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Global Mangrove Watch (GMW) v3.0

Background

The Global Mangrove Watch (GMW) dataset (v3.0) was released in 2022 and depicts the global extent and changes of mangrove forests for eleven annual epochs between 1996 and 2020, derived using a combination of L-band Synthetic Aperture Radar (SAR) and optical satellite data.

The Global mangrove Watch was established in 2011 under the Japan Aerospace Exploration Agency’s (JAXA) Kyoto & Carbon Initiative by Aberystwyth University, solo Earth Observation and the International Water Management Institute, with the aim to provide open access geospatial information about mangrove extent and changes to the Ramsar Convention on Wetlands. In collaboration with Wetlands International and with support from DOB Ecology, the first GMW baseline maps were released in 2018 at the Ramsar COP13. In 2019 the GMW teamed up with The Nature Conservancy, Wetlands International, NASA, and a host of partners to develop the Global Mangrove Watch Platform. The effort is supported by the Oak Foundation, COmON Foundation, DOB Ecology and the Dutch Postcode Lottery.

The GMW maps also constitute the official mangrove datasets used by UNEP for reporting on Sustainable Development Goal 6.6.1 (change in the extent of water-related ecosystems over time).

Methodology

The GMW mangrove maps were derived in two steps:

  • generation of a baseline map of global mangrove extent for the year 2010, and,
  • detection of changes (both gains and losses) between the 2010 baseline and each of the other six annual epochs, respectively.

(1) Generation of 2010 global mangrove extent baseline map:

The 2010 baseline map was derived by Random Forest Classification of a combination of radar (ALOS PALSAR) and optical (Landsat-5, -7) satellite data. Approximately 15,000 Landsat scenes and 1,500 ALOS PALSAR (1 x 1 degree) mosaic tiles were used to create optical and radar image composites covering the coastlines along the tropical and sub-tropical coastlines in the Americas, Africa, Asia and Oceania.

The classification was confined using a mangrove habitat mask, which defined regions where mangrove ecosystems can be expected to exist. The mangrove habitat definition was generated based on geographical parameters such as latitude, elevation and distance from ocean water. Training for the habitat mask and classification of the 2010 mangrove mask was based on randomly sampling some 38 million points using historical mangrove maps for the year 2000 (Giri et al., 2010; Spalding et al., 2010), water occurrence maps (Pekel et al, 2017), and Digital Elevation Model data (SRTM-30).

(2) Generation of 1996, 2007–2009, and 2015–2020 maps of mangrove extent and changes:

The maps for the other ten epochs were derived by detection and classification of mangrove losses (defined as a decrease in radar backscatter intensity) and mangrove gains (defined and a backscatter increase) between the 2010 ALOS PALSAR data on one hand, and JERS-1 SAR (1996), ALOS PALSAR (2007, 2008 & 2009) and ALOS-2 PALSAR-2 (2015 – 2020) data on the other. The change pixels for each epoch in question were then added or removed from the 2010 baseline raster mask (buffered to allow detection of mangrove gains also immediately outside of the mask) to produce the new yearly extent maps.

Accuracy assessment

The overall map accuracy of the 2010 baseline dataset was estimated to be 95.1% with a 95th confidence interval of 93.8–96.5%, as assessed using 50,750 reference points located across 60 globally distributed sites. The accuracies of the mangrove extent of the other ten annual epochs were estimated to 87.4% (95th conf. int.: 86.2–88.6%).

Classification accuracies of the individual annual gain and loss change classes were estimated to 58.1% (52.4–63.9%) and 60.6% (56.1–64.8%), respectively.

Next GMW Data Release

Release of the next GMW dataset (version 4), comprising a new baseline dataset for the year 2020, and change maps between 1992 and 2023, is scheduled for release in mid 2024.

GMW Data Access

UNEP-WCMC (vector shapefiles):  https://data.unep-wcmc.org/datasets/45

JAXA Earth Observation Research Center (raster format): https://www.eorc.jaxa.jp/ALOS/en/dataset/gmw_e.htm

GMW Online Vieweing

References

Bunting P., Rosenqvist A., Hilarides L., Lucas R.M., Thomas N., Tadono T., Worthington T.A., Spalding M., Murray N.J., Rebelo L-M., Global Mangrove Extent Change 1996–2020: Global Mangrove Watch Version 3.0. Remote Sens. 2022, 14, 3657; doi.org/10.3390/rs14153657

Bunting P., Rosenqvist A., Hilarides L., Lucas R., Thomas N., Global Mangrove Watch: Updated 2010 Mangrove Forest Extent (v2.5). Remote Sens. 2022, 14, 1034;  doi.org/10.3390/rs14041034