Climate Feedback

Watching peat dry

Peat_Lewis.jpgIt’s a buried time bomb of greenhouse emissions – and it’s even less photogenic than melting permafrost. A team of researchers led by Takeshi Ise of the Japan Agency for Marine-Earth Science and Technology has been watching peat dry.

Peat – “an accumulation of partially decayed vegetation matter”, in Wikipedia’s appetizing phrase – forms in bogs and swamps where the acidic, waterlogged, oxygen-poor soil smothers the decomposition process, just as permafrost freezes it out. That makes it a big sink for carbon that would otherwise have joined the atmosphere as the plants composted. But peat’s not just a sink, it’s a sump – and a snowballing one. The large amount of water peat can hold lowers the oxygen available, which makes more peat accumulate, which sucks up more groundwater and blocks it from draining.

It’s this feedback process, as it occurs in the northern bogs of Manitoba, Canada, that Ise et al. succeeded in accurately modeling for the first time in a paper published this week in Nature Geoscience (subscription required). Their bad news is that warming air temperatures reverse the loop: the peat dries and decays, then can’t hold as much water and dries and decays some more.

As Joseph Romm points out on Grist and Climate Progress, that potentially makes the peat loop a link in a bigger, and climatically more important, vicious circle – the one where temperatures raised by human emissions start an uncontrollable release of methane and carbon dioxide from natural stores like peat and permafrost.

Romm writes:

[The study] projects that “a warming of 4 degrees C causes a 40 percent loss of soil organic carbon from the shallow peat and 86 percent from the deep peat” of Northern peatlands. And that amplifying carbon cycle feedback is dangerous for three reasons:

1. The northern peatlands are believed to store some 320 (+/- 140) billion metric tons of carbon, roughly half of what the atmosphere contains.

2. Peatlands tend to emit much of their carbon in the form of methane, which is more than 20 times as powerful a greenhouse gas as carbon dioxide.

3. A warming of 4 degrees C this century is all but inevitable if we don’t sharply reverse emissions trends quickly.

There’s at least one key figure missing from this summary: the hundreds of years that the authors say it would take to complete those carbon losses. But according to the Ise et al. model, twenty-first century climate change predicted from the IPCC’s medium-emissions A2 scenario would indeed wreak some havoc on the peat-bog components that decay most easily, such as leaves and fine roots. Each dry spell a few years long knocks out more than 20% of the carbon in that small portion of the peat, they report.

What fraction of northern peat’s 320 billion tonnes of of carbon would such droughts release? There’s no estimate here, though even a small one could represent Romm’s “point of no return beyond which it becomes all but impossible to stop catastrophic global warming”. Because of the centuries-long window of change, however, the authors explicity note that

The massive soil organic carbon loss induced by the soil-condition–carbon feedback can be prevented if the temperature rise is reversed within a few hundred years.

The important take-home may not be the 40% and 86% numbers themselves, but that the peat layers now seem much more responsive to temperature than they did in the previous generation of models. Too often, learning more about the subtle physical and biogeochemical processes behind climate change makes it even scarier.

Anna Barnett

Photo: Peat in Scotland / Wikipedia user Lysy


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