The Global Mangrove Watch—A New 2010 Global Baseline of Mangrove Extent

TLDR

Artefacts from cloud cover and Landsat‑7 SLC‑off errors affect some areas, especially West Africa, due to missing Landsat‑5 data and persistent cloud cover. This study establishes the first 2010 global baseline of mangrove extent (137,600 km²) using a globally consistent automated mapping method, and it will serve as the foundation for a monitoring system that tracks mangrove change from 1996 to present and informs future baselines with Sentinel‑2 data. The authors used a globally consistent, automated workflow that first generated a habitat mask from ALOS PALSAR and Landsat imagery, then classified mangroves with an Extremely Randomized Trees classifier. The resulting map achieved 94.0 % accuracy (99 % confidence interval 93.6–94.5%) based on 53,878 validation points, and revealed that Asia contains 38.7 % of global mangroves, followed by Latin America (20.3 %), Africa (20.0 %), Oceania (11.9 %), North America (8.4 %) and European Overseas Territories (0.7 %). Users should note that the product recommends a minimum mapping unit of 1 ha, and that error rises in disturbed areas and where mangroves are narrow or fragmented.

Abstract

This study presents a new global baseline of mangrove extent for 2010 and has been released as the first output of the Global Mangrove Watch (GMW) initiative. This is the first study to apply a globally consistent and automated method for mapping mangroves, identifying a global extent of 137,600 km 2 . The overall accuracy for mangrove extent was 94.0% with a 99% likelihood that the true value is between 93.6–94.5%, using 53,878 accuracy points across 20 sites distributed globally. Using the geographic regions of the Ramsar Convention on Wetlands, Asia has the highest proportion of mangroves with 38.7% of the global total, while Latin America and the Caribbean have 20.3%, Africa has 20.0%, Oceania has 11.9%, North America has 8.4% and the European Overseas Territories have 0.7%. The methodology developed is primarily based on the classification of ALOS PALSAR and Landsat sensor data, where a habitat mask was first generated, within which the classification of mangrove was undertaken using the Extremely Randomized Trees classifier. This new globally consistent baseline will also form the basis of a mangrove monitoring system using JAXA JERS-1 SAR, ALOS PALSAR and ALOS-2 PALSAR-2 radar data to assess mangrove change from 1996 to the present. However, when using the product, users should note that a minimum mapping unit of 1 ha is recommended and that the error increases in regions of disturbance and where narrow strips or smaller fragmented areas of mangroves are present. Artefacts due to cloud cover and the Landsat-7 SLC-off error are also present in some areas, particularly regions of West Africa due to the lack of Landsat-5 data and persistence cloud cover. In the future, consideration will be given to the production of a new global baseline based on 10 m Sentinel-2 composites.