Up to 15% of Sundarbans losing ability to recover from climate stress over 25 years, research shows

The Sundarbans, a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site spanning India and Bangladesh, is seeing a marked decline in its ability to recover from environmental stress like climate driven and anthropogenic disturbances, a new study has found.

Researchers estimate that around 10-15 per cent of the forest, roughly 610 to 990 square kilometres, has experienced a loss of resilience over a 25-year period from February 2000 to December 2024. The Sundarbans stretches across more than 10,000 sq km.

The study, published in the journal Communications Earth & Environment, identifies this weakening through a phenomenon known as “critical slowing down”, where ecosystems take longer to recover from disturbances, show increased fluctuations in productivity, and become more unstable over time.

Cyclones and climate pressures reshape the forest

The findings show that major cyclones have played a significant role in reducing resilience. Between 2007 and 2009, following cyclones Sidr, Rashmi and Aila, large areas of forest shifted to lower resilience categories. For example, areas measuring 820.97 sq km, 732.54 sq km and 1,138.74 sq km respectively moved from resilient to moderately resilient. At the same time, other large tracts shifted further, from moderately resilient to weakly resilient.

The most affected zones are in the central and south-eastern parts of the Sundarbans, particularly in seaward regions stretching from the Satkhira range to the Sharankhola range. These include parts of the Chandpai and Sharankhola ranges.

Another hotspot identified is the Shala River corridor, which lies between the Chandpai and Sharankhola ranges. Despite receiving freshwater, this region remains highly vulnerable due to its exposure to the paths of severe cyclones such as Sidr.

In India’s western Sundarbans, mangrove stands were found to be less resilient than those in Bangladesh, particularly near the northern boundaries where significant declines were recorded.

The study also identifies a new hotspot of resilience loss in the north-eastern region, while long-term analysis shows that northern and coastal boundary areas have seen steep declines.

Human pressures and ecological shifts

The study highlights that both climate change and human activity are contributing to the weakening of the ecosystem. Forests along boundary regions contain a mix of fast-growing species such as Avicennia officinalis and Excoecaria agallocha, and slower-growing species including Heritiera fomes, Bruguiera sexangular and Xylocarpus mekongensis. These diverse stands, often characterised by taller canopy heights, are now under stress. Some species like Excoecaria agallocha grow taller in the northern areas, while in the saltier seaward and western parts, they remain shorter.

Rising temperatures were found to reduce resilience by affecting species richness and key traits such as canopy height. While rainfall can help maintain hydrological balance, erratic patterns combined with extreme weather events are undermining this benefit.

The researchers note that higher rainfall typically reduces salinity and water stress, allowing mangroves to grow more efficiently. However, changing climate patterns are disrupting these processes.

“Higher rainfall plays a crucial role in alleviating water stress, increasing freshwater flow and diluting porewater salinity, which, in turn, reduces the energetic cost of osmoregulation and improves overall physiological performance. This allows mangrove trees to allocate more energy into vertical growth. Moreover, consistent precipitation helps maintain a hydrological balance and stabilises sediment conditions,” the study said.

The study also links the trends in the Sundarbans to broader patterns seen in other tropical forests, such as the Amazon, where resilience is also declining.

In the Sundarbans, resilience depends on factors such as structural complexity, species diversity and functional traits. The study found that as canopy height and leaf characteristics decline, forests are becoming more uniform with fewer species, reducing overall diversity.

Disease is another factor. The “top dying” condition has affected the dominant Sundari tree (Heritiera fomes) since the 1960s, reducing timber value and turning parts of the forest from a carbon sink into a source of emissions. The study estimates an annual loss of about $16.72 million in timber value.

Human pressures are also intensifying the problem. Reduced freshwater flow due to upstream dams increases salinity, forcing plants to expend more energy on survival rather than growth. Excess phosphorus in sediments further disrupts nutrient balance and can create oxygen-deficient soil conditions.

Encroachment, shrimp farming and illegal logging, particularly of large, dominant trees, are also degrading the forest’s structural complexity and recovery capacity.

The researchers say improving forest management will be key. They recommend protecting mature trees from poachers, strengthening patrolling, involving local communities for awareness, and incorporating climate impacts into policy can play a key role in protecting the mangroves.