The Earth is breathing heavily – and that’s a problem
Meta-analysis throws doubt on the ability of soil to act as a carbon sink. Natalie Parletta reports.
The ripple effects of rising global temperatures on Earth’s ecosystems extend to accelerated release of carbon from the soil, according to observations by US scientists.
Warmer temperatures are increasing the activity of microbes that feed on dead leaves and fallen trees, according to a report published this week in the journal Nature by Ben Bond-Lamberty of the Joint Global Change Research Institute in Maryland, and co-authors.
When bacteria and fungi consume the decaying matter, they convert the carbon to carbon dioxide which is then released into the atmosphere.
The global study drew from 25 years of converging data from over 1500 individual studies and information gathered from more than 500 towers around the world fitted with meteorological sensors that continually measure atmospheric variables such as temperature, humidity, wind speed, rainfall and carbon dioxide.
Bond-Lamberty says the breakdown of carbon stored in soils worldwide “has never been observed and quantified on such a large scale”.
Soil is a significant carbon reservoir, containing around twice as much as the Earth’s atmosphere. As it “breathes”, the carbon is released into the atmosphere – a process known as “soil respiration”.
The research confirms that microbes are key players in this process. Responsible for 54% of the soil’s respiration at the study’s inception in 1990, warmer temperatures increased the microbial contribution to 63% by 2014.
The microbes’ release of carbon from the soil increased by 1.2% over that period, surpassing plants’ conversion of carbon dioxide to oxygen.
Although that might not seem like much, on a global scale such an increase over a relatively short period is not trivial. “We’re talking about a huge quantity of carbon,” says soil scientist and co-author Vanessa Bailey.
Bond-Lamberty explains: “Because soil respiration is so large, even small percentage shifts translate into large amounts of carbon moving around.
“Specifically, soil respiration is probably 85 to 95 petagrams (with one petagram equalling a trillion kilograms) of carbon every year, compared to 10 to 12 put out by human industrial activities. So a small change in the former can have a big impact.”
The findings indicate that climate-driven release of soil carbon is happening across several ecosystems, and shows a distinctive global trend, according to the report.
This research generates new data for inclusion in climate change forecasting models.
Like other gases, carbon dioxide traps heat and prevents it from escaping the Earth’s atmosphere. Without carbon dioxide, the Earth would be uninhabitable, much like Mars. But scientists now agree that the excesses produced by human activity are causing extreme weather patterns and warming the planet.
Bond-Lamberty says their discovery that global warming impacts soil respiration is not unexpected. “But our finding that it’s rising faster than photosynthesis is troubling,” he adds, because it calls into question how robust the earth’s “carbon sink” will be.
Of further concern is the spiralling impact on climate change. Bond-Lamberty explains that as global warming increases microbial activity in soil, more carbon dioxide will be released, which in turn warms the atmosphere.
“Any release of long-stored carbon from global soils will thus constitute a feedback effect,” he says. But how strong that effect will be, they don’t know yet.