The Global Flood Protection Benefits of Mangroves

Abstract

Coastal flood risks are rising rapidly. We provide high resolution estimates of the economic value of mangroves forests for flood risk reduction every 20 km worldwide. We develop a probabilistic, processbased valuation of the effects of mangroves on averting damages to people and property. We couple spatially-explicit 2-D hydrodynamic analyses with economic models, and find that mangroves provide flood protection benefits exceeding $US 65 billion per year. If mangroves were lost, 15 million more people would be flooded annually across the world. Some of the nations that receive the greatest economic benefits include the USA, China, India and Mexico. Vietnam, India and Bangladesh receive the greatest benefits in terms of people protected. Many (>45) 20-km coastal stretches particularly those near cities receive more than $US 250 million annually in flood protection benefits from mangroves. These results demonstrate the value of mangroves as natural coastal defenses at global, national and local scales, which can inform incentives for mangrove conservation and restoration in development, climate adaptation, disaster risk reduction and insurance. Coastal flooding impacts are increasing due to coastal development, population growth 1 , climate change 2,3 , and habitat loss 4-7. In 2017 alone, overall storm damages were more than $US 170 billion in the North Atlantic 8. However, development choices often neglect flood risks 3,9,10 and there is growing pressure to adopt flood mitigation and adaptation strategies to reduce these impacts and economic losses 9,11,12. In many tropical and subtropical regions mangroves reduce waves and storm surges, and serve as a first line of defense against flooding and erosion. These benefits are provided through bottom friction, the cross-shore width of forests, tree density and shape. The aerial roots of a mangroves forest retain sediments, stabilizing the soil of intertidal areas and reducing erosion 13. Roots, trunk and canopy dissipates storm surge 14 and waves 15. Previous studies have shown that mangroves can reduce up to 66% of wave energy in the first 100 m of forest width 15,16. Mangroves can also provide adaptive defenses as they can, under the right conditions, keep pace with sea-level-rise through vertical accretion 17-19. Yet, mangroves have experienced significant losses over the last decades, declining globally from 139,777 km 2 in 2000 to 131,931 km 2 in 2014 20 , with even greater losses before 2000. Most of this loss has happened through the conversion for aquaculture or agriculture and coastal development 21. The loss of these habitats can contribute to increasing coastal risk 22 , particularly in developed areas with great exposure of coastal populations 23,24. Quantifying the value of mangroves as natural coastal defenses is crucial for incentivizing their conservation and restoration for the benefit of nature and people 25. The economic value of mangroves for services that rely on conserving them, such as flood protection, is typically not included within national...

Estuarine and Coastal Ecosystems as Defense Against Flood Damages: An Economic Perspective

“…Nevertheless, many studies confirm that the protective value of ECEs are one of the more significant benefits sacrificed when these habitats are lost or degraded. Global assessments for both coral reefs and mangroves are also illustrating the economic significance of this protective benefit (Beck et al, 2018;Hochard et al, 2019;Menéndez et al, 2020).…”

“…More recently, some valuation studies of the protective value of estuarine and coastal ecosystems (ECEs) have developed the expected damage function approach as an alternative to the replacement cost method (Barbier, 2007;Barbier and Enchelmeyer, 2014;World Bank, 2016;Beck et al, 2018;Highfield et al, 2018;Menéndez et al, 2020;Rezaie et al, 2020). This method assumes that an ECE provides a nonmarketed service, such as "protection" of economic activity, property and even human lives, which benefits individuals through limiting damages.…”

“…Finally, because of the growing interest in the storm protection service provided by estuarine and coastal ecosystems, global analyses of this benefit are beginning to emerge. Table 1 lists one example for coral reefs (Beck et al, 2018) and two for mangroves (Hochard et al, 2019;Menéndez et al, 2020). For example, Beck et al (2018) estimate that the absence of the protective benefit of coral reefs would double the annual expected damages from flooding globally and triple the costs from frequent (Badola and Hussain, 2005;Das and Vincent, 2009;Das and Crépin, 2013, mangroves, India) (Sathirathai and Barbier, 2001;Barbier, 2007Barbier, , 2012Barbier et al, 2008;Barbier and Lee, 2014, mangroves, Thailand) (Farber, 1987(Farber, , 1996Landry et al, 2011;Petrolia and Kim, 2011;Barbier et al, 2013;Kim and Petrolia, 2013;Barbier and Enchelmeyer, 2014;Petrolia et al, 2014;Boutwell and Westra, 2016, marsh The 41 studies selected are from peer-reviewed academic journals only, and are grouped by type of estuarine and coastal ecosystem and its location.…”

“…For a community with an average cover of 6.3 m of mangroves extending inland from the seaward edge, direct cyclone exposure can reduce economic activity permanently by 5.4-6.7 months, whereas for a community with 25.6 m of mangroves extending inland from the shoreline, the loss in activity is 2.6-5.5 months. Menéndez et al (2020) value the global flood protection benefits of mangroves at over $US 65 billion per year, and estimate that the loss of all mangroves would mean that 15 million more people worldwide would be susceptible to annual flooding. The countries benefiting the most include the United States, China, India, Mexico, Vietnam, and Bangladesh.…”

“…These include specific studies of marshes (Shepard et al, 2011;Rupprecht et al, 2017), mangroves (Cochard et al, 2008;Zhang et al, 2012;Dasgupta et al, 2019;Montgomery et al, 2019), near-shore coral reefs (Ferrario et al, 2014;Reguero et al, 2018) and seagrass beds (Paul et al, 2012;Christianen et al, 2013;Ondiviela et al, 2014). This protective value of ECEs is increasingly used to justify coastal conservation and restoration efforts worldwide (Temmerman et al, 2012;Arkema et al, 2013;Duarte et al, 2013;Barbier, 2014;Elliott et al, 2016;Narayan et al, 2016;Ruckelshaus et al, 2016;Hochard et al, 2019;Menéndez et al, 2020;Newton et al, 2020).…”