PARIS, March 25, 2021 (BSS/AFP) – The world is counting too heavily on
soil and plants to soak up planet-ravaging carbon pollution, researchers
cautioned Wednesday.
Climate projections mistakenly assume that land and what grows on it are
able to absorb the CO2 humanity loads into the atmosphere, they reported in
the journal Nature.
In reality, there’s a trade off.
“Either soil or plants, but not both, will absorb more CO2 as carbon
levels rise,” lead author Cesar Terrer, a researcher at Lawrence Livermore
National Laboratory in California, told AFP.
It is tempting, he said, to hang hopes on supercharged plant growth and
massive tree-planting campaigns to reduce CO2 produced by burning fossil
fuels, agriculture and destroying forests.
But researchers said that when elevated carbon dioxide levels boost forest
and grassland growth, the accumulation of CO2 in soil slows down.
“Soils store more carbon worldwide than is contained in all plant
biomass,” said senior author Rob Jackson, a professor at Stanford’s School of
Earth, Energy and Environmental Sciences.
So far, Earth’s terrestrial ecosystems have kept pace with rapidly
increasing CO2 emissions, consistently absorbing some 30 percent even as
those emissions have more than doubled over the last 50 years.
Oceans have also syphoned off a steady 20-odd percent of CO2 pollution
during the same period.
Without these natural sponges, the amount of CO2 in the atmosphere today
might be double preindustrial levels, enough to heat up the planet’s surface
by four to six degrees Celsius, according to a new generation of climate
models.
With only 1.1C of warming so far, the planet has seen a crescendo of
deadly heatwaves, flooding and other extreme weather.
The new study adds to growing evidence that the terrestrial carbon sink is
weaker than once thought.
Terrer and colleagues analysed data from more than 100 published
experiments on soil carbon levels, plant growth and CO2 concentrations, which
have risen by half since pre-industrial times. – Put the bar higher –
They were surprised by the results.
“It proved much harder than expected to increase both plant growth and
carbon soil,” said Jackson.
Researchers found that soils only accumulated more carbon in experiments
where plant growth remained fairly steady, despite high levels of CO2 in the
air.
The findings highlight a key difference between two types of ecosystem,
and suggest that grasslands may turn out to be more important than long
assumed when it comes to stocking away carbon.
“In forests, additional CO2 mainly increases above-ground carbon storage,”
Terrer explained.
“But the acquisition of additional nutrients needed to fuel plant growth
increases soil carbon losses,” cancelling out the benefit.
In grasslands, by contrast, elevated CO2 causes a relatively modest bump
in biomass, while loss of carbon from the soil remains low.
“This new paper puts the bar higher for those models to capture the
additional complexities of above-ground versus below-ground carbon storage,”
said Pep Canadell, executive director of the Global Carbon Project.
“Carbon stored in the soils is probably better long term protected than
carbon in plants, which are susceptible to fire and other disturbances,” he
told AFP.
In 2019, the same researchers estimated that a doubling of CO2 compared to
mid-19th century levels — as expected by the end of this century — will
increase plant biomass by only 12 percent, far less than previously
predicted.
Other research has warned that forests are losing their effectiveness in
mopping up CO2.
In part, that’s because a football pitch of old-growth, primary forest is
destroyed every six seconds, releasing CO2 and reducing the areas left for
absorbing it.
And beyond a certain temperature threshold the capacity of plants to
absorb CO2 also declines, according to a study earlier this year.