Rising Seas: Climate Impact on Philippine Coasts

Mangrove forests cover less than 1 percent of the Earth’s surface, yet they hold roughly 15 percent of all carbon stored in ocean ecosystems. That disproportionate role makes them one of the planet’s most efficient natural carbon sinks — but new research suggests their ability to lock away carbon could weaken as sea levels continue to rise. For a country like the Philippines, which has the fifth longest coastline in the world and extensive mangrove stands, the stakes are unusually high.

These figures frame a tension that coastal nations are only beginning to reckon with. Mangroves are not just carbon vaults — they also buffer storm surges, support fisheries, and sustain livelihoods. But a new computer model developed by researchers at the University of Exeter, in collaboration with partners in Colombia and the United States, indicates that the very mechanism that makes mangroves valuable may be undermined by the climate pressures those forests are expected to mitigate. The findings, published in the journal Earth’s Future, suggest that while some parts of a mangrove forest may temporarily accumulate more carbon as water levels rise, the overall capacity of the forest to store carbon is likely to decline over the next century. For a deeper look at how climate change is reshaping food systems in the archipelago, see our analysis on climate threats to Philippine food security.

How Mangroves Store Carbon — and Why That Could Change

The core mechanism is straightforward but often overlooked. Mangrove plants are highly specialised: they need a precise duration of tidal flooding to survive. When sea levels rise beyond that tolerance, the plants drown. As they die, the carbon-rich soils they once stabilised begin to erode, releasing stored carbon back into the atmosphere. Dr. Arya Iwantoro, who led the research at the University of Exeter and is now based at the University of Plymouth, explained that field observations have sometimes shown carbon storage increasing as sea levels rise — but those localised gains may not reveal what is happening across the entire forest. The new model was designed to capture the full landscape-scale picture.

The distinction between local and landscape-scale effects matters because policy decisions are often based on plot-level studies. If those studies show carbon gains in one patch of mangroves, the assumption may be that the forest as a whole is thriving. The Exeter model challenges that assumption directly. It shows that rising seas may initially boost carbon accumulation in some locations — but as conditions continue to change, the broader forest could lose its ability to store carbon effectively. That nuance is easy to miss when data is collected only from healthy-looking stands.

What Happens When Mangroves Cross the Threshold

The researchers evaluated several sea-level rise scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). The pattern was consistent: higher levels of sea-level rise produced increasingly negative effects on mangrove carbon storage. This is not a linear relationship where a little more water causes a little more damage. Instead, there appears to be a threshold beyond which the system shifts from net carbon absorption to net carbon release.

Consider a hypothetical mangrove forest in a Philippine delta. For decades, sediment from upstream rivers has kept pace with gradual sea-level rise, allowing the mangroves to maintain their elevation relative to the water. But if sea-level rise accelerates — as IPCC projections indicate it will — the sediment supply may no longer be sufficient. The mangroves become waterlogged for longer periods each day. Seedlings fail to establish. Mature trees begin to die from the seaward edge inward. As the roots decay, the carbon-rich peat they once held together erodes into the water column. What was a carbon sink becomes a carbon source.

Dr. Barend van Maanen, who leads the mangrove and carbon project at Exeter, noted that mangroves face an uncertain future not only from climate change but also from other human impacts on rivers and coasts. Dams that trap sediment upstream, for example, can starve downstream mangroves of the material they need to keep pace with rising seas. This compounding of pressures — climate-driven sea-level rise plus human alterations to sediment flows — makes the outlook for many mangrove forests worse than either factor alone would suggest. The broader context of environmental degradation in the Philippines, including how waste mismanagement harms coastal ecosystems, adds another layer of stress to these already vulnerable systems.

What Gets Missed in the Mangrove Carbon Debate

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Several points in this debate are frequently misunderstood. First, the scale of observation matters enormously. Plot-level studies that show carbon gains under rising seas are not wrong — they are incomplete. The Exeter model demonstrates that those gains can occur in specific locations even as the forest as a whole declines. Second, the transition from carbon sink to carbon source is not gradual. Once mangroves die and soils erode, the carbon release can be rapid and difficult to reverse. Third, the role of sediment is underappreciated. Mangroves need a steady supply of sediment to build their soil elevation. When upstream dams or river diversions cut that supply, the mangroves lose their ability to keep up with rising water levels.

