Wetlands, swamps and coastlines

Thatched-roof houses mounted on wooden piles above the water of a lake. — Reconstruction of Neolithic stilt houses at the village of Unteruhldingen, on Lake Constance (photograph from 2023). Rufus46, CC BY-SA 3.0, source

Peatlands cover 3 to 4% of the land surface, but store up to a third of the planet’s soil carbon.

Wetlands are also the environments that have been drained the most.

The exception

Our page on tropical forests (6.8) establishes a fact that holds for almost every environment in the world.

A forest produces oxygen and consumes roughly as much: the trees respire, and micro-organisms decompose what falls. The net balance is close to zero.

The oxygen present in the atmosphere actually comes from organic matter that, over geological time, escaped decomposition and was buried.¹

And that is exactly what happens in waterlogged environments.

Under water, oxygen is almost absent. The decomposers, which need oxygen, can barely function. Plant matter accumulates instead of rotting: it forms peat, then ever deeper layers of carbon.²

These environments do not recycle everything. They accumulate. On a global scale, peatlands cover only 3 to 4% of the land surface, but they store up to one third of the world’s soil carbon — roughly twice the carbon contained in the biomass of all the world’s forests combined — the wood, not their soils. It is by trapping this carbon outside the short cycle that these environments have allowed oxygen to remain in the atmosphere.³

The vocabulary, and what it produced

The other half of this page concerns a recurring mechanism.

How were these environments named in the legal and technical texts of the eighteenth and nineteenth centuries?

Marsh. Swamp. Unhealthy land. Unproductive land.⁴

These words do not merely describe: they judge. “Unhealthy” decrees a threat to health. “Unproductive” decrees zero economic value.

For centuries, this terminology legitimised massive drainage policies.

This site has listed some thirty cases in which a word manufactures a belief. This one has a peculiarity: it is the first in which the word directly triggers earthworks.

In France, the law of 16 September 1807 on the drainage of marshes illustrates this shift: the legal text systematically describes these expanses as “uncultivated and unhealthy lands” in order to authorise pre-emption and finance drainage canals. The administrative watchwords did not merely plant a false idea in people’s heads: they moved millions of tonnes of earth and drained entire basins.⁵

The day a semicolon declassified thousands of hectares

This is a story of legal exegesis with direct ecological consequences.

To protect a wetland, regulations must first define it. Since a 2007 decree, French law has based its definition on two criteria: the morphology of the soil (hydromorphy) and the nature of the vegetation (hygrophilous plants).⁶

However, the syntax of the original text left room for doubt: did the two conditions have to be combined, or was one of them enough?

Three changes in three years

In 2008, a ministerial order leans towards alternative criteria: the soil or the vegetation. One of the two is enough to classify the area.
On 22 February 2017, the Council of State interprets the punctuation of the regulatory definition as imposing cumulative criteria (“and”). It now becomes compulsory to prove the simultaneous presence of suitable soil and vegetation.
Immediate consequence: thousands of hectares lose their legal protection overnight, even though no physical change has taken place on the ground.
In July 2019, the law “Establishing the French Biodiversity Office” states expressly that the criteria are alternative. The alternative character is restored.
In June 2020, the Council of State confirms this clarification for the future.⁷

The reality of the decline

Beyond the legal adjustments, the physical toll remains heavy — but the figure itself has changed, and that is worth telling.

In 2018, the Ramsar Convention’s Global Wetland Outlook reported around 35% of wetlands lost since 1970. This is the figure that circulated widely for years.⁸

The 2025 edition gives another number for the same period: 22% since 1970, or 411 million hectares, with an ongoing decline of 0.52% per year.⁹

This is where the most instructive detail of this page lies.

The 35% figure has not disappeared from the report. It is still there — but with a different starting year. Since 1700, over 35% of natural wetlands have been lost.¹⁰

The point deserves emphasis: the same number, two starting dates, two different claims — and both of them accurate.

A figure for change means nothing without its starting year. The rule holds for wetlands, glaciers and forest cover alike. Any figure for change calls for an immediate check of the chosen baseline.

Someone who remembered “35%” but not the year could therefore, even today, quote a correct figure while attaching it to the wrong period. This is the mechanism of this entire page, caught in the act.

To this must be added a finding that the first figure did not provide: around a quarter of the remaining wetlands are in poor ecological condition, and this proportion is increasing in all regions. The report puts it this way: degradation now rivals outright loss.¹¹

One last sentence conveys the scale of the problem: wetlands are disappearing faster than any other ecosystem.¹²

The health factor: a historical duality

This analysis would be incomplete without its medical context.

Historically, marshes were centres of transmission of malaria (or “marsh fever”), carried by Anopheles mosquitoes, as well as of other endemic diseases. In Western Europe and North America, many nineteenth-century drainage schemes responded to well-founded public-health imperatives and genuinely reduced infant mortality and endemic malaria locally.¹³

There are therefore two simultaneous realities:

Wetlands were systematically disparaged through language, in pursuit of agricultural and property expansion.
The draining of certain foci responded to a real health emergency in the face of vector-borne diseases.

Historical analysis requires establishing, case by case in the archives, which motive (economic, land-related or health-related) determined the development decision.

The physical and biological functions of coastlines

Coastal wetlands fulfil measurable roles in protection and reproduction.

Mangroves and mechanical attenuation

Mangroves — tropical coastal forests growing in saline environments — act as natural civil-engineering works. Physical oceanography measurements indicate that a band of mangrove 100 metres wide reduces the height of waves (wind and swell waves) by 13% to 66%. They retain sediment, limit erosion of the shoreline and attenuate the impact of storm surges.¹⁴

Estuarine zones and marine nurseries

Marshy coastlines, salt marshes and seagrass meadows are essential nursery areas for the ocean. A large proportion of the fish species caught worldwide — estimates range from half to three quarters depending on the region — depend on coastal or estuarine wetlands for at least one stage of their life cycle (reproduction, growth of fry, feeding).¹⁵

Technical adaptation: lake dwellings

Occupying wetlands has not always meant destroying them.

The lake city of Ganvié, in Benin, built on Lake Nokoué, is home to tens of thousands of inhabitants — around 50,000 according to a recent estimate — living in structures built on stilts. Founded by the Tofinu people, who were fleeing slave raids, in the seventeenth or eighteenth century depending on the source, this settlement shows that an aquatic environment can be suitably developed without first being drained.¹⁶

Living on the water relies on precise knowledge of the environment, the use of rot-resistant timber and a reversible construction system.

Note 1

Without burial, this oxidation reaction causes dead plant material on Earth's surface to burn. That causes the carbon it contains, which originated in the atmosphere, to bond with oxygen to form carbon dioxide. And for oxygen to build up in our atmosphere, plant organic matter must be protected from oxidation. […] Continual burial of carbon is needed to keep the atmosphere pumped up with oxygen.

Tenenbaum, David, “Fossil fuel formation: Key to atmosphere’s oxygen?”, College of Letters & Science, University of Wisconsin–Madison, 30 December 2016