How do trees affect climate change?

P.people breathe out carbon dioxide, trees breathe carbon dioxide. It is one of the first things children learn about the carbon cycle, the paths that carbon takes as it moves between the living and non-living things that make up the planet. This may be part of why trees and forests have long been a focal point of the carbon sequestration conversation. Dozens of companies have committed to planting and protecting trees as part of their efforts to tackle greenhouse gas emissions, and by 2030, the Trillion Trees campaign aims to increase the number of trees in the world by a third.

Tree planting sounds great and creates stunning photos of CEOs and presidents turning the land with shovels of gold, and there is compelling evidence that both new trees and existing forests can help reduce carbon dioxide concentrations in the ‘atmosphere. But the role of trees and forests in global warming is more complex than it seems. Anyone hoping to harness the power of trees in the fight against global warming must appreciate this complexity.

Forest protection and tree planting projects predate the idea of ​​net-zero: the Trillion Trees campaign is a continuation of the Billion Trees campaign from the early 2000s, which was inspired by the Green Belt Movement that started in Kenya in the 1950s. 70. The current number comes from a much-cited document from 2015 which calculated that planting another 1.2 trillion trees would absorb the equivalent of 10 years of carbon emissions. A subsequent document from 2019 calculated that 1 trillion trees could fit about 2.2 million acres of available land, although its definition of “available” has been contested.

Remembering the basic science

How would trees do this feat? In a word, photosynthesis. Carbon dioxide (CO2) and water (H2O) are the ingredients of this recipe; light acts as energy that helps the plant reassemble hydrogen, oxygen and carbon into carbohydrates (CH2O) and oxygen (O2).

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Plants use some of the carbohydrates they produce through breathing. This is the same process that people use when we convert the food we eat into energy; like people, plants exhale some carbon dioxide when they breathe. On average, plants emit about half of the carbon dioxide which they absorb and store the rest in their bodies as biomass while they are alive. Trees can store more carbon in their bodies and hold it longer than most plants because they are larger, denser, and live longer than the average blade of grass.

For nearly 100 million years after trees evolved in the Carboniferous period, nothing was able to break down the hard lignin that gives the wood its stiffness, so dead trees have accumulated in swampy deposits that have hardened under pressure and over time. Some of these deposits have become the coal deposits that are now being mined and burned, releasing the carbon stored by the ancient forests again. The Carboniferous period gets its name from these carbon-rich coal deposits, surrounded by layers of rock where geologists can find fossils of trees, ferns, marine animals and other creatures from a bygone world.

Today, however, fungi have evolved to be able to break down lignin and trees eventually decompose after death like the rest of us. Fungi and other decomposers also produce carbon dioxide through respiration, so the carbon stored by trees can be released back into the atmosphere as they decompose. Trees also release their carbon as they burn, in fires (which have increased in frequency and intensity with global warming) or in slash-and-burn practices employed by farmers and ranchers clearing forests for agriculture. This is a key detail to keep in mind when considering the role of forests in the fight against global warming.

A vital carbon sink at risk

Despite these disturbances and the slower decay process, the earth’s forests remain a sink of carbon dioxide. The planet is currently home to approximately 4 billion hectares of forest, which collectively emit 8.1 billion tons of carbon annually and absorb 16 billion tons. The net absorption of 7.6 billion tons is more than what the United States emits in a year and about 30% of the amount the world emits in a year.

It could be assumed that the most significant carbon sinks are tropical rainforests, the most biodiverse biomes on earth. But the tropical rainforests of Southeast Asia, one of the three largest systems in the world, are now a net source of carbon emissions from wildfires, plantation remediation and peaty soil drainage. The Amazon rainforest is on the verge of becoming a net source due to similar disturbances. The second largest tropical rainforest in the world, located in the Congo River Basin, is the only rainforest in the top three that is still a significant carbon sink. These dire statistics are part of why protecting forests, especially rainforests, has become a key point of discussion about decarbonising the atmosphere and slowing global warming.

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“Whether it’s the Amazon or Alaska’s Tongass Rainforest … those are all the lungs of our planet,” says Dominick DellaSala, chief scientist at Wild Heritage, an environmental organization based in Berkeley, California. “The deforestation and development that occurs in those forests, which forever changes their ability to absorb and cling to carbon.” DellaSala says companies can avoid being part of the problem by avoiding wood and fiber from centuries-old forests.

Many other products traded globally are grown on land cleared of rainforests, including beef, cocoa and palm oil. The complex commodity market can make it difficult to keep track of which products are grown on former rainforest land, but companies like Nestlé and IKEA have released ‘positive forest’ plans to reduce the amount of deforestation involved in their supply chains. through efforts such as satellite monitoring and supply chain mapping.

The problem of carbon offsetting

Some companies are investing directly in forest protection through the carbon offsetting market. Organizations like the Coalition for Rainforest Nations and the Rainforest Trust sell the opportunity to protect the rainforest for as little as $ 5 per acre – money they say goes to indigenous people, local governments and other groups they might otherwise choose. to reduce the rainforest for economic reasons. Companies can purchase these credits to offset their greenhouse gases as part of the carbon accounting required to reach net zero.

However, ProPublica reported in 2019 that several forest protection projects that had received money from carbon credit sales were not keeping their promises; some protected plots were cleared even though people had been paid to keep them wooded. Even when the people involved live up to their commitments, forests set aside for carbon offsetting can be burned by fires, releasing their carbon.

Furthermore, there is not enough land available for carbon dioxide projects (and with no impact on food security). A 2021 report from Oxfam notes that “the total amount of land required for planned carbon removal could potentially be five times the size of India, or the equivalent of all agricultural land on the planet.”

Some carbon offsetting projects involve planting new trees, but these plantations do not absorb as much carbon as mature natural forests. However, each tree can absorb tens of pounds of carbon dioxide in a year and carbon credit sellers, governments and organizations are all involved in tree planting “to the point that we are now also concerned about the supply chain for tree planting. to make sure we are able to have enough seeds to meet that demand, “says Joe Fargione, chief scientist for North America at The Nature Conservancy.

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Fargione says the most effective tree-planting projects focus on restoring existing forests, rather than trying to create new ones. If planted in the wrong environment, trees can cause increased carbon emissions through side effects that can be difficult to predict in advance. For example, planting trees in grasslands can increase the risk of fire by releasing the naturally stored carbon into the plants and soil of that environment. Peatland drainage to plant trees releases the carbon that those wetland ecosystems can hold back for centuries.

As much as they love trees and forests, scientists like Fargione and DellaSala agree that we can’t rely on them to take care of the excess carbon dioxide emissions humans have added to the atmosphere. To maintain the current role of trees as a sink for a large share of carbon dioxide emissions, the priority should be to restore and maintain mature forests that still exist, by finding better ways to protect them from ourselves.

—With referral from Jennifer Junghans

This article is part of a series on the key topics of the climate crisis for time.com and CO2.com, a division of TIME that helps companies reduce their impact on the planet. For more information, go to co2.com

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