1998 and 2014, human disruption, including agriculture and years of dam building affected the natural flow of freshwater as well as salty sea water in the surrounding area of Lake Urmia. Water quality has also been adversely affected, with salinity fluctuating over time, but in recent years reaching a maximum of 340 g L <sup>–1</sup> (similar to levels in the Dead Sea). This has rendered the underground water unfit for drinking and agricultural purposes and risky to human health and livelihoods. Adverse impacts of global climate change as well as direct human impacts have caused changes in land use, overuse of underground water resources and construction of dams over rivers, which resulted in the drying-up of the lake in large part. This condition created sand, dust and salt storms in the region which affected many sectors including agriculture, water resources, rangelands, forests and health, and generally presented desertification conditions around the lake (Karbassi et al. 2010 <sup>[[#fn:r1526|1526]]</sup> ; Marjani and Jamali 2014 <sup>[[#fn:r1527|1527]]</sup> ; Shadkam et al. 2016 <sup>[[#fn:r1528|1528]]</sup> ).

Rapid irrigation expansion in the basin has, however, indirectly contributed to inflow reduction. Annual inflow to Lake Urmia has dropped by 48% in recent years. About three-fifths of this change was caused by climate change and two-fifths by water resource development and agriculture (Karbassi et al. 2010 <sup>[[#fn:r1529|1529]]</sup> ; Marjani and Jamali 2014 <sup>[[#fn:r1530|1530]]</sup> ; Shadkam et al. 2016 <sup>[[#fn:r1531|1531]]</sup> ).

In the drylands of Mexico, intensive production of irrigated wheat and cotton using groundwater (Halvorson et al. 2003 <sup>[[#fn:r1532|1532]]</sup> ) resulted in sea water intrusion into the aquifers of La Costa de Hermosillo, a coastal agricultural valley at the centre of Sonora Desert in Northwestern Mexico. Production of these crops in 1954 was on 64,000 ha of cultivated area, increasing to 132,516 ha in 1970, but decreasing to 66,044 ha in 2009 as a result of saline intrusion from the Gulf of California (Romo-Leon et al. 2014 <sup>[[#fn:r1533|1533]]</sup> ). In 2003, only 15% of the cultivated area was under production, with around 80,000 ha abandoned due to soil salinisation whereas in 2009, around 40,000 ha was abandoned (Halvorson et al. 2003 <sup>[[#fn:r1534|1534]]</sup> ; Romo-Leon et al. 2014 <sup>[[#fn:r1535|1535]]</sup> ). Salinisation of agricultural soils could be exacerbated by climate change, as Northwestern Mexico is projected to be warmer and drier under climate change scenarios (IPCC 2013a <sup>[[#fn:r1536|1536]]</sup> ).

In other countries, intrusion of seawater is exacerbated by destruction of mangrove forests. Mangroves are important coastal ecosystems that provide spawning bed for fish, timber for building, and livelihoods to dependent communities. They also act as barriers against coastal erosion, storm surges, tropical cyclones and tsunamis (Kalhoro et al. 2017 <sup>[[#fn:r1537|1537]]</sup> ) and are among the most carbon-rich stocks on Earth (Atwood et al. 2017 <sup>[[#fn:r1538|1538]]</sup> ). They nevertheless face a variety of threats: climatic (storm surges, tidal activities, high temperatures) and human (coastal developments, pollution, deforestation, conversion to aquaculture, rice culture, oil palm plantation), leading to declines in their areas. In Pakistan, using remote sensing, the mangrove forest cover in the Indus delta decreased from 260,000 ha in 1980s to 160,000 ha in 1990 (Chandio et al. 2011 <sup>[[#fn:r1539|1539]]</sup> ). Based on remotely sensed data, a sharp decline in the mangrove area was also found in the arid coastal region of Hormozgan province in southern Iran during 1972, 1987 and 1997 (Etemadi et al. 2016 <sup>[[#fn:r1540|1540]]</sup> ). Myanmar has the highest rate (about 1% yr <sup>–1</sup> ) of mangrove deforestation in the world (Atwood et al. 2017). Regarding global loss of carbon stored in the mangrove due to deforestation, four countries exhibited high levels of loss: Indonesia (3410 GgCO <sub>2</sub> yr <sup>–1</sup> ), Malaysia (1288 GgCO <sub>2</sub> yr <sup>–1</sup> ), US (206 GgCO <sub>2</sub> yr <sup>–1</sup> ) and Brazil (186 GgCO <sub>2</sub> yr <sup>–1</sup> ). Only in Bangladesh and Guinea Bissau was there no decline in the mangrove area from 2000 to 2012 (Atwood et al. 2017 <sup>[[#fn:r1541|1541]]</sup> ).

Frequency and intensity of average tropical cyclones will continue to increase (Knutson et al. 2015 <sup>[[#fn:r1543|1543]]</sup> ) and global sea level will continue to rise. The IPCC (2013) <sup>[[#fn:r1544|1544]]</sup> projected with ''medium confidence'' that the sea level in the Asia Pacific region will rise from 0.4 to 0.6 m, depending on the emission pathway, by the end of this century. Adaptation measures are urgently required to protect the world’s coastal areas from further degradation due to saline intrusion. A viable policy framework is needed to ensure that the environmental flows to deltas in order to repulse the intruding seawater.

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