sPeople breathe carbon dioxide, and trees breathe carbon dioxide. It’s one of the first things kids learn about the carbon cycle, and the paths carbon takes as it travels between the living and nonliving things that make up the planet. This may be part of the reason 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.
Planting trees looks great and leads to stunning photos of CEOs and chiefs turning the soil with golden shovels – and there is compelling evidence that new trees and existing forests can help reduce the concentration of carbon dioxide in the atmosphere. But the role of trees and forests in global warming is more complex than it might seem. Anyone hoping to harness the power of trees to combat global warming needs to 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 in the early 2000s, which was inspired by the green belt movement that began in Kenya in the 1970s. The current figure comes from a much-cited 2015 research paper that calculated that planting an additional 1.2 trillion trees would absorb the equivalent of 10 years of carbon emissions. A subsequent 2019 paper calculated that a trillion trees could fit into about 2.2 million acres of available land, though its definition of “available” has been contested.
Remember the basic sciences
How will trees succeed in this feat? In a word, photosynthesis. Carbon dioxide (CO2) and water (H2O) are the ingredients for this recipe; The light acts as energy that helps the plant recombine hydrogen, oxygen and carbon into carbohydrates (CH2O) and oxygen (O2).
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Plants use some of the carbohydrates they make through respiration. This is the same process that people use when we turn the food we eat into energy; Like humans, plants breathe in some carbon dioxide when they breathe in. On average, plants release about half of the carbon dioxide they absorb and store the rest in their bodies as biomass while they are alive. Trees can store more carbon in their bodies and retain it for 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, nothing could break down the tough lignin that gives wood its hardness, so dead trees accumulated in swamp sediments that hardened under stress and over time. Some of these deposits became coal seams that are now mined and burned, releasing carbon stored in ancient forests. Named after the carbon-rich coal seams, the Carboniferous period is surrounded by layers of rock where geologists can find fossils of trees, ferns, marine animals, and other creatures from the ancient world.
However, today’s fungi have evolved to be able to break down lignin, and trees eventually decompose after dying like the rest of us. Fungi and other decomposers also produce carbon dioxide through respiration, so the carbon that trees store can be re-released into the atmosphere as they decompose. Trees also release their carbon if they burn, either in wildfires (which have increased in frequency and intensity with global warming) or slash-and-burn practices used by farmers and ranchers that clear forests for planting. These are key details to keep in mind when considering the role of forests in combating global warming.
Bio carbon sink in danger
Despite these disturbances and a slow decomposition process, Earth’s forests remain a net sink for carbon dioxide. The planet is currently home to about 4 billion hectares of forest, which together emit 8.1 billion metric tons of carbon each year and absorb 16 billion metric tons. The net uptake of 7.6 billion metric tons is more than what the United States exports in a year and about 30% of the amount the world exports in a year.
One might assume that the most important carbon sinks would be tropical rainforests, the most diverse 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 due to fires, farm clearing, and peat soil drainage. The Amazon rainforest is on the verge of becoming a net exporter 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 an important carbon source. These terrible statistics are part of the reason why protecting forests, especially rainforests, has become a major talking point about removing carbon from the atmosphere and slowing global warming.
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“Whether it’s in the Amazon or in the Alaskan Tongas rainforest…these are all the lungs of our planet,” says Dominic Delasala, chief scientist at Wild Heritage, an environmental organization based in Berkeley, California. “The logging and development that takes place 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 sourced from ancient forests.
Many other globally traded products are grown on land cleared of rainforests, including beef, cocoa, and palm oil. A complex commodity market can make it difficult to determine which products are grown on former rainforest lands, but companies like Nestlé and IKEA have published “forest positive” plans to reduce the amount of deforestation included in their supply chains through efforts such as satellite monitoring and mapping supply chain.
Carbon offset problem
Some companies invest directly in forest protection through the carbon offset market. Organizations like the Coalition for Rainforest Nations and the Rainforest Trust sell the opportunity to protect rainforests for as little as $5 an acre, money they say goes to Indigenous Peoples, local governments and other groups who might otherwise choose to lower the rainforest for economic reasons. Companies can purchase these credits to offset their greenhouse gases as part of the carbon accounting involved in reaching net zero.
However, ProPublica reported in 2019 that several forest protection projects that received funds from carbon credit sales did not deliver on their promises; Some protected plots have been cleared despite people paying to keep them forested. Even when the people involved stick to their commitments, forests designated to offset carbon can burn due to wildfires, releasing carbon.
Additionally, there is not enough land available for carbon projects (and without affecting food security). A 2021 report from Oxfam notes that “the total land area required for planned decarbonization could be five times the size of India, or the equivalent of all agricultural land on the planet.”
Some carbon offset projects involve planting new trees, but these plantings do not absorb as much carbon as mature natural forests. However, each tree can absorb tens of pounds of carbon dioxide per year, and carbon credit vendors, governments and organizations are all so involved in planting trees “that we are now concerned about the supply chain for planting trees. To make sure we will be able to get enough of the seeds to meet that demand,” says Joe Fargeon, North American chief scientist 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 an increase in carbon emissions with side effects that can be difficult to anticipate ahead of time. For example, planting trees in grasslands can increase the risk of fires and release carbon stored naturally in that environment’s plants and soil. Drying peatlands to grow trees releases carbon that wetland ecosystems can hold 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 abundance of carbon dioxide emissions humans have added to the atmosphere. To preserve the current role of trees as a sink for a large slice of carbon dioxide emissions, the priority must be restoring and maintaining mature forests that still exist, and finding better ways to protect them than ourselves.
– With a report by Jennifer Jennings
This article is part of a series on key topics in 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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