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Global warming makes tropical soils leak CO2: study
PARIS, Aug 12, 2020 (BSS/AFP) – Tropical forest soil warmed in
experiments to levels consistent with end-of-century temperature
projections released 55 percent more CO2 than control plots, exposing
a previously underestimated source of greenhouse gas emissions,
researchers reported Wednesday.
Before humanity began loading the atmosphere with carbon pollution
by burning fossil fuels, the input and outflow of CO2 into soil — one
key element in Earth’s complex carbon cycle — remained roughly in
balance.
Gases emitted by deadwood and decaying leaves, in other words, were
cancelled out by microorganisms that feed on such matter.
But climate change has begun to upset that balance, according to a
new study, published in Nature.
“Carbon held in tropical soils is more sensitive to warming than
previously recognised,” lead author Andrew Nottingham, a researcher at
the University of Edinburgh’s School of Geosciences, told AFP.
“Even a small increase in respiration from tropical forest soils
could have a large effect on atmospheric CO2 concentrations, with
consequences for global climate.”
The quantity of carbon cycling each year through soils worldwide is
up to 10 times greater than human-generated greenhouse gas emissions.
Just a one-percent imbalance — with more carbon going out than in
— “would equal about ten percent of global anthropogenic (manmade)
carbon emissions,” noted Eric Davidson, a researcher at the University
of Maryland Center for Environmental Science.
Earth’s average surface temperature has risen just over one degree
Celsius (1C) above preindustrial levels, enough to boost the severity
of droughts, heatwaves and superstorms made more destructive by rising
seas.
But the increase in temperatures over land alone — excluding
oceans, which cover 70 percent of the planet — has been nearly 2C, or
double the global average.
– Carbon ‘sink’ to ‘source’ –
MORE/MRU/2251hrs
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In the experiments, Nottingham and colleagues placed heating rods
in a one-hectare plot of undisturbed primary forest on Barro Colorado
Island, Panama.
They warmed the soil to a depth of just over one metre (three feet)
by 4C over a period of two years.
Soil temperature is usually about a degree warmer than air temperature.
While such experiments have been conducted in higher latitude
forests, none had been carried out up to now in the tropics.
Climate models seeking to take into account the potential carbon
leakage from soil due to rising temperatures have relied on
theoretical calculations that underestimate outputs compared to the
field tests reported in Nature.
Extrapolating from the new findings, the study estimates that if
all the world’s tropical soils warmed by 4C for a two-year period some
time before 2100, it would release 65 billion tonnes of carbon —
equivalent to about 240 billion tonnes of CO2 — into the atmosphere.
“That is more than six times the current annual emissions from
human-related sources,” Nottingham said.
“This could be an underestimation, because we might see large
continued loss beyond the two years in our experiment.”
Nor are deeper stores of carbon — below two metres — taken into
account, he added.
No sweeping conclusions can be drawn on the basis of a single
experiment, the researchers caution.
“But the study adds to recently accumulating evidence that tropical
forests are unlikely to continue indefinitely to be carbon sinks as
the world warms,” said Davidson, who was not among the study’s
authors.
Up to now, tree cover and the ocean have together consistently
absorbed about half of the excess carbon emissions from human
activity, but there are signs that some forests may be experiencing
CO2 fatigue.
Stored CO2 is also released when trees are cut down.
Last year, a football pitch of primary, old-growth trees was
destroyed every six seconds, about 38,000 square kilometres (14,500
square miles) in all, according to Global Forest Watch.
BSS/AFP/MRU/2251hrs