Global declines in human‐driven mangrove loss

Abstract

Global mangrove loss has been attributed primarily to human activity. Anthropogenic loss hotspots across Southeast Asia and around the world have characterized the ecosystem as highly threatened, though natural processes such as erosion can also play a significant role in forest vulnerability. However, the extent of human and natural threats has not been fully quantified at the global scale. Here, using a Random Forest‐based analysis of over one million Landsat images, we present the first 30 m resolution global maps of the drivers of mangrove loss from 2000 to 2016, capturing both human‐driven and natural stressors. We estimate that 62% of global losses between 2000 and 2016 resulted from land‐use change, primarily through conversion to aquaculture and agriculture. Up to 80% of these human‐driven losses occurred within six Southeast Asian nations, reflecting the regional emphasis on enhancing aquaculture for export to support economic development. Both anthropogenic and natural losses declined between 2000 and 2016, though slower declines in natural loss caused an increase in their relative contribution to total global loss area. We attribute the decline in anthropogenic losses to the regionally dependent combination of increased emphasis on conservation efforts and a lack of remaining mangroves viable for conversion. While efforts to restore and protect mangroves appear to be effective over decadal timescales, the emergence of natural drivers of loss presents an immediate challenge for coastal adaptation. We anticipate that our results will inform decision‐making within conservation and restoration initiatives by providing a locally relevant understanding of the causes of mangrove loss.

Future carbon emissions from global mangrove forest loss

“…The drivers of mangrove loss for each province (2000-2016) were obtained from changes in mangrove area and a decision-tree model that separated drivers into five categories: a) conversion to commodities, such as agriculture or aquaculture, b) coastal erosion, c) clearing due to various activities including logging or hydrological modifications, d) extreme climatic events, such as tropical storms and fluctuations in sea level, and e) conversion to human settlements (Goldberg et al, 2020). Briefly, mangrove loss was estimated from the Surface Reflectance Tier-1 Landsat 5TM, 7ETM+, and 8OLI imagery within Google Earth Engine.…”

“…3A). for the marine provinces of the world, and (B) the proximate driver responsible for the largest CO2 emissions for each marine province (Goldberg et al 2020). The names for all marine provinces can be found in Fig.…”

“…These site-scale models have supported globally comprehensive spatial models of carbon storage (Rovai et al, 2018;Sanderman et al, 2018;Simard et al, 2019). Simultaneously, global efforts to accurately map and monitor mangrove cover and health have provided unprecedented knowledge on the risks that mangrove forests face (Bunting et al, 2018;Goldberg et al, 2020;Hamilton & Casey, 2016). These studies have enabled global-scale estimation of mangrove carbon storage and its historical loss across different nations (Murdiyarso et al, 2015;Serrano et al, 2019) and globally (Atwood et al, 2017).…”

“…These predictions should overcome several key limitations in past studies (Macreadie et al, 2019). First, carbon emission estimates have yet to associate particular land-use changes with CO2 emissions, as mapping of global drivers of mangrove loss has just recently become available (Goldberg et al, 2020). Second, many global estimates have included only the first meter of soil, thus, underestimating the total carbon content of mangroves and the emissions that arise from their conversion to other land uses (Kauffman et al, 2020).…”

“…Our model synthesised information from multiple newly available global datasets, including carbon stocks (Kauffman et al, 2020;Sanderman et al, 2018;Simard et al, 2019), mangrove distribution (Bunting et al, 2018), deforestation rates (Hamilton & Casey, 2016), drivers of land-use change (Goldberg et al, 2020) and emissions factors (Sasmito et al, 2019). We provide predictions of future CO2 emissions from mangrove loss, accounting for the effect of proximate drivers of land-use change including: a) conversion to commodities, such as agriculture or aquaculture, b) coastal erosion, c) clearing, d) extreme climatic events, and e) conversion to human settlements (Goldberg et al, 2020). Importantly, we account for the foregone opportunity of soil carbon sequestration when mangroves are lost (Maxwell et al, 2019).…”