By Tim Radford
As the world warms so does much of the planet’s basic matter, thanks to its subterranean citizens, the soil microbes, intent on putting more energy into the important business of decay and recycling.
As a consequence, everywhere, more carbon dioxide escapes into the atmosphere, according to a new study. And since there is at least twice as much carbon in the soil – mostly in the form of plant detritus – as there is in the atmosphere, the discovery is ominous.
With more of the greenhouse gas escaping from soil to air, so the rate at which the world warms picks up speed, to accelerate yet further soil respiration.
US researchers report in the journal Nature that they looked at results from more than 1,500 studies and data from more than 500 monitoring towers around the world that measure temperature, rainfall and other evidence, to conclude that between 1990 and 2014 the rate of soil respiration has increased by 1.2%.
“It’s important to note that this is a finding based on observations in the real world. This is not a tightly controlled lab experiment,” said first author Ben Bond-Lamberty of the Joint Global Change Research Institute, partnered by the US Pacific Northwest National Laboratory and the University of Maryland.
With rising soil respiration rates, you won’t have an intact land carbon sink forever
“Soils around the globe are responding to a warming climate, which in turn can convert more carbon into carbon dioxide which enters the atmosphere. Depending on how other components of the carbon cycle might respond due to climate warming, these soil changes can potentially contribute to even higher temperatures due to a feedback loop.”
The role of soil microbes remains one of the great unknowns in the climate conundrum. A microbe – bacterium or fungus – is tiny but vital. Single-celled creatures managed the living world for two billion years before the first complex animals and plants, turning carbon dioxide into carbon and oxygen, and then converting dead lifeforms back into new raw materials for life.
And their numbers are huge: so vast that terrestrial bacteria add up to 70 billion tons of living carbon, and fungi another 12 billion tons, according to a recent estimate. Together these microbes weigh 40 times as much as all the animals on the planet.
And for years climate scientists have puzzled about the impact of warming on the silent living things under foot and out of sight. They identified the microbial world as key to the puzzle of the carbon budget and wondered about the rate at which soils could absorb carbon and store it. They have also warned that, in a warmer world, soil microbes might become more active.
The latest study seems to suggest that they are right. Carbon dioxide is being exhaled back into the atmosphere at a faster rate. Around 25 years ago, microbes accounted for 54% of soil respiration. Now they account for 63%.
If correct, this is a case of what engineers call positive feedback: humans burn fossil fuels, emit carbon stored deep underground 100 million years ago, and raise planetary temperatures. And as this happens, carbon once buried in the shallow soils – some that might otherwise in many millions of years become fossilised as coal – escapes to the surface as greenhouse gas, to create even more warming.
“We know with high precision that global temperatures have risen,” Dr Bond-Lamberty said. “We’d expect that to stimulate microbes to be more active. And that is precisely what we’ve detected.
“Land is thought to be a robust sink of carbon overall, but with rising soil respiration rates, you won’t have an intact land carbon sink forever.” – Climate News Network
This post was previously published on climatenewsnetwork.net and under a Creative Commons license CC BY-ND 4.0.
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