When most people think about carbon storage, they picture forests. Trees pulling CO2 from the atmosphere, locking it into wood and soil. It’s an accurate picture - terrestrial forests store enormous amounts of carbon. But there’s a less visible carbon sink that operates with far greater efficiency per unit area, and Australia has some of the largest reserves of it on Earth.

Blue carbon refers to the carbon captured and stored by coastal and marine ecosystems: mangroves, seagrass meadows, and tidal salt marshes. These ecosystems cover a tiny fraction of the ocean floor, but they punch dramatically above their weight in carbon sequestration. Understanding blue carbon - what it is, where it sits, and why it matters - is becoming increasingly important as Australia looks for every available tool to address climate change.

The Numbers

The carbon storage capacity of blue carbon ecosystems is remarkable. Per hectare, mangroves, seagrass, and salt marshes store carbon at rates 2-4 times higher than tropical forests, according to research published by the International Union for Conservation of Nature (IUCN).

The difference is even more dramatic when you account for soil carbon. Terrestrial forests store most of their carbon in above-ground biomass - trunks, branches, leaves. Blue carbon ecosystems store most of their carbon below ground, in waterlogged soils that accumulate organic material over centuries to millennia. Because these soils are low in oxygen, decomposition is extremely slow, meaning carbon remains locked in the sediment for thousands of years.

A single hectare of mangrove forest may contain 1,000 tonnes of carbon in its soil. A hectare of seagrass meadow may contain 500-800 tonnes. By comparison, a hectare of tropical forest typically contains 150-250 tonnes in above-ground biomass and soil combined.

Australia is estimated to hold approximately 5-11% of the world’s blue carbon stocks, spread across its extensive coastline. The exact figure is uncertain because comprehensive mapping and carbon quantification of Australian blue carbon ecosystems is still underway.

Where It Is

Australia’s blue carbon ecosystems are concentrated in several key regions.

Northern Australia has the most extensive mangrove forests, stretching across the coasts of Queensland, the Northern Territory, and Western Australia. The Gulf of Carpentaria and the Kimberley coast support some of the largest and least disturbed mangrove systems in the world.

Queensland hosts the majority of Australia’s seagrass, particularly within and adjacent to the Great Barrier Reef Marine Park. The seagrass meadows of Torres Strait, the northern and central Great Barrier Reef, and Moreton Bay are among the most significant in terms of both area and carbon storage.

Southern Australia has extensive salt marsh systems, particularly along the coasts of South Australia, Victoria, and Tasmania. The Coorong in South Australia is a nationally significant salt marsh and wetland system. Corner Inlet in Victoria and the coastline of western Tasmania also support substantial salt marsh areas.

Shark Bay in Western Australia contains one of the world’s largest seagrass meadows, estimated at approximately 4,300 square kilometres. Research published in Nature Geoscience estimated that Shark Bay’s seagrass alone stores approximately 1.3 billion tonnes of CO2 equivalent in its sediments - a staggering figure for a single location.

Why It Matters for Climate

Blue carbon ecosystems provide a double climate benefit. When healthy, they actively remove CO2 from the atmosphere and lock it into long-term soil storage. When damaged or destroyed, they release their stored carbon back into the atmosphere, becoming a source of emissions rather than a sink.

This second point is critical. When a mangrove forest is cleared for coastal development, or a seagrass meadow is destroyed by poor water quality, the carbon that accumulated in the soil over centuries begins to oxidise and escape as CO2. The emission can continue for decades after the initial disturbance.

Globally, it’s estimated that blue carbon ecosystems are being destroyed at a rate of 1-7% per year, depending on the ecosystem type and region. Each year of destruction releases stored carbon while simultaneously removing the ecosystem’s capacity to absorb future emissions. It’s a one-two punch that makes blue carbon destruction one of the most carbon-intensive forms of land use change.

