Our Warming World, Another Feedback Loop

26 years of study by researchers from New England have revealed yet another global warming feedback loop underway, the release of CO2 from the warming of soil. Think of it as cooking our dirt.

The team from the Marine Biological Laboratory (MBL) in Massachusetts, the University of Massachusetts, and the University of New Hampshire, says rising temperatures cause a two-stage cycle where carbon output increases for several years then levels off – probably due to soil microbes adjusting to the warmer conditions.

After that readjustment, carbon release levels start rising again. The worry is that warmer soil would cause a warmer atmosphere, in turn heating up the ground and perpetuating temperature rise we have no control over.

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Way back in 1991, heating coils similar to those used to keep sports pitches from freezing were buried about 10 centimetres (3.9 inches) underground in the Harvard Forest, keeping certain parts of the forest soil surface 5°C (9°F) warmer than control plots.

Since then, the heated patches have lost 17 percent of the carbon stored in the top 60 centimetres (23.6 inches) of soil, where most organic matter is held. Carbon loss in warmer soil outpaced the rate in the control plots in two bursts, 1991-2000 and 2008-2013.

"If a significant amount of that soil carbon is added to the atmosphere, due to microbial activity in warmer soils, that will accelerate the global warming process," says Melillo. "And once this self-reinforcing feedback begins, there is no easy way to turn it off. There is no switch to flip."

As the team points out, every year we pump about 10 billion tonnes of carbon into the atmosphere, mostly through burning fossil fuels. About 3,500 billion tonnes are thought to be stored in the world's soil.

A significant part of that is stored in big pools of frozen soil in the Arctic – the carbon there is easily decomposed, and we know temperatures at the poles are on the rise.

This report highlights the disconnect between world leaders committed to slashing man-made greenhouse gas emissions and the warming targets they've set.  Their aspirational quotas fail to take into account these natural feedback loops from the loss of Arctic sea ice to the retreat of ice caps and glaciers, to the melting of the permafrost and the release of the CO2 and methane once safely sequestered. Despite our best efforts, and "best" is unduly generous, our cuts in man-made emissions may be overwhelmed by natural emissions, feedback loops that we have already triggered.