Aspen trees in the Castle Creek valley.
The relationship between forests and carbon
Humans benefit from forests in myriad ways. Many of them are very direct gains from the physical material of forests. Large numbers of us live in buildings made of wood, we use paper products every day, and we use fires for ambiance, cooking, and warmth. Other benefits are less direct. For example, the shade and evaporative cooling provided by trees make our cities more livable, trees prevent erosion, and a walk in a forest has been shown to improve mental health. In addition to these advantages, one of the often-overlooked benefits we receive from Earth’s many trees is a livable climate. Over hundreds of millions of years, the connections between carbon in the atmosphere and forests has resulted in a climate in which the human species has thrived. This is a complex relationship—to better understand it, we can look closely at the key components: the molecule of carbon, forests and their evolution, and how forests and carbon interact.
Carbon
Carbon is often referred to as the building block of life on Earth. Chemically, carbon atoms are the backbone of organic compounds—molecules like amino acids, carbohydrates and hydrocarbons that make life on Earth possible. Some of the molecules containing carbon in the atmosphere, such as carbon dioxide, methane, and fluorocarbons, are also the principal drivers of climate change. Of these gasses, carbon dioxide is responsible for the majority of present-day global warming. Climate change is primarily the result of human activity increasing the concentration of carbon dioxide in the atmosphere.
The amount of carbon on Earth doesn’t change, but it can move between different reservoirs, or areas with large quantities of carbon. Movement between these reservoirs is described as the carbon cycle. The vast majority (over 99 percent) of carbon is found in minerals below the planet’s crust. This reservoir is extremely stable and changes over geologic time scales, which is millions of years. Less than one percent of Earth’s carbon is found in and above the Earth’s crust. This carbon is found in the oceans, soils, the atmosphere, plants, animals, and fossil carbon (fossil fuels such as coal, oil, and fossil gas). Human activities including land use change and fossil fuel combustion move carbon from soils, plants, and fossil carbon into the atmosphere, resulting in global warming.
For almost one million years, Earth’s climate has oscillated between long ice ages and relatively shorter warm periods. During this time, the amount of carbon in each reservoir was relatively stable. Atmospheric carbon would go down during ice ages and back up during warm periods. At the start of the agricultural revolution, 12,000 years ago, humans began disrupting this cycle by clearing land for agriculture and burning forests. In doing so, humans moved carbon from soil, plants, and trees into the atmosphere. As human populations grew and technology advanced, carbon in the atmosphere increased. Disruptions to the carbon cycle accelerated dramatically during the Industrial Revolution, because at that time humans began burning large amounts of fossil carbon.
Fossil fuels are primarily composed of carbon and hydrogen molecules. When combusted in the presence of oxygen, the result is water and carbon dioxide. Today, atmospheric carbon is 50 percent higher than it was at the start of the Industrial Revolution in the mid-1700s. Most of the additional carbon in the atmosphere is from the combustion of fossil carbon, but about 23 percent of carbon emissions are from agriculture, forestry, and other land uses.
Forests
Forests are ecosystems dominated by trees. Currently, forests cover about one-third of the Earth’s land surface (10,000 years ago, they covered nearly double that), and are home to 80 percent of terrestrial biodiversity. Earth’s forests contain roughly three trillion trees and can be found on every continent except Antarctica. The trees that make up forests evolved about 385 million years ago, 150 million years before the first mammals. The evolution of trees set Earth on a course to become the planet it is today.
As trees and other land plants evolved and spread across Earth’s continents, the amount of carbon dioxide in the atmosphere decreased by a factor of 10. Photosynthetic activity from the additional plants increased the amount of oxygen in the atmosphere, setting the stage for the evolution of present-day animal life. Today, forests continue to play a pivotal role in shaping Earth’s climate.
Forests and Carbon
The relationship between forests and carbon is central to Earth’s climate, biodiversity, and human civilization. Such a massive and consequential relationship starts very small. Half of a tree’s biomass is composed of carbon. The carbon in trees comes from the air. Trees absorb air through their leaves and then, with the help of sunlight, they separate the carbon atom from carbon dioxide and combine it with water to create glucose, which is the initial building block of, and fuel for, a tree’s cells.
Over time, most of the carbon used by a tree becomes the trunk and branches. All the trees on Earth account for 57 percent of all carbon in living biomass. If we include the carbon found in forest soils and dead organic matter, forests contain almost 50 percent more carbon than is found in the atmosphere. This means that changes in Earth’s forests have profound impacts on the rest of the carbon cycle and the climate.
The role of forests in Earth’s climate system is complicated and often contradictory. In recent history, forests have both warmed and cooled the planet. When forests die, whether that death is natural or human-caused, they stop absorbing carbon dioxide from the atmosphere and begin decomposing. When trees decompose or burn, the carbon that was sequestered in their biomass is released into the atmosphere, causing global warming. Scientists estimate that 30 percent of all human carbon emissions since the Industrial Revolution are a consequence of deforestation.1
Trees have evolved to thrive in specific temperature ranges. That means that as temperatures rise, trees face additional stresses and lower survival rates. Increased stress is due to both higher temperatures and lower water availability (warmer air increases evaporation and decreases the water available to plants). These conditions lead to higher susceptibility of trees to insects, pathogens, and wildfire. More carbon in the atmosphere causes higher temperatures, and higher temperatures increase tree mortality. That increased tree mortality then releases even more carbon into the atmosphere. This pattern, negative as it is for the Earth, is called a positive feedback loop, which is when a disturbance in nature results in a reaction that leads to more of the same.
While forest loss has increased the amount of carbon in the atmosphere, the remaining trees have slowed global warming by sequestering larger amounts of carbon. As the relative amount of carbon in the atmosphere increases, trees and other plants are more efficient. The process is akin to a person running a race at sea level after living at a high elevation. Trees have adapted to a certain amount of carbon in the atmosphere. When that quantity increases, they are able to get more carbon with each “breath.” This process is referred to as carbon fertilization. Increased carbon uptake by trees since the Industrial Revolution has sequestered 30 percent of human carbon emissions. Unfortunately, this process isn’t limitless. At a certain point, the amount of carbon doesn’t keep up with the water and nutrients a tree needs. To return to the analogy of a person running a race, it doesn’t matter how much oxygen is in the air if the racer hasn’t eaten for several days.
Forests and the Climate Crisis
With their central role in the carbon cycle and in shaping Earth’s climate, forests are a key component in addressing the climate crisis. While this much is clear, there are many questions as to the nature of their role. Is it possible to increase the amount of carbon being sequestered by forests? How stable is carbon sequestered by forests? How will forests respond to climate change, and will they continue to sequester carbon at a similar rate?
These questions and others are current topics of scientific research and debate. As with any question touching on the future, there are no definitive answers. Ecosystems are incredibly complex, and human choices around climate mitigation are unknowable. Through research, we can begin to understand what is possible, and in some cases, even what is probable.
