World relies too heavily on soil, plants in CO2 fight

The world is counting too heavily on soil and plants to soak up planet-ravaging carbon pollution, researchers cautioned on 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 tradeoff.

“Either soil or plants, but not both, will absorb more CO2 as carbon levels rise,” said lead author Cesar Terrer, a researcher at Lawrence Livermore National Laboratory in California.

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 siphoned 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 pre-industrial levels, enough to heat up the planet’s surface by 4 to 6 degrees Celsius, according to a new generation of climate models.

With only 1.1 degrees Celsius of warming so far, the Earth 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 analyzed 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. 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 crucial 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,” canceling out the benefit.