What Is Blue Carbon? How Are Marine Ecosystems Calculated?

When people think of carbon sequestration, forests usually come to mind first — but science shows that coastal ecosystems are far more powerful carbon reservoirs per hectare. In this guide, we explain the concept of "Blue Carbon," which ecosystems sequester how much carbon, and how the difference between protecting and destroying these areas is calculated.

What Is Blue Carbon?

Blue Carbon refers to the carbon captured and stored by coastal and marine ecosystems (mangrove forests, seagrass meadows, salt marshes). These ecosystems can sequester 3-5 times more carbon per hectare than land forests — because carbon accumulates not only in plant tissue but also in the anaerobic (oxygen-free) sediment layer, where it can remain undissolved for thousands of years.

→ Bibliography: full data tables and methodology note in the Marine & Biogenic section

Three Ecosystems, Three Different Values

It's no coincidence that mangrove forests have both the highest sequestration and the highest loss value: this ecosystem has the densest root/sediment biomass, so it both stores the most carbon and releases the most carbon back when destroyed.

How Do You Interpret Your Result?

• A negative value (e.g. -11.0): This means a net "sink" — the ecosystem is PULLING carbon from the atmosphere and storing it. The more negative the value, the higher the climate value of protecting that area.
• A positive value (e.g. +18.0): This shows that the ecosystem has been DESTROYED and the carbon it had stored is being released back into the atmosphere — the larger the number, the higher the climate cost of losing that area.
• The difference between protection and loss (e.g. 11.0+18.0=29.0 for mangroves): This shows the total climate value of "protecting a hectare of mangrove compared to losing it" — for decision-makers, this is usually the single most meaningful figure.

Why a Separate Category?

Blue Carbon is treated as a category separate from land forests by the IPCC Wetlands Supplement and the Blue Carbon Initiative, because both the carbon density and the permanence profile differ: carbon in a land forest is mainly held in living biomass (subject to fire/logging risk), while most of the carbon in coastal ecosystems is stored under anaerobic sediment, an environment that — as long as it's not exposed to oxygen — dramatically slows the dissolution/ release of carbon.

Worked Example

The difference between protecting a 10-hectare seagrass meadow for 5 years versus destroying it over the same period:

Protection scenario: 10 ha × -6.5 ton CO2e/ha/year × 5 years = -325 ton CO2e (net sequestration)
Destruction scenario: 10 ha × 11.0 ton CO2e/ha/year × 5 years = +550 ton CO2e (net release)
Total difference = 325 + 550 = 875 ton CO2e — meaning protecting this area creates a climate value difference of 875 ton CO2e over 5 years compared to destroying it.