In Australia, historical losses have been significant. An estimated 50% of Australia’s original salt marsh has been lost since European settlement, primarily to coastal development, drainage, and reclamation. Mangrove loss has been lower (approximately 11% nationally) but concentrated in urban and agricultural coastal areas. Seagrass loss is harder to quantify but documented in multiple regions including the Great Barrier Reef, Moreton Bay, and Shark Bay.

Policy and Markets

Australia has been a global leader in recognising blue carbon in climate policy, at least on paper.

In 2022, the Australian Government included blue carbon in the national Emissions Reduction Fund, allowing eligible blue carbon projects to generate Australian Carbon Credit Units (ACCUs). This means that protecting or restoring mangrove, seagrass, or salt marsh ecosystems can generate carbon credits that have monetary value.

The Clean Energy Regulator has approved methodologies for blue carbon projects, though the number of registered projects remains small. The main challenge is proving “additionality” - demonstrating that the carbon storage wouldn’t have occurred without the project intervention. For protection projects (preventing destruction of existing ecosystems), this requires establishing a credible threat of loss.

Restoration projects face different challenges. Replanting mangroves or restoring tidal flow to degraded salt marsh is technically feasible, but the carbon sequestration takes years to decades to accumulate in quantities sufficient to generate meaningful credits. The economics depend on carbon credit prices remaining high enough to justify the upfront restoration costs.

Several pilot projects are underway. Bush Heritage Australia and other conservation organisations are developing blue carbon restoration projects along the coast. State governments in Queensland and South Australia have blue carbon strategies that aim to map, protect, and restore coastal ecosystems.

Some AI development for research groups are working on satellite-based monitoring systems that can track changes in mangrove extent, seagrass coverage, and salt marsh condition at continental scale. Accurate, regular monitoring is essential for both conservation management and carbon accounting, and the scale of Australia’s coastline makes ground-based monitoring alone insufficient.

The Research Gaps

Despite progress, significant knowledge gaps remain.

Carbon stock quantification for many Australian blue carbon ecosystems is based on limited sampling. The estimates for total blue carbon storage vary by a factor of two or more, depending on the assumptions used. More comprehensive soil sampling across different ecosystem types, regions, and conditions is needed to narrow these estimates.

Seagrass mapping is particularly incomplete. Unlike mangroves and salt marshes, which are visible in aerial and satellite imagery, seagrass is submerged and often difficult to detect remotely. Large areas of northern Australia’s seagrass have never been systematically mapped.

Permanence - how long stored carbon remains sequestered under changing climate conditions - is a key uncertainty. If rising sea levels, marine heatwaves, or intensifying storms destroy blue carbon ecosystems faster than they can migrate or recover, the carbon stored in their soils will be released. This creates a feedback loop where climate change releases carbon from coastal ecosystems, which accelerates further climate change.

Connectivity between blue carbon ecosystems and other marine habitats is not well understood. Mangroves, seagrass, and coral reefs function as connected systems - juveniles of many reef fish species use mangroves as nursery habitat, for example. Managing blue carbon in isolation from the broader marine landscape risks missing important ecological relationships.

The Opportunity

Australia is in an unusual position. We have vast blue carbon resources, many of which are still relatively intact. We have the scientific expertise to study them. We have a policy framework that, at least in principle, values their carbon storage. And we have an economy that needs every available pathway to reduce net emissions.

The opportunity is to protect what remains, restore what’s been degraded, and account for blue carbon properly in national emissions reporting. The risk is that we lose more of these ecosystems to coastal development, agricultural runoff, and climate impacts before we’ve even finished measuring what they’re worth.

There’s nothing flashy about protecting a mangrove swamp or restoring a salt marsh. These aren’t projects that generate headlines or inspire viral campaigns. But the carbon maths is clear: per dollar invested, protecting and restoring blue carbon ecosystems is one of the most cost-effective climate interventions available. Australia has the resources to lead on this globally. Whether we do depends on treating these ecosystems as the critical infrastructure they are, rather than empty coastal land waiting for a higher-value use.