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

=== 4.9.6 Management of land degradation induced by tropical cyclones ===

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Tropical cyclones are normal disturbances that natural ecosystems have been affected by and recovered from for millennia. Climate models mostly predict decreasing frequency of tropical cyclones, but dramatically increasing intensity of the strongest storms, as well as increasing rainfall rates (Bacmeister et al. 2018 <sup>[[#fn:r1482|1482]]</sup> ; Walsh et al. 2016b <sup>[[#fn:r1483|1483]]</sup> ). Large amplitude fluctuations in the frequency and intensity complicate both the detection and attribution of tropical cyclones to climate change (Lin and Emanuel 2016b). Yet, the force of high-intensity cyclones has increased and is expected to escalate further due to global climate change ( ''medium agreement, robust evidence'' ) (Knutson et al. 2010 <sup>[[#fn:r1484|1484]]</sup> ; Bender et al. 2010 <sup>[[#fn:r1485|1485]]</sup> ; Vecchi et al. 2008 <sup>[[#fn:r1486|1486]]</sup> ; Bhatia et al. 2018 <sup>[[#fn:r1487|1487]]</sup> ; Tu et al. 2018 <sup>[[#fn:r1488|1488]]</sup> ; Sobel et al. 2016 <sup>[[#fn:r1489|1489]]</sup> ). Tropical cyclone paths are also shifting towards the poles, increasing the area subject to tropical cyclones (Sharmila and Walsh 2018 <sup>[[#fn:r1490|1490]]</sup> ; Lin and Emanuel 2016b <sup>[[#fn:r1491|1491]]</sup> ). Climate change alone will affect the hydrology of individual wetland ecosystems, mostly through changes in precipitation and temperature regimes with great global variability (Erwin 2009 <sup>[[#fn:r1492|1492]]</sup> ). Over the last seven decades, the speed at which tropical cyclones move has decreased significantly, as expected from theory, exacerbating the damage on local communities from increasing rainfall amounts and high wind speed (Kossin 2018 <sup>[[#fn:r1493|1493]]</sup> ). Tropical cyclones will accelerate changes in coastal forest structure and composition. The heterogeneity of land degradation at coasts that are affected by tropical cyclones can be further enhanced by the interaction of its components (for example, rainfall, wind speed, and direction) with topographic and biological factors (for example, species susceptibility) (Luke et al. 2016 <sup>[[#fn:r1494|1494]]</sup> ).

Small Island Developing States (SIDS) are particularly affected by land degradation induced by tropical cyclones; recent examples are Matthew (2016) in the Caribbean, and Pam (2015) and Winston (2016) in the Pacific (Klöck and Nunn 2019 <sup>[[#fn:r1495|1495]]</sup> ; Handmer and Nalau 2019 <sup>[[#fn:r1496|1496]]</sup> ). Even if the Pacific Ocean has experienced cyclones of unprecedented intensity in recent years, their geomorphological effects may not be unprecedented (Terry and Lau 2018 <sup>[[#fn:r1497|1497]]</sup> ).

