How long is carbon is stored in plants? Not as long as we think, says study challenging efficacy of tree planting in climate fight

Researchers have thrown a wrench into plans to combat climate change through large-scale tree planting. A new study has raised concerns about the effectiveness of nature-based carbon removal projects, specifically those focused on forestation.

The study, published in journal Science, highlighted shortcomings in current climate models used to predict the longevity of carbon stored in trees. The models, according to the research, may be overestimating the amount of time carbon stays trapped in plants and underestimating the impact of climate change on forests themselves.

This means that the plants absorb more carbon than expected, but they remain stored for a shorter period of time because it returns to the atmosphere sooner than previously predicted. 

“Many of the strategies being developed by governments and corporations to address climate change rely on plants and forests to draw down planet-warming carbon dioxide and lock it away in the ecosystem,” Heather Graven, reader in climate physics at Imperial’s Department of Physics, said in a statement.

The potential for such nature-based carbon removal projects, she explained, is limited. The expert called for a quick ramp-down of fossil fuel emissions to minimise the impact of climate change.

Plants and soils absorb 30 per cent of carbon dioxide emissions from human activities each year, thereby mitigating climate change and its consequences, according to studies.

However, the new study highlighted some gaps in our understanding. For example, it is unclear how this storage happens and its stability is not well understood.

Various models have estimated that global net primary productivity, or the rate of creation of new plant tissues and products, ranges between 43 and 76 petagrammes (billion tonnes) of carbon (PgC) per year. These models did not show a strong trend throughout the twentieth century, the paper said.

To find more concrete answers on carbon cycling, researchers turned to a radioactive isotope (variant) of carbon called radiocarbon (Carbon-14 or C-14). Though radiocarbon is produced naturally, nuclear bomb testing in the 1950s and 1960s increased its levels in the atmosphere. 

This extra C-14 was absorbed by plants all over the world, allowing researchers to track its accumulation in the terrestrial biosphere and assess rates of carbon uptake and turnover. This was compared to the accumulation of C-14 in plants between 1963 and 1967, when there were no major nuclear detonations.

They fed this into the model to simulate how plants use carbon dioxide at a global scale and the interactions between the atmosphere and the biosphere.

Their analysis revealed that the net primary productivity is likely at least 80 PgC per year presently, compared with the 43 to 76 PgC per year predicted by current models. The C-14 accumulation in vegetation over 1963-67 was 69 ± 24 ×10 raised to the power 26.

As for the storage of carbon from human activities in terrestrial vegetation, the study found it is likely more short-lived and vulnerable than previously predicted.

“The observations show that the growth of plants at the time was faster than current climate models estimate that it was. The significance is that it implies that carbon cycles more rapidly between the atmosphere and biosphere than we have thought, and that we need to better understand and account for this more rapid cycling in climate models,” Charles Koven from Lawrence Berkeley National Laboratory said in a statement.

The paper called for a longer analysis of subsequent decades after 1967 to find critical insights into whole-ecosystem cycling, including litter and soil.