Peat

Peat is dead plant material that has not decomposed, and the reason it has not is water. A waterlogged bog is anoxic below a few centimetres, the bacteria and fungi that would break the material down cannot work without oxygen, and so organic matter accumulates instead of returning to the air.

It accumulates slowly — on the order of a millimetre a year — so a metre of peat is roughly a thousand years of vegetation, and the deep bogs are ten thousand years old and have been forming since the last glaciation ended.

It is also the first step of the sequence that makes coal. Bury peat, compress it, heat it, and it becomes lignite, then bituminous coal, then anthracite, losing water and volatiles and concentrating carbon at every stage. Every coal seam in the world was a peat bog, and peat is the only stage of that sequence still visibly in progress.

Two conditions, and both must hold: more water than drains away, and enough plant growth to supply material.

Blanket bog forms where rainfall is high and steady enough to waterlog even sloping ground, which is why it covers so much of western Ireland and the Scottish Highlands. Raised bog grows upward from a filled lake basin until the surface is domed above the water table and fed only by rain — genuinely rain-fed, so acidic and nutrient-starved, which is why bog plants are adapted the way they are and why some of them eat insects.

Sphagnum moss does most of the work and does it actively rather than incidentally. It holds many times its own weight in water, acidifies its surroundings chemically, and its dead lower parts are the peat while the living top continues upward. A raised bog is, in effect, a single organism's waste heap that it is still living on top of.

The anoxia and acidity together are why bogs preserve. Bog bodies — Tollund Man, Lindow Man — retain skin, hair and stomach contents after two thousand years, tanned rather than decayed, by the same chemistry that stops the moss rotting.

Fuel, where there was nothing else. Cut by hand with a slane, stacked to dry through a summer, and burned through a winter — the domestic economy of much of Ireland and the Hebrides within living memory, and industrially significant in Ireland and Finland into the 2000s, where peat-fired power stations ran on milled peat harvested by machine.

Horticulture, which is now the larger use in Britain and much of Europe. Peat is the base of most potting compost because it holds water, drains, and has an open structure — not for nutrition, of which it supplies almost none.

Whisky, in a small volume with a disproportionate effect. Burning peat under germinated barley in the kiln deposits phenolic compounds on the grain, and those survive the whole of malting, mashing, fermentation and distillation to give Islay whiskies their character. A few tonnes of peat a year flavours a very large amount of spirit.

Growing media and filtration take the rest, and peat's cation exchange capacity makes it useful as a soil conditioner where the structure matters more than the fertility.

Peatlands cover roughly three per cent of the world's land and hold somewhere near a third of all soil carbon — more than all the world's forests combined, in a fraction of the area. That single comparison is the whole of the case against extracting them.

And the carbon released is not mainly the carbon burned. Draining a bog to cut it lets oxygen into peat that has been anoxic for thousands of years, and the whole exposed profile begins to decompose and oxidise. A drained peatland emits continuously, for decades, whether or not anything is ever taken off it — which is why peat's emissions per unit of energy are worse than coal's once the drainage is counted, and why the horticultural use is a climate problem despite nothing being set on fire.

The secondary effects are large too. Bogs regulate river flow, and drained catchments flood faster. Drained peat burns, and peat fires — Indonesia's in 1997 and 2015, spreading underground through drained tropical peat swamp — are among the largest single carbon release events on record.

The direction of policy has turned accordingly: Ireland ended peat-fired generation in 2020, the UK is phasing out horticultural peat, and large-scale rewetting is now funded as carbon abatement. Coir, bark, wood fibre and composted green waste are the horticultural substitutes; they behave differently enough that growers have had to change practice rather than swap a bag, which is why the transition has been slower than the policy.