The Compost Bomb: Peat and Global Warming

The Compost Bomb: Peat and Global Warming

06 Dezember 2010

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USA —  Peat, or turf, is an accumulation of partially decayed vegetation matter. Peat forms in wetland bogs, moors, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world. Peat has a high carbon content and can burn under low moisture conditions. Once ignited by the presence of a heat source, it smolders. These smoldering fires can burn undetected for very long periods of time (months, years and even centuries) propagating in a creeping fashion through the underground peat layer. The rate of global warming could lead to a rapid release of carbon from these peat lands that would then further accelerate global warming. Two recent studies published by the Mathematics Research Institute at the University of Exeter highlight the risk that this ‘compost bomb’ instability could pose, and calculate the conditions under which it could occur.

Peat fires are emerging as a global threat with significant economic, social and ecological impacts. Recent burning of peat bogs in Indonesia, with their large and deep growths containing more than 50 billion tons of carbon, has contributed to increases in world carbon dioxide levels. Peat deposits in Southeast Asia could be destroyed by 2040.

In 1997, it is estimated that peat and forest fires in Indonesia released between 0.81 and 2.57 Gt of carbon; equivalent to 13-40 percent of the amount released by global fossil fuel burning, and greater than the carbon uptake of the world’s biosphere. These fires may be partially responsible for the increase in carbon dioxide levels since 1998.

Underground peat fires are fairly common and world wide. In 2008 there was an underground peat fire in North Carolina triggered by a lightning strike and aided by a prolonged drought. In Australia (Victoria province)there has been a peat fire raging for 13 years. In 2010 Russia is suffering from a prolonged peat fire.

A peat fire, like any fire, requires fuel (peat), air, and ignition.

The first Exeter paper by Catherine Luke and Professor Peter Cox describes one of the basic potential ignition sources. When soil microbes decompose organic matter they release heat — this is why compost heaps are often warmer than the air around them.

The compost bomb instability is a runaway feedback that occurs when the heat is generated by microbes more quickly than it can escape to the atmosphere. This in turn requires that the active decomposing soil layer is thermally-insulated from the atmosphere.

Catherine Luke explains: “The compost bomb instability is most likely to occur in drying organic soils covered by an insulating lichen or moss layer.”

The second paper led by Dr Sebastian Wieczorek and Professor Peter Ashwin, also of the University of Exeter, proves there is a dangerous rate of global warming beyond which the compost bomb instability (i.e. ignition or spontaneous combustion occurs) occurs.

Spontaneous combustion occurs when materials self-heat to a temperature high enough to cause them to ignite. Typically, composting materials ignite at temperatures between 150 and 200°C.

As the temperature rises, the speed of temperature increase also rises. For example, heat is generated about 16 times faster at 100°C than at 60°C because the reaction rate approximately doubles with each 10°C rise in temperature.

The Exeter team is now modeling the potential impact of the compost bomb instability on future climate change, including the potential link to the Russian peat land fires.It is also working to identify other rate-dependent tipping points.

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