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Blue Carbon

Threats to blue carbon ecosystems

Mangroves, salt marshes, and seagrass meadows are vanishing faster than tropical forests -- and when they go, centuries of stored carbon go with them.

Blue carbon ecosystems are among the most productive and carbon-dense habitats on Earth. They are also among the most threatened. Global coastal wetland loss has accelerated over the past century, driven by a combination of direct human pressures and climate-driven stressors that compound each other in ways that are increasingly difficult to reverse.1

1 Sea-level rise and coastal squeeze

Sea-level rise is often framed as a future threat, but its effects on coastal wetlands are already measurable. The primary danger is not simply that water levels rise, but that wetlands cannot migrate inland when human infrastructure blocks the way. Roads, seawalls, and coastal development trap habitats between rising water and fixed barriers -- a phenomenon researchers call coastal squeeze.2

Under high coastal development pressure, modeling suggests global mangrove sediment carbon stocks could decline by 15 to 30% by 2100. That is not just a loss of future sequestration capacity -- it represents the release of carbon stored over centuries.

2 Mangrove drowning

Even where migration is theoretically possible, the pace of sea-level rise can outstrip the ability of mangrove ecosystems to adjust. New modeling work shows that while some mangroves may initially accumulate more sediment in response to rising seas, across entire forests the trajectory is toward declining carbon storage as flooding exceeds species tolerance thresholds.3 When mangroves drown, their carbon-rich soils erode and oxidize -- flipping the ecosystem from a carbon sink to a carbon source.

3 Coastal development and habitat conversion

Direct habitat destruction remains the most widespread and immediate threat. Blue carbon ecosystems are cleared for resorts, marinas, ports, housing, aquaculture ponds, and agricultural expansion. Globally, an estimated 2.4 million acres of coastal wetland are lost each year.4

The pattern is visible across regions. In the Caribbean, hotel developers clear mangroves and seagrass to create the clear-water beach aesthetics that tourists expect -- unaware, or unconcerned, that the ecosystems they are removing protect the very coastlines their businesses depend on. Venice has lost over 75% of its historic salt marshes to dredging and embankment construction, a loss that has measurably weakened its natural flood defences.5

4 Resource extraction and overharvesting

In many coastal regions, mangroves are harvested for timber, charcoal, construction materials, and fuel. This is rarely a marginal activity. In Kenya, surveys found that 20% of mangrove cover was lost between 1985 and 2010, with peri-urban areas losing up to 70% in the same period -- almost entirely due to harvesting pressure from growing urban populations with limited alternative fuel sources.6

5 Pollution, microplastics, and ocean acidification

Marine pollution is now ubiquitous, reaching even the most remote coastal and ocean environments. Microplastics are present in mangrove sediments, salt marsh soils, and seagrass beds across the globe, including in Antarctic ecosystems far from any direct human activity. Their effects on coastal ecosystem function are still being quantified, but early evidence suggests they alter sediment chemistry, stress filter-feeding organisms, and disrupt the biological processes that support blue carbon accumulation.7

Ocean acidification -- driven by the absorption of atmospheric CO2 -- compounds this by harming the shell-forming organisms that underpin coastal food webs, including the oysters and mussels whose reef structures stabilize sediments in marsh and mangrove systems.

6 Climate-driven extreme events

Storms and hurricanes can uproot mangrove forests, erode marsh sediments, and smother seagrass beds with suspended sediment. Individual events are often survivable -- many ecosystems have evolved with periodic disturbance. The problem is frequency. As extreme weather events become more common under climate change, recovery periods between events shorten, and ecosystems that might have recovered fully from one storm are struck again before they can stabilize.

7 Barriers to landward migration

Even under moderate sea-level rise scenarios, the long-term survival of coastal wetlands depends on their ability to shift inland as water levels rise. Where seawalls, roads, urban development, and agricultural land block this movement, ecosystems become stranded. This is not a projected future problem -- it is already constraining the distribution of salt marshes and mangroves in heavily developed coastlines across North America, Europe, and Southeast Asia.8

The compound risk

The most dangerous aspect of these threats is how they interact. Sea-level rise reduces the area available to coastal wetlands; development removes what remains; pollution and acidification weaken what survives; and extreme events finish the job. Each pressure makes the ecosystem less resilient to the next. When blue carbon ecosystems collapse under this combination, they do not just stop absorbing carbon -- they begin releasing it, accelerating the very climate change that is driving them toward collapse.

References
  1. Hamilton, S.E. and Casey, D. (2016). Creation of a high spatio-temporal resolution global database of continuous mangrove forest cover for the 21st century. Global Ecology and Biogeography, 25(6), 729-738.
  2. Borchert, S.M. et al. (2018). Coastal wetland resilience to sea level rise. Estuarine, Coastal and Shelf Science, 212, 260-272.
  3. Saintilan, N. et al. (2020). Thresholds of mangrove survival under rapid sea level rise. Science, 368(6495), 1118-1121. doi:10.1126/science.aba2656
  4. Valiela, I. et al. (2001). Mangrove forests: one of the world's threatened major tropical environments. BioScience, 51(10), 807-815.
  5. Solidoro, C. et al. (2010). Understanding dynamic of the Venice lagoon ecosystem under human forcing over a broad temporal scale. Estuarine, Coastal and Shelf Science, 88(4), 455-468.
  6. Kairo, J.G. et al. (2002). Structural development and productivity of mangrove forests in Mida Creek, Kenya. Forest Ecology and Management, 169(1-2), 131-143.
  7. Galgani, L. et al. (2019). Microplastics increase the marine production of particulate forms of organic matter. Environmental Research Letters, 14(12).
  8. Temmerman, S. et al. (2013). Ecosystem-based coastal defence in the face of global change. Nature, 504, 79-83. doi:10.1038/nature12859