Case Study: What Do Forests Have To Do With Global Warming?

The Keeling Curve

The red curve shows average monthly concentrations since March of 1958, and the blue curve shows the 12-month average. Click the graph for a larger view.

Earth's atmosphere is changing. Since measurements began in the late 1950s, the average concentration of carbon dioxide (CO₂) in the atmosphere has climbed steadily. The increase in CO₂ is attributed to the human activities of burning coal, oil, and wood.

Increased concentrations of CO₂ mean that the atmosphere now traps more heat from the sun's energy than it did at lower concentrations. The connection between increased levels of carbon dioxide and global warming has politicians, scientists, students, and households wondering what they can do to reduce the amount of carbon dioxide in the air.

Show me more about this graph

The graph is known as the Keeling curve, named after Charles David Keeling, the scientist who supervised the long-term effort to collect the data. The regular changes in the red line are caused by seasonal variations. Averaging the monthly data with the six months before and the six months after it produces the blue curve. The curve indicates that CO₂ levels have risen substantially over the past 50 years.
The figure was produced by Robert Rohde; it is a part of the Global Warming Art collection.

Carbon's Pathways

Click the image for a larger view. Diagram courtesy of GLOBE Carbon Cycle.

The Carbon Cycle is similar to the concept of the Water Cycle. Carbon exists in different forms in different places, and natural processes move it from one place to another.

The "storage" places for carbon are called stocks. They are labeled in blue in this diagram. The numbers for each stock are estimates of the amount of carbon stored in them for the whole Earth.
The largest stock for carbon is Earth's crust. Over millions of years, the carbon-rich remains of plants and the calcium carbonate shells of marine organisms have been buried and become a part of Earth's crust.
The processes that move carbon from one stock to another are called fluxes. The red arrows indicate which stocks the carbon moves from and to. The numbers show the amount of carbon that moves through each flux in a year.
Plant growth is one example of carbon moving through the carbon cycle. During photosynthesis, plants use carbon dioxide from the atmosphere, water and nutrients from the soil, and sunlight for energy to build their structures. The carbon becomes a part of the plant's trunk, roots, or leaves; it has moved from the atmosphere into the biosphere.
Another example of a flux in the carbon cycle is when plant materials burn; oxygen reacts with the carbon that was stored in the plant, releasing carbon dioxide into the atmosphere. Carbon has moved from the biosphere to the atmosphere.

The Forest Connection

The amount of carbon that moves into the atmosphere from burning fossil fuels in cars and industry is small compared to the fluxes that involve plants. However, evidence shows that this human-induced flux is increasing the stock of carbon in the atmosphere. Reducing the amount of carbon dioxide humans produce is the most direct way to stop increasing atmospheric carbon dioxide. Another possibility would be to increase one or more of the fluxes that remove carbon from the atmosphere. As photosynthesis in forests represents a large flux of carbon out of the atmosphere, scientists wonder if adjusting forest management practices could increase this flux.

A number of questions need to be answered in order to consider this solution. For instance:

What controls how much carbon a forest absorbs? Do different types of trees absorb more carbon from the atmosphere than others? Does the age of a forest control its ability to absorb carbon?
What forest management practices could increase the amount of carbon a forest absorbs? Would fertilizing the forest work? Would the practice of harvesting old trees and growing new ones absorb more carbon than leaving the forest alone?
In addition to absorbing more carbon, what other consequences would result from new forest management practices?
Could a change in current practices absorb enough carbon to counteract the flux from burning fossil fuels?

Answering these questions by carrying out full-scale experiments in actual forests, while effective, takes a lot of resources including money, people, and tools, especially when you consider the amount of forest area that would need to be measured. Field data collection and experiments are also often limited by the long amount of time it takes to get meaningful results. As a supplement or sometimes an alternative, scientists have created models to help them better understand forest ecosystems. Models can vary widely in their intention, limitations, and implementation, and are typically designed with specific goals in mind.

In this chapter you will use one type of model that shows how biomass and carbon storage accumulates in a forest ecosystem over time. As an example, you'll use the model to explore the carbon storage by forests in New Hampshire; then, you'll use data from the U.S. Forest Service to explore the forests of your own state. Setting different conditions in the model, examining the outputs, and answering follow-up questions will help you understand the role that your own state's forests play in the uptake of carbon dioxide from Earth's atmosphere and storage of carbon in the biosphere.

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