Mark Kinver BBC News 30 Nov 16;
A warmer world will release vast volumes of carbon into the atmosphere, potentially triggering dangerous climate change, scientists warn.
Writing in journal Nature, they project that an increase of 1C (1.8F) will release an additional 55 billion tonnes of carbon into the atmosphere by 2050.
This could trigger a "positive feedback" and push the planet's climate system past the point of no-return.
Previous assessments have not taken carbon released by soil into account.
In their Nature paper, an international team of scientists said that the majority of the Earth's terrestrial store of carbon was in the soil.
They warned that as the world warmed, organisms living in the planet's soils would become more active, resulting in more carbon being released into the atmosphere - exacerbating warming.
"There have been concerns about this positive feedback for a long, long time," said lead author Thomas Crowther, who conducted the research while based at Yale University, US, but now at the Netherlands Institute of Ecology.
"For the past two or three decades there have been literally thousands of studies trying to address this topic and trying to identify whether there are going to be increases or decreases in carbon uptake of the soil in relation to warming or increases in carbon loss."
Considerable losses
Dr Crowther said the uncertainty surrounding the exchange of carbon between the atmosphere and the planet's soils had led to "sizeable differences in the projections of future climate conditions".
He told BBC News: "We are the first study to take a global perspective and then map the variability and able to say that in these areas there are going to be huge losses and in these areas there are going to be some gains.
"Using this approach we can get a robust idea of the whole picture. We show that, actually, the losses are going to be really considerable."
Using data stretching over 20 years from 49 sites across the globe, the team observed that global carbon stocks would fall by up to 55 petagrams (55 billion tonnes) under a business-as-usual scenario, which is roughly equivalent to adding the emissions from a nation the size of the US.
Dr Crowther, whose team had produced a short video on the subject, added: "I do not positive as in 'good' but positive as in it is reinforcing, so it is a process that once it has kicked off, it leads to the acceleration of itself.
"Carbon comes out of the soil, which leads to more warming, which leads to more carbon out of the soil, it is a reinforcing cycle. The concerning thing is that our projection is that we are going to lose 55 petgrams, that's 55 trillion kilograms by 2050. This process is only going to accelerate and accelerate.
In the global carbon cycle, soils act as a depository, a place where carbon is stored in a state that does not directly influence the global climate system.
He observed: "The carbon is trapped in the soil because it is taken from the atmosphere by plant material through photosynthesis. Particularly in cold places, it get stored in the soil for a very long time, and this minimises the atmospheric concentrations.
"In the soil, there are microbes and soil animals, as well as plant roots, and they all use that soil carbon for their growth and activity.
"Where it is really cold, the activity and growth is limited but when it warms, and warming is likely to be disproportionately happening in cold areas, then the more active they are set to become."
Dr Crowther said the increased activity by the organisms would mean that they would consume greater volumes of the carbon in the soil, and this would be released as carbon dioxide, a greenhouse gas.
"It is very similar to the way we respire and produce carbon dioxide. Because there is such a huge biomass of microbes and soil animals, that respiration really can be massive," he said.
Map of temperature change
One of the latest milestone in the global effort to curb climate change was the Paris Agreement, which was signed at the UN Framework Convention on Climate Change (UNFCCC) summit, known as COP21, in the French capital city in December 2015.
In it, nations agreed to keep the increase of the average global temperature to below 2C (3.6F) of pre-industrialisation levels.
Dr Crowther said that the soil carbon study highlighted the importance of politicians and policymakers to heed the results of scientific studies on the issue of climate change.
"I really do want to get the message across that the strength of feedbacks like this really do stress the need to meet the targets of COP21," he urged.
"The feedback will exist and it will occur even if we do meet these targets but the magnitude of this feedback is going to be minimised hugely and it is really going to dampen the strength of it and it would prevent these enormous losses we expect by the end of the century if greenhouse gases are cut.
"These findings really do reinforce the necessity to meet those targets."
