Peat accumulation

How can a bog that gains a millimeter a year store more carbon than a forest?

Peatlands cover roughly three percent of the land surface. A rainforest, by contrast, is the image everyone reaches for when asked where carbon is stored. Yet the peatlands hold something on the order of six hundred gigatonnes of carbon — comparable to, and by most estimates greater than, all the carbon held in the world's forest vegetation put together.

The bog is not growing fast. A northern peatland typically deepens by something like half a millimetre to a millimetre a year, and a mossy hummock produces far less plant matter annually than a stand of trees. So the puzzle is sharp: the slower, smaller system holds the larger store. What is a bog doing that a forest is not?

Begin with the forest, because it is the clearer case. A forest fixes an enormous amount of carbon each year — and returns almost all of it. Leaves fall, fungi and bacteria oxidise them back to carbon dioxide within a season or two, and even the wood is recycled within decades. A mature forest's store is large but roughly steady: it is a big standing balance, not a growing one. The trees are a reservoir, not an accumulator.

Now ask what would have to change for a store to grow. Not the input. If a system takes in a hundred units a year and returns a hundred, it accumulates nothing however large the hundred. The store grows only on the difference between two flows — and if that difference is small and positive, the store still grows, provided nothing resets it.

That is the whole of it, but the two halves are worth separating, because they are the two things a bog actually does.

The first half is why decay stops. A bog is waterlogged, and water carries very little dissolved oxygen and replenishes it slowly. Below the thin surface layer that the water table rises and falls through, the peat is effectively anoxic. Anaerobic decomposers extract far less energy per unit of substrate than aerobic ones do, so they work far more slowly, and the material they cannot break down simply stays. There is a further mechanism, proposed as the "enzymic latch": the enzyme that degrades phenolic compounds requires oxygen, so without oxygen those phenolics accumulate, and they in turn inhibit the enzymes that would break down everything else. The latch is a leading hypothesis rather than a settled quantity, and how much of the slowdown it accounts for is still argued — but the waterlogging itself is not in doubt. Sphagnum mosses sharpen every part of this: they hold many times their weight in water, acidify their surroundings, and are built of tissue that decays reluctantly even in air.

The second half is time. Suppose only about a tenth of what a bog fixes each year escapes decay — a small residue, perhaps twenty to thirty grams of carbon per square metre. That is nothing in a year. But most northern peatlands began forming after the last glaciers withdrew and have been running that residue for eight to eleven thousand years without interruption. Multiply a trivial annual surplus by ten thousand years and you get metres of peat and a continental-scale store. The forest, meanwhile, has been near balance the whole time. Accumulation is not a rate; it is a rate that was never reset.

A few refinements matter. First, the bog is not a place where nothing decays; it is a place where decay is slow and incomplete. Peat is partially decomposed plant matter, and methane bubbling out of a bog is proof that anaerobic decomposers are working. The residue is what is left over from an inefficient process, not what a stalled one failed to touch.

Second, the depth of peat records the whole history, not the current rate. Peatlands have a thin living surface layer where the water table fluctuates and most decay happens, and a permanently saturated layer beneath where material is essentially archived. Whether a given year adds to the archive depends on how long material lingers in that surface layer before it is buried below the water table.

Third, this reframes the conservation stake. Because the store is a physical accumulation rather than a standing crop, the asymmetry between building it and losing it is extreme: a drained peatland, or a peat fire, releases in years what took millennia to lay down, and no management can rebuild it on a human timescale. That asymmetry, not the annual uptake rate, is the reason intact peatlands are treated as irreplaceable.

• Why tropical peatlands, which accumulate faster, are also the most vulnerable to drainage and fire.
• Whether warming turns northern peatlands from a slow sink into a source, and what the field measurements currently show.
• Methane against carbon dioxide: how a bog can be a carbon sink and a potent greenhouse-gas source at the same time.

Every term the collection defines is gathered in the glossary.