Another layer of complexity involves the interaction between mangroves and other coastal ecosystems. Seagrass beds and salt marshes also store blue carbon, and they respond differently to sea-level rise. A mangrove dieback might be partially offset by expansion of salt marsh into areas that become too wet for mangroves — but salt marshes store less carbon per hectare, and the transition period can release significant amounts of stored carbon. The net effect depends on local conditions, rates of sea-level rise, and the availability of space for ecosystems to migrate inland. That last factor — inland migration space — is often blocked by coastal development, seawalls, and agriculture. The Philippines, with its dense coastal populations, faces particular challenges in this regard. The pollution of waterways from medical waste and other sources further degrades the habitat quality that mangroves and associated ecosystems depend on.

What Coastal Communities and Planners Can Do

Prioritise Sediment Management

The single most actionable step for preserving mangrove carbon storage is maintaining sediment supply. This means evaluating upstream dams, mining operations, and land-use changes that reduce the amount of sediment reaching the coast. In watersheds where sediment flow has been disrupted, restoration of natural sediment transport — or managed relocation of sediment — may be necessary. Local government units in the Philippines can integrate sediment management into their coastal resource management plans, working with the Department of Environment and Natural Resources (DENR) and the Department of Public Works and Highways (DPWH) on watershed-level assessments.

Create Inland Migration Corridors

Mangroves need room to move inland as sea levels rise. This requires identifying coastal areas where development can be restricted or modified to allow ecosystem migration. Zoning changes, easements, and buyout programs for high-risk coastal properties are tools that have been used in other countries. In the Philippine context, the DENR’s role in environmental regulation could be expanded to include coastal setback requirements that account for projected sea-level rise over the next 50 to 100 years.

Invest in Landscape-Scale Monitoring

Plot-level studies are not enough. The Exeter model shows that local carbon gains can mask forest-wide decline. Planners need monitoring systems that track mangrove extent, health, and carbon storage at the landscape scale — using satellite imagery, drone surveys, and ground-truthing. The Philippines’ Department of Science and Technology (DOST) and the National Mapping and Resource Information Authority (NAMRIA) already have some capacity in this area, but sustained funding and technical support are needed to operationalise landscape-scale monitoring for all major mangrove areas.

Prepare for the Sink-to-Source Transition

In some areas, mangrove dieback may be unavoidable. Planners should identify which forests are most at risk and develop contingency plans. These could include assisted migration of mangrove species to more suitable locations, restoration of degraded mangrove areas that still have a chance of recovery, and — where dieback is inevitable — monitoring of carbon release to account for it in national greenhouse gas inventories. The Philippines’ Nationally Determined Contribution (NDC) under the Paris Agreement includes land-use and forestry targets; accurate accounting for mangrove carbon is essential for meeting those commitments.

• 1
  Assess Sediment Supply

  Evaluate upstream dams, mining, and land-use changes that reduce sediment reaching mangrove areas. Coordinate with DPWH and DENR on watershed assessments.

• 2
  Map Migration Space

  Identify coastal areas where mangroves can move inland as seas rise. Work with local government units on zoning and easement policies.

• 3
  Deploy Landscape Monitoring

  Use satellite imagery and drone surveys to track mangrove extent and health at scale. Integrate data into national carbon accounting systems.

• 4
  Plan for Dieback Zones

  Identify high-risk forests and prepare contingency plans for assisted migration, restoration, or carbon release monitoring.

Frequently Asked Questions About Mangroves and Sea-Level Rise

The evidence from the Exeter model is clear: mangroves are not a permanent carbon solution if sea-level rise accelerates beyond moderate projections. They remain one of the most valuable ecosystems on the planet — for carbon storage, coastal protection, and biodiversity — but their capacity to deliver those benefits is conditional on how quickly and how high the seas rise. For the Philippines, which depends heavily on its coastal ecosystems for food security, storm protection, and livelihoods, the message is that protecting mangroves means protecting the conditions that allow them to survive — sediment supply, migration space, and water quality. That is a far more complex task than simply planting trees, but it is the only approach that matches the scale of the challenge. If this was useful, you might also want to read how soil degradation compounds environmental risks in the Philippines.

Sources

Climate change threat to Philippine food security — Explores how rising temperatures and shifting rainfall patterns affect agricultural production across the archipelago.

Dirty water hurts Filipino communities — Examines the links between water pollution, public health, and ecosystem degradation in Philippine watersheds.

Rising seas may reduce mangrove carbon storage. ScienceDaily, 2026.

Can AI help coastal cities prepare for rising seas and extreme events? The Conversation, 2026.

Iwantoro, A. P., et al. “The Importance of Scale in the Future of Mangrove Blue Carbon Under Sea‐Level Rise.” Earth’s Future, vol. 14, no. 6, 2026. DOI: 10.1029/2025EF006984.