Characterising the impact of tropical cyclones on mangroves using a multi-decadal Landsat archive

Tropical cyclones exert a strong driving force for change in mangrove systems. The potential increase in cyclone intensity globally is concerning as this may lead to significant changes in species composition, forest complexity and loss of ecosystem services, including important climate mitigation services. This has implications for the efficacy of blue carbon offsetting in the tropics. There is an urgent need to understand and quantify the effects of cyclones on mangrove ecosystems and the services they provide. The Landsat archive within Digital Earth Australia provides an unprecedented opportunity to quantify cyclone impacts at a national scale and beyond. The aim of this study was to quantify the short- and long-term impacts of Category 3-5 cyclones on mangroves in Australia. This was achieved using wind-field modelling (Geoscience Australia’s Tropical Cyclone Risk Model) and the Landsat archive to a)  establish the degree of recovery in canopy cover of mangroves following cyclones of different category strength; b) quantify the differential response on mangroves, accounting for varying cyclone intensity, mangrove composition, and recovery time since cyclone; and c) conceptualise the likely impacts of future cyclones given predictions of future change, including that associated with anthropogenic driven climatic fluctuation. Windspeeds > 165-224 km/hr, typical of category 3 cyclones, caused the most widespread damage, suggesting a critical windspeed threshold was exceeded. Patterns of short-term damage reflected location and exposure, with the greatest damage observed along open coastlines and fringing forests. Assessments indicated persistent loss of forests when the impact was high over the short-term. Areas experiencing a minor reduction in cover, and to a lesser extent major reduction in cover, exhibited signs of recovery, but the duration of recovery may be prolonged (>10 years). Where cyclones were frequent recovery was impeded by subsequent cyclones, and this may lead to a shift in ecosystem type. The approach used Geoscience Australia’s Open Data Cube and Jupyter Notebooks, which have been published online as open-source code to allow users to repeat the assessments for future cyclones in their area of interest. This is an important feature as Data Cubes are developed and operationalised globally. This analysis demonstrates the utility of Data Cubes for assessing impacts of coastal natural hazards and provides crucial information regarding the long-term resilience of mangroves and their ecosystem services, particularly in the context of a changing climate and variation in cyclone intensity and frequency.