What Is a Wetland? The Most Valuable Land Nobody Wanted

The defining feature is that water saturates the soil for long enough to produce low-oxygen conditions, which changes the chemistry and selects for plants adapted to it. The categories differ by water source and chemistry:

• •Marshes, dominated by soft-stemmed plants such as reeds and sedges, fed by surface water and often seasonally flooded
• •Swamps, dominated by trees and shrubs, including mangrove swamps in the tropics and cypress swamps in the American south
• •Bogs, fed only by rainfall, therefore acidic and nutrient-poor, dominated by sphagnum moss and accumulating peat
• •Fens, fed by groundwater that has passed through rock, therefore less acidic and far richer in species
• •Floodplains and riparian zones alongside rivers, which are wet periodically rather than permanently
• •Estuaries and salt marshes where fresh and salt water mix, among the most biologically productive systems measured anywhere

The functions are measurable and several have been costed. Flood control comes from storage and from friction: a floodplain holds water that would otherwise move downstream in a peak, and reeds and mangroves slow flow substantially, with studies after the 2004 Indian Ocean tsunami and after Atlantic hurricanes finding markedly reduced damage behind intact mangrove and marsh. Water purification happens because slow flow drops sediment, plants take up nitrogen and phosphorus, and bacteria in low-oxygen sediment convert nitrate to nitrogen gas, which is why constructed wetlands are used as tertiary sewage treatment at a fraction of the cost of equivalent plant. Carbon storage is the function that has changed the policy conversation: peatlands cover around three percent of the land surface and hold roughly twice as much carbon as all the world's forests, because waterlogging prevents the decay that would release it. Fisheries depend on them, since a large proportion of commercially caught fish use estuaries or mangroves as nurseries. And they are the principal stopover habitat for migratory birds, whose flyways are strings of wetlands.

The reasons were consistent across centuries and mostly rational given what was known. Drained wetland makes excellent farmland, because the accumulated organic matter is fertile, which is why the English Fens, the Dutch polders and much of the American Midwest were converted. Marshes were associated with disease, and the association was correct in the case of malaria, which is transmitted by mosquitoes that breed in standing water, so drainage was a public health measure before the mechanism was understood. Wetlands were regarded as wasteland in law and in language, and drainage was subsidised by governments well into the twentieth century, with agricultural policy in Europe and North America paying farmers directly to convert them. Urban expansion took coastal marshes for ports and airports. The cumulative loss is enormous and continuing, with the steepest current rates in Southeast Asia, where mangroves have been cleared for shrimp aquaculture, and in tropical peatlands drained for plantations, which then dry, oxidise and burn.

Drained peat is a special case because it does not simply stop storing carbon, it starts releasing it. Peat accumulates only while waterlogged, and once drained the exposed organic matter oxidises, so a drained peatland emits carbon dioxide continuously for decades and the ground surface subsides, sometimes by centimetres a year, which is why parts of the Fens are now several metres below sea level and why drainage pumps must run permanently. Dried peat also burns, and peat fires smoulder underground through wet seasons and are extremely difficult to extinguish, with the Indonesian fires of 1997 and 2015 releasing quantities of carbon comparable to the annual emissions of major industrial economies and producing haze across the region. Degraded peatlands are estimated to account for a few percent of global emissions from a very small share of the land area. Rewetting reverses the emission, which is why peatland restoration has become one of the cheapest available emissions measures and why several countries have banned peat extraction for horticulture.

The international framework is the Ramsar Convention, agreed in the Iranian city of that name in 1971, which is unusual in being a treaty about a single ecosystem type and in predating almost all modern environmental agreements. Its parties designate sites of international importance, of which there are now over two and a half thousand covering a very large area, and commit to their wise use. National protection has followed in most developed countries, with wetland loss slowing sharply where drainage subsidies were removed and permitting introduced. Restoration works better than most habitat restoration, because reinstating the hydrology is often enough: block the drains, let the water table rise, and the vegetation and the species return over years rather than decades. Large projects have reconnected floodplains to rivers in Europe, rewetted blanket bog in Britain and Ireland, and replanted mangroves along tropical coasts, with the last of these having a poor early record from planting the wrong species in the wrong places and a better one where the tidal hydrology is restored first and the trees are allowed to arrive on their own.

A wetland is land where saturated soil produces low-oxygen conditions, and the types divide by water source, from rain-fed acidic bogs to groundwater-fed fens and tidal salt marshes. They store more carbon per hectare than forest, slow floodwater, filter nitrogen and phosphorus and serve as fish nurseries and migratory stopovers, and around eighty-five percent have been lost since 1700 to drainage for farmland, disease control and building. Drained peat emits carbon for decades and burns underground, and rewetting reverses both.