How excess CO2 is bulking up trees

WE know that trees guzzle up CO2. Their roots and leaves absorb the emissions released into the atmosphere, which makes them valuable in the fight against global warming.

But scientists have now discovered that excess carbon dioxide is contributing to increasing the wood volume of forests in the US, and at the same time increasing the capacity of forests to store CO2.

Between 700 and 800 million tonnes of carbon dioxide has been sequestered by US forests each year for the past two decades (or about 10 to 11% of the country's total CO2 emissions), estimates a new study published in the journal Nature Communications.

The research also shows that high levels of carbon dioxide in the atmosphere have increased the volume of trees in these US forests. According to the scientists' estimates, modern vegetation could be between 20% and 30% larger than before.

"If applied to the Coast Redwood forests -- home to some of the largest trees in the world --, even a modest percentage increase means a lot of additional carbon storage in forests," the scientists explain in a news release.

This phenomenon also applies to large, older trees, which continue to increase in size as they age. High levels of carbon dioxide actually appear to have contributed to an increase in the mass of most trees, both those that have grown naturally and those that have been planted by humans.

Studied between 1970 and 2015 in this research, this significant increase in tree wood volume caused by a net increase in carbon emissions has a name: "carbon fertilization." Simply put, an influx of carbon dioxide increases a plant's rate of photosynthesis and accelerates its growth, drawing on energy from the sun, water, and nutrients from the ground and air.

The study authors, who estimate that this wood volume response to carbon dioxide in Earth's ecosystem could be "even higher" than described in previous work, see this as an opportunity to reduce the cost of global warming mitigation.

"Carbon fertilization certainly makes it cheaper to plant trees, avoid deforestation, or do other activities related to trying to enhance the carbon sink in forests. We should be planting more trees and preserving older ones, because at the end of the day they’re probably our best bet for mitigating climate change," explains Brent Sohngen, co-author of the study and professor at The Ohio State University, USA