On the second day of the Glasgow UN Climate Summit COP 26, a total of 100 countries pledge to halting and reversing deforestation. Why do forests matter to the climate?
Land-clearing by humans accounts for almost a quarter of greenhouse gas emissions, largely deriving from deforestation for agricultural products such as palm oil, soy bean, and beef.
To keep climate change at a safe level, reducing future emissions through renewable energy and energy efficiency will not be enough. Reaching net-zero requires actively removing CO2 from the atmosphere because of (1) there are economic sectors such as aviation that will continue using fossil fuels and (2) the existing CO2 level is already enough to cause significant adverse impacts.
There are two non-exclusive approaches to draw down atmospheric CO2—natural and technological. The natural approach of planting forests will buy us time to develop technological carbon capture-and-store solutions that are economically feasible.
Photosynthesis removes CO2 from the atmosphere naturally, and trees are especially good at storing carbon removed from the atmosphere by photosynthesis. Trees are 30% carbon. According to World Resources Institute (WRI), existing global forests absorb a net 8.4 billion tons of CO2 out of the 40 billion tons of global emission per year. On the other hand, deforestation is the second biggest source of CO2 emission, after the burning of fossil fuels. When forests are cleared, they are often just being burned down. This biomass burning directly releases CO2 into the atmosphere. Decomposing of wood or forest-floor biomass by microorganisms also releases CO2. Even when forest is turned into timber for construction, the wood will eventually be decomposed by microorganism and CO2 is released.
Tropical rainforests collectively sequester more carbon from the atmosphere than other types of forest, but they’re also increasingly destroyed for agricultural expansion. The world’s three largest tropical rainforests are in the Amazon, Congo River basin, and Southeast Asia. Over the past 20 years, Southeast Asia rainforests have collectively become a net source of carbon emissions due to clearing and fires. The Amazon River basin, which has experienced extensive fires and clearing, is now barely a net carbon sink. Only Congo has enough standing forest left to remain a strong net carbon sink.
How do trees help fight global warming? The approach to remove CO2 through forests and trees can be relatively inexpensive compared to technological carbon removal options (currently about $50 per metric ton) and yield cleaner water and air in the process.
A recent article, “Mapping carbon accumulation potential from global natural forest regrowth” (in the journal Nature), shows that “natural forest regrowth”—allowing forests to grow back on lands previously cleared for agriculture and other purposes—has the potential to absorb up to nearly 10 billion tons of CO2 from the atmosphere. That’s the equivalent of soaking up 25% of global emissions from the atmosphere every year. This is on top of the carbon sequestration already provided by existing forests, which currently absorb around 30% of CO2 emissions each year. (See WRI figure below)
Besides forests, wetlands (such as peatlands, prairie potholes, bogs, river, lake and coastal wetlands) store significant amounts of carbon—up to 35% of the carbon stored in the U.S. So restoring forests and wetlands are very important tools in fighting climate change.
The restoration of forested land at a global scale could help capture atmospheric carbon and mitigate climate change. World ecosystems could support an additional 2.2 billion acres of continuous forest. This would represent a greater than 25% increase in forested area, including more than 200 billion tons of additional carbon at maturity. Such a change has the potential to store an equivalent of 25% of the current atmospheric carbon pool. The number of trees required to absorb an entire year of global carbon emission would probably run into over tens of billions—a mind boggling number, but not impossible.
A success story is that since 1978, forest coverage in China has increased from 12% to 22%, making China a world leader in converting bare land into forests. As part of the commitment to reaching net-zero carbon by 2060, China will plant 36,000 sq. km. (nearly 9 million acres, larger than the size of Belgium) of new forest a year from 2021 to 2025.
Besides fighting climate change, trees’ environmental benefits include (1) reducing air temperature by shading sunlight and cool the space around it through evapotranspiration; (2) reducing soil moisture loss; (3) reduce surface water runoff from storms and reduce soil erosion, and other others. So planting trees is a no-regret action, does not need to involve trade-offs, and planting more trees the better.