What is the carbon cycle, anyway? Most people are familiar with the basics: trees and other plants absorb CO₂ from the atmosphere and convert it back to oxygen. Since CO₂ is a molecule that traps the sun’s heat, trees and other carbon-sequestering organisms help to offset and cool rising temperatures by reducing atmospheric CO₂. This is the system that has operated on planet Earth for billions of years, since the first carbon-based life forms began.
Rotch professor of atmospheric and environmental science Steven Wofsy is a bonafide tree expert. In this interview, he explains how trees function within Earth’s natural carbon cycle and the geological processes of heating and cooling, and how they are being impacted by contributors to accelerated climate change—including deforestation, increased wildfires, and urban expansion.
The story of trees in the carbon cycle begins with photosynthesis, one of the oldest and most elegant energy transformations on earth. Chlorophyll, a natural compound present in all green plants, draws in sunlight and uses it as a fuel to split CO₂ into sugars and oxygen. The sugars produced become both energy and building materials, powering the tree’s metabolism while forming the woody stems, roots, and leaves. When trees die, they fall to the forest floor, and the carbon they once held is released back to the soil—where microbial life goes to work, breaking down organic material and leaving behind carbon-rich residue.
In the tropics, carbon is stored in the dense wood of palms and other trees. In the Arctic, however, it goes elsewhere: it becomes locked in permafrost, the layers of soil and ice that cover around a quarter of the Northern Hemisphere. This frozen carbon, accumulated during thousands of years, is stable until climate warming activates soil microbes. These communities of soil bacteria slowly break the organic matter down, releasing it to the atmosphere as CO₂—generating more heat, and further thawing the surrounding ground.
The Arctic’s permafrost holds roughly twice the amount of carbon in the atmosphere. “So, one way that climate change affects carbon stored in forests and in tundra and other ecosystems of the north,” Wofsy explains, “is by mobilizing that carbon that’s been essentially frozen or stuck in the permafrost for decades, centuries, and millennia” (See “Plants on a Changing Planet,” May-June 2024).
The Impact of Deforestation on Earth’s “Lungs”—the Rainforest
What about other parts of the world where trees help to regulate CO₂? Nowhere are the impacts of deforestation more pronounced than in the Amazon. Once a bastion of biodiversity, vast tracts of the rainforest are now being cleared for agriculture, leaving behind a patchwork of soybeans and pasture grasses where towering Brazil nut trees—home to black spider monkeys and poison dart frogs—once grew (see “Models—and Mud—in Amazonia,” January-February 2011). These fields then become pasture or feed for cattle, which contribute methane to the atmosphere.
But these deforestation practices disturb more than just the carbon balance. In the rainforest, trees absorb water from the soil and release it into the air. This produces the rainfall that sustains the ecosystem. Without forests, this cycle collapses, leaving the Amazon drier, more susceptible to drought, and ultimately more vulnerable to wildfires. The self-perpetuating cycle of deforestation and climate change that ensues could have serious consequences. “We could expect to see some of the tropical forests,” says Wofsy, “convert to savanna.”
Managing Fires for Carbon Conservation
As temperatures rise, wildfires in forests—especially in North America, as Californians know well—are becoming more frequent and intense (See “When Wildfires Make Your Air Unhealthy,” June 2023). Once part of natural cycles, these fires are now burning larger, hotter, and in some cases, permanently transforming ecosystems. “As you start to raise the temperatures,” Wofsy says, “fires can get hotter, and they can destroy the forest in a way that they once couldn’t. And that can lead to essentially a degradation of the forest overall, turning forests into grassland.”
Boreal forests, the planet’s largest biome (composed of deciduous trees like pines and spruce), may also shift to grasslands as fire zones expand northward. Given the impact of forested areas on converting carbon, what will happen when what was once forest is turned to grassland?
The answer? The carbon that would normally be sequestered as the forest regrows will instead be re-captured much more slowly by grasses, further contributing to climate change.
Human efforts to control wildfires have, in many places, made things worse. Some amount of fire is natural and integral to forest health, aiding and promoting biodiversity; some plant species require fires to germinate. But fires made more extreme by greenhouse gases—and fires burning into new areas—have an outsized effect on these patterns, burning faster than habitats can renew. Improper forest management has exacerbated wildfire risks, especially in the western U.S., where past practices such as fire suppression allowed fuel to build up, leading to catastrophic fires. “That’s something that one could solve with prescribed burns,” Wofsy says.
Controlled fires, along with forest thinning, can restore balance to fire-prone landscapes. In the short term, Wofsy says, “There’s a lot that we can do to manipulate that system, to improve things, to get out of this situation where we have very large fires and very large amounts of smoke inundating where people live every year.”
Trees in Urban Spaces and the “Heat Island Effect”
In cities, trees play a different role, helping to moderate the sun’s impact. ‘Heat islands,’ or urbanized areas that experience higher temperatures, can harm health and safety. While urban trees don’t sequester carbon at the scale large forests do, they provide cooling shade, improve air quality, and enhance public health, particularly in disadvantaged neighborhoods. How do trees mitigate the effects of city heat?
“When you have a nice leafy area in a city,” Wofsy explains, “the temperatures that people experience in their homes and walking around outside will often be five degrees, maybe ten degrees cooler than in those parts of the city that are all asphalt.” This effect is principally the result of a combination of shade and evaporation.
In less affluent neighborhoods and districts, where green spaces are scarcer, high temperatures compound poor living conditions—and, in a larger sense, contribute to long-term social and economic inequalities. “If you use a heat-sensing camera and take a picture of a city from a satellite or an airplane,” Wofsy says, “those hot areas are very often the places where the lower income groups are living and suffering under the heat.”
Planting more trees in cities, then, could serve to mitigate some of these issues (see “Parks for Tomorrow,” July-August 2024). The good news is that urban planning efforts have begun to consider how trees can improve the health of cities and their inhabitants, beyond just making lived spaces more beautiful.