Cyclone impacts on coastal areas is not restricted to SIDS, but a problem for all low-lying coastal areas (Petzold and Magnan 2019 <sup>[[#fn:r1498|1498]]</sup> ). The Sundarbans, one of the world’s largest coastal wetlands, covers about one million hectares between Bangladesh and India. Large areas of the Sundarbans mangroves have been converted into paddy fields over the past two centuries and, more recently, into shrimp farms (Ghosh et al. 2015 <sup>[[#fn:r1499|1499]]</sup> ). In 2009, cyclone Aila caused incremental stresses on the socio-economic conditions of the Sundarbans coastal communities through rendering huge areas of land unproductive for a long time (Abdullah et al. 2016 <sup>[[#fn:r1500|1500]]</sup> ). The impact of Aila was widespread throughout the Sundarbans mangroves, showing changes between the pre- and post-cyclonic period of 20–50% in the enhanced vegetation index (Dutta et al. 2015 <sup>[[#fn:r1501|1501]]</sup> ), although the magnitude of the effects of the Sundarbans mangroves derived from climate change is not yet defined (Payo et al. 2016 <sup>[[#fn:r1502|1502]]</sup> ; Loucks et al. 2010 <sup>[[#fn:r1503|1503]]</sup> ; Gopal and Chauhan 2006 <sup>[[#fn:r1504|1504]]</sup> ; Ghosh et al. 2015 <sup>[[#fn:r1505|1505]]</sup> ; Chaudhuri et al. 2015 <sup>[[#fn:r1506|1506]]</sup> ). There is ''high agreement'' that the joint effect of climate change and land degradation will be very negative for the area, strongly affecting the environmental services provided by these forests, including the extinction of large mammal species (Loucks et al. 2010 <sup>[[#fn:r1507|1507]]</sup> ). The changes in vegetation are mainly due to inundation and erosion (Payo et al. 2016 <sup>[[#fn:r1508|1508]]</sup> ).

Tropical cyclone Nargis unexpectedly hit the Ayeyarwady River delta (Myanmar) in 2008 with unprecedented and catastrophic damages to livelihoods, destruction of forests and erosion of fields (Fritz et al. 2009 <sup>[[#fn:r1509|1509]]</sup> ) as well as eroding the shoreline 148 m compared with the long-term average (1974–2015) of 0.62 m yr <sup>-1</sup> . This is an example of the disastrous effects that changing cyclone paths can have on areas previously not affected by cyclones (Fritz et al. 2010 <sup>[[#fn:r1510|1510]]</sup> ).

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==== 4.9.6.1 Management of coastal wetlands ====

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Tropical cyclones mainly, but not exclusively, affect coastal regions, threatening maintenance of the associated ecosystems, mangroves, wetlands, seagrasses, and so on. These areas not only provide food, water and shelter for fish, birds and other wildlife, but also provide important ecosystem services such as water-quality improvement, flood abatement and carbon sequestration (Meng et al. 2017 <sup>[[#fn:r1511|1511]]</sup> ).

Despite their importance, coastal wetlands are listed amongst the most heavily damaged of natural ecosystems worldwide. Starting in the 1990s, wetland restoration and re-creation became a ‘hotspot’ in the ecological research fields (Zedler 2000 <sup>[[#fn:r1512|1512]]</sup> ). Coastal wetland restoration and preservation is an extremely cost-effective strategy for society, for example, the preservation of coastal wetlands in the USA provides storm protection services, with a cost of 23.2 billion USD yr <sup>–1</sup> (Costanza et al. 2008 <sup>[[#fn:r1513|1513]]</sup> ).

There is a ''high agreement'' with ''medium evidence'' that the success of wetland restoration depends mainly on the flow of the water through the system, the degree to which re-flooding occurs, disturbance regimes, and the control of invasive species (Burlakova et al. 2009 <sup>[[#fn:r1514|1514]]</sup> ; López-Rosas et al. 2013 <sup>[[#fn:r1515|1515]]</sup> ). The implementation of the Ecological Mangrove Rehabilitation protocol (López-Portillo et al. 2017 <sup>[[#fn:r1516|1516]]</sup> ) that includes monitoring and reporting tasks, has been proven to deliver successful rehabilitation of wetland ecosystem services.

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'''Figure 4.10'''

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'''Decision tree showing recommended steps and tasks to restore a mangrove wetland based on original site conditions. (Modified from Bosire et al. 2008.)'''
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[[File:3efd3985210292342c41b92880022bc4 Figure-4.10-1024x667.jpg]]

Decision tree showing recommended steps and tasks to restore a mangrove wetland based on original site conditions. (Modified from Bosire et al. 2008. <sup>[[#fn:r1656|1656]]</sup> )

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