Peat Bogs and the Carbon They Keep

A To the casual traveller, a peat bog can seem the least remarkable of places: a flat, soggy expanse of dull brown ground, treeless and often shrouded in mist, with little to catch the eye. For centuries such wetlands were dismissed as wastelands, fit only to be drained and put to some more profitable use. Yet scientists have come to understand that these unglamorous landscapes are among the most valuable on Earth, for locked within their waterlogged ground lies an immense store of carbon, and their fate has a direct bearing on the climate of the whole planet.

B A peat bog forms with extraordinary slowness, and only where certain conditions come together. In cold, wet, acidic ground that is starved of oxygen, the plants that die each year, chiefly a spongy plant known as sphagnum moss, do not rot away in the usual manner. The tiny organisms that would normally break down dead vegetation cannot thrive in such sour and airless conditions, and so the plant matter is only half-decayed. Layer upon layer of this partly rotted material builds up, compressing under its own weight into the dark, dense substance we call peat. A bog may deepen by less than a millimetre in a year, so that a metre of peat can represent a thousand years of growth.

C The importance of all this lies in what the peat contains. While they grew, the mosses and other bog plants drew the gas carbon dioxide out of the air, as all plants do, and used the carbon to build their tissues. Normally, when a plant dies and decays, that carbon is released back into the atmosphere; but in a bog the half-rotted plants never fully decay, and so their carbon stays locked in the ground instead of returning to the air. Over thousands of years the amount held in this way becomes enormous. Although peatlands cover only a small share of the world's land, they hold more carbon than all its forests combined, which makes them one of the planet's most important carbon stores.

D This great reservoir is safe only for as long as the bog stays wet. When people drain a bog to create farmland or to dig out the peat for fuel, air is let into ground that has been sealed off from it for centuries. The buried plant matter, exposed at last to oxygen, begins to rot in earnest, and as it does so the carbon that was locked away for thousands of years is released back into the atmosphere as carbon dioxide. A damaged, drying bog thus turns from a quiet store of carbon into a steady source of it, adding to the very warming that threatens the climate. Burning the drained peat releases its carbon faster still.

E A healthy bog is valuable for more than the carbon it keeps. Its wet, acidic ground is home to unusual plants and animals found in few other places, and it acts like a giant sponge, soaking up heavy rain and releasing it slowly, so reducing the risk of floods downstream and helping to keep river water clean. Bogs are also remarkable keepers of the past: because the airless, acid peat halts decay, objects and even human bodies thousands of years old have been recovered from them almost perfectly preserved, offering a rare window onto ancient life.

F For all these reasons, the draining and destruction of peatlands, once thought a mark of progress, is now widely regarded as a serious mistake, and efforts are under way to reverse it. The chief method is a process called rewetting, in which the drains cut into a bog are blocked so that the water table rises again and the ground is returned to its natural sodden state. Once rewetted, a bog stops releasing its carbon and, in time, the sphagnum moss returns and it can slowly begin to store carbon once more. Restoring these overlooked wetlands has come to be seen as one of the cheaper and more effective ways of helping to hold back a warming world.