Massive forest restoration could greatly slow global warming. The right trees, planted in the right places, could store 205 gigatons of carbon dioxide.
By Mark Fischetti
We have heard for years that planting trees can help save the world from global warming. However, that mantra was primarily a statement of faith. Now the data finally exists, and it shows us that if the right tree species are planted in the right types of soil across the planet, emerging forests could capture 205 gigatons of carbon dioxide in the next 40 to 100 years. That's two-thirds of all the CO2 humans have generated since the industrial revolution. "Forest restoration is by far our most powerful planetary solution today," says Tom Crowther, professor of global ecosystem ecology at the Swiss Federal Institute of Technology in Zurich, and author of a study published Thursday in Science that generated the revelation. number.
The study team analyzed nearly 80,000 satellite photo measurements of tree cover around the world and combined them with huge global databases on soil and climate conditions, evaluating one hectare at a time.
The exercise generated a detailed map of the number of trees the Earth could naturally support: where forests grow now and where they could grow, outside of areas like deserts and savannas that are home to very few or no trees.
The team then subtracted existing forests as well as urban areas and land used for agriculture. That left 0.9 billion hectares that could be forested but have not been. If those spaces were filled with trees already blooming nearby, the new growth could store 205 gigatons of carbon for when the forests mature.
Of course, after 40 to 100 years, the storage rate will flatten as forest growth slows, but the researchers say the 205 gigatons would remain as old trees die and new ones grow. There would be "a bank of excess carbon that is no longer in the atmosphere," says Crowther.
Credit: "The global potential for tree restoration." Jean-Francois Bastin et al. in Sciences, vol. 365, number 6448, July 5, 2019.
The Earth could naturally support 4.4 billion hectares of forest (colors on the top map; gray represents areas like desert that have no potential). When existing forests, agricultural lands, and urban areas are subtracted from potential forest lands, 0.9 billion hectares (colors on the map below) remain where new forests could grow, removing 205 billion tons of CO2 from the atmosphere.
The team also created a planning tool linked to the map that will be open to the public starting July 5. Individuals and organizations can zoom into any location to see where new forests can be started.
Crowther hasn't studied other carbon sequestration techniques that have been much discussed lately, such as ocean fertilization (growing algae to absorb carbon) or direct air capture (machines that extract CO2 from the atmosphere), but he thinks they would be much more. expensive than growing trees. He estimates that it could cost the world $ 300 billion to plant the 0.9 billion hectares. And the new forests provide another strong benefit: they restore biodiversity, which is crucial because many species of plants and animals are disappearing.
Crowther says he started studying reforestation because he was really looking for ways to stop species loss. The huge benefits beyond carbon sequestration "come from biodiversity: the supply of food, medicine, clean water and all kinds of things for humans," he says.
However, removing all that carbon from the atmosphere could take longer than anticipated. Forests can take more than 70 to 100 years to reach full maturity, says Robin Chazdon, an ecologist and evolutionary biologist at the University of Connecticut, who was not involved in the study.
However, he says that any replanting should start as soon as possible because climate change could compromise the ability of forests to grow. Higher temperatures increase the tree's respiration, causing them stress. And the drought will widen, reducing the growth of trees. Crowther adds that while climate change will allow more trees to grow in northern latitudes, it will also dry out tropical latitudes. Tree loss in the tropics, he says, will exceed gains in the high north.
Chazdon cautions that replanting may not be as simple as it sounds, and wonders if 0.9 billion new hectares will be possible, given the priorities of the competition. More trees consume more water, and this could threaten agriculture or other human activities in dry areas.
And local people may not want forests if they need to generate income from the land, for example from agriculture or ranching. Some prominent reforestation programs, like those in the Philippines, have failed "because there was no local participation," he says.
The best places to start reforestation are where multiple benefits can easily be obtained. In a July 3 Science Advances article, Chazdon and his colleagues identified a number of locations in the tropics that have higher-than-average potential for profit, as well as ease of getting started.
All the work on new trees, says Chazdon, notes that "we are entering the practicality stage" of smart reforestation. “We can bring in a great deal of interdisciplinary science. I hope there will be more interaction between scientists and politicians, realizing that the tools we have now can guide reforestation which is the most profitable, and has multiple benefits and the fewest trade-offs. "