Global vulnerability of peatlands to fire and carbon loss

11 Peatland ecosystems accumulate thick organic soil layers because plant production exceeds decomposition throughout the entire organic soil column (Fig. 1). Peatlands cover only about 2–3% of the Earth’s land surface but store around 25% of the world’s soil carbon1. They are most abundant at northern high latitudes (Fig. 2a), where they cover roughly 4,000,000 km2 of land1 and store an estimated 500–600 gigatonnes of carbon (1 Gt = 1015 g). Tropical peatlands store an additional ~100 GtC across 400,000 km2, primarily in Southeast Asia1,2. Hence the global peat carbon pool exceeds that of global vegetation (~560 GtC) and may be of similar magnitude to the atmospheric carbon pool (~850 GtC)3. Peat is defined as an organic soil composed of partially decayed plant remains with less than 20–35% mineral content. Slow decomposition rates created by anaerobic conditions are viewed as a necessary condition for peatland development4. Plant remains are deposited into the upper peat layer, which usually is located above the mean water table for at least part of the year, and undergo aerobic decomposition. The remaining organic matter is buried and transferred to the saturated peat layer below the water table where decomposition is minimal. Thus, water-table depth is a key regulator of peatland decomposition and peat accumulation rates. If warming or disturbance lowers the water table in peatlands, removal of anaerobic constraints on decomposition will stimulate loss of peat carbon to the atmosphere5. Moreover, a lower water table also will stimulate the loss of peat carbon through combustion during wildfires2,6, which we discuss in more detail in the sections below.