Mangroves are usually at the centre of the discussion when talking about protecting Australia from rising sea levels and coastal erosion. But new research suggests climate change poses an increasing risk to this usually hardy species in Northern Australia.
This new study ‘Future climate change will increase risk to mangrove health in Northern Australia’ featuring in Nature investigates how rising temperatures and sea level variability are projected to impact the resilience of coastal ecosystems. The consequences extend beyond mangroves and are expected to impact species diversity, coastal protection and carbon sequestration in the region.
Understanding the 2015 mangrove dieback
In 2015, a devastating dieback event hit the Gulf of Carpentaria, leading to the loss of nearly 40 million mangrove trees along 2000 kilometres of coastline. The mangroves have not fully recovered from the extreme event and die-backs are likely to continue in the future.
Bureau of Meteorology Senior Researcher and the study’s lead author, Dr Christine Chung explains, “The 2015 dieback event in the Gulf was linked to hot dry conditions and sudden, multi-season sea level drop”. Author Dr Pandora Hope explains this is related to the “‘almost El Nino’ of 2014 and the strong El Nino of 2015/16”.
The latest research from Extreme events explained looks at general mangrove health. As global temperatures continue to rise and sea levels fluctuate, the risks to mangrove health are set to intensify.
Identifying risk factors for mangrove health
To understand the factors influencing mangrove health, researchers studied two key risk factors: sea level variability and maximum daily temperatures.
“If there’s an extended period of too-low sea level, the mangroves can die of ‘thirst’. Unusually high sea level can also cause ‘drowning’ dieback, so either too-low or too-high sea level is a risk to mangrove health,” says Dr Chung.
Additionally, mangroves struggle to photosynthesise effectively above temperatures of approximately 35°C, highlighting the delicate balance between sea level and moderate temperatures needed for mangroves to survive.
How different warming scenarios impact mangroves
The research analysed climate projections for the Gulf region, focusing on two scenarios: The middle-of-the-road (SSP245) and high emissions (SSP585) scenarios based on CMIP6 models. These results revealed an increase in high-risk events (though not necessarily dieback events) under both scenarios. However, the high emissions scenario (SSP585) demonstrated a significant rise in the projected number of low and high sea level occurrences and high maximum temperature events.
The impact of natural variability on mangroves
Although the 2015 dieback event was one of the most severe recorded, other mangrove diebacks have occurred and affected different species. The Gulf of Carpentaria’s mangroves, which consist of the most resilient species, must endure an exceptionally harsh environment.
“Even under ideal conditions, only three to four species can withstand the area’s challenging climate, which is significantly lower than the biodiversity seen in other areas like Darwin Harbour, which boasts over 35 mangrove species,” notes Dr Chung.
Implications for coastlines and ecosystems
The long-term consequences of mangrove loss on individual ecosystems are not yet fully understood. However, it is evident that the absence of mangroves significantly impacts the species relying on them for food and shelter. Meanwhile, coastal erosion is likely to escalate, leading to potential hazards for coastal communities. The loss of mangroves as carbon sinks also exacerbates sequestration.
Mitigating mangrove risk under climate change
The 2015 dieback event was just the beginning, and the Gulf of Carpentaria’s mangroves remain at risk from rising temperatures and sea level variability. The level of devastation will depend on which emissions pathway the global population follows.
Dr Chung emphasises that the study highlights the ever-increasing risks climate change has on mangrove health, even beyond triggering dieback events.
“The disparity in negative outcomes to mangrove populations between SSP245 and SSP585 is sobering, and can be somewhat reduced by mitigating emissions.”