Tree planting to restore natural foliage can help to ease the climate crisis. So someone has to pay its massive price.

LONDON, 16 December, 2020 − The world can grow out of its climate emergency − but at a price. Enough tree planting around the world could achieve a 10% reduction in carbon emissions − but only if landowners are paid to plant and protect them.

And by 2055, the bill for planting trees to keep global heating from going any higher than the internationally-agreed target of 1.5°C above the average for most of human history could be US$393bn (£297bn) a year.

Grassland restoration, on the other hand, can pay dividends. And since grasslands are home to 40% of the planet’s natural vegetation, the rewards could be substantial, a second study suggests.

The challenge is simple: to keep the world from climate catastrophe, nations must think of a way to absorb from the atmosphere, and sequester, six billion tonnes of carbon dioxide every year. This is roughly the same as the emissions from 1.3 billion passenger vehicles in one year. Forests can do this, if planted, nurtured and protected.

The costs however will be considerable, according to a new study in the journal Nature Communications. “The global forestry sector can provide a really substantial chunk of the mitigation needed,” said Justin Baker of North Carolina State University, one of the researchers.

“The physical potential is there, but when we look at the economic costs, they are non-linear. That means the more we reduce emissions − the more carbon we’re sequestering − we’re paying higher and higher costs for it.”

Scientists involved in this kind of research must solve a range of “what-if” puzzles. Repeatedly, they have examined the appalling price to be paid by nations, regions, cities and citizens if nothing is done and global temperatures rise uncontrollably.

They have calculated the implicit rewards of prompt action to limit fossil fuel use and contain climate change: these too, in terms of economies and political stability, are enormous.

They have calculated that there is enough exploitable land on which to plant new forests or extend existing ones, while still feeding the extra 2bn or more population rise expected before the century’s end.

But for the most part, they have not examined in detail the financial costs implicit in planting the world out of a climate crisis. And, it seems, the higher the ambition, the greater the cost. This cost seems to rise exponentially as it gets nearer the limits of the possible.

The researchers looked at the challenge of avoiding deforestation; of forest management; of stepping up harvest rotation; and of reforestation or afforestation, in 16 regions of the planet.

It could cost $2bn a year to sequester 600,000 tonnes of carbon dioxide in the foliage, trunks and roots of trees. But to absorb 10 times that could cost almost 200 times as much: a ten-fold increase could add up to an annual bill of $393bn.

The scientists also established that tropical rainforest nations would − if they restored or protected the forests of the Amazon, Indonesia and the Congo basin − contribute the largest share, in the race for global mitigation: from 72% to 82%. The southern US, too, could make a significant contribution.

But so too could, in a different way, the restoration of some of the African savannah, and other semi-arid landscapes. These important ecosystems already support perhaps a billion humans and their livestock; they regulate water cycles and support a wide variety of animals and plants. Importantly for climate scientists, they cover and enrich a substantial proportion of the planet’s soil organic carbon.

Researchers write in the journal Scientific Reports that they looked at data from a degraded sample of grassland in Kenya − invaded by a Mexican tree species, Prosopis juliflora, a kind of mesquite − to find that 40% of the life-enhancing soil organic carbon had disappeared. Thirty years of a restoration programme replenished soil organic carbon to a depth of a metre at the rate of 1.4% a year.

“The importance of managing grasslands to optimise carbon sequestration for climate change mitigation is widely recognised. Soils are the largest terrestrial carbon reservoir, containing more carbon than the vegetation and the atmosphere combined,” said Purity Rima Mbaabu, of the Kenya Forestry Research Institute, who led the study.

“Yet soil organic carbon, which makes up about two thirds of global soil carbon, is sensitive to land degradation, with significant negative consequences for soil quality and productivity, and an exacerbation of greenhouse gas emissions.” − Climate News Network