Is Afforestation a Meaningful Remedy to Global Warming?

Most ecologists would agree that the buildup of carbon dioxide (CO2) in Earth’s atmosphere is among the most pressing of environmental issues. The general public has also been sensitized to this issue.

What can be done to reduce the amount of CO2 in the atmosphere? Reexamine the carbon cycle to recall the normal reservoirs of CO2and the mechanisms which regulate this system. We recognize that the ocean, one of the main mechanisms for removing CO2, is an overloaded system. It cannot take up enough CO2quickly enough to solve the problem. What are some other avenues? Photosynthesis is another major mechanism with the potential for removing CO2. Recall that the general formula for photosynthesis is

CO2 + H20  C6H12O6 (sugar) + O2

Some fossil fuel-burning electrical utilities have turned to afforestation, the planting of trees, as a way to compensate for the gases which they emit as a normal product of combustion. Recall that the burning of fossil fuels can simplistically be described by the formula

C6H12O6 + O2  CO2 + H20

Is afforestation really a meaningful remedy to this problem?

We will explore the carbon cycle in a reduced scale in which the carbon content of a single tree will be estimated and compared to the carbon content of exhaust emissions from a single automobile.

This exercise consists of four activities:

  1. estimation of the wood volume of a tree
  2. calculation of tree carbon content
  3. estimation of carbon in a car’s exhaust emissions
  4. comparison of carbon sequestration during tree growth and carbon additions to the atmosphere during combustion.

Procedures

1. Estimation of wood volume of a tree. We will estimate the volume of a typical tree. To do this we will make several assumptions. The volume of a trunk of a tree can be estimated as a cylinder. (Do you accept the assumption that a tree trunk is a cylinder?) The volume of a cylinder can be calculated by the formula

V = (dbh/2)2 x  x (tree height)

where dbh = diameter at breast height (a standard forestry measurement)

and  = 3.14159 etc.

We can measure the height of a tree using some of the math you used (and forgot) in high school. Standing a known distance from the tree, use a protractor to measure the angle from your eye to the top of the tree. This creates a right triangle (as pictured below).

From geometry, we know that the tangent of an angle is equal to the opposite side divided by the adjacent side. For example, if the angle measured by the observer is 30o, and the distance to the base of the tree is 33 meters (~100 feet), then

tangent 30o = tree height (h) / 33 m

Rearrange this to solve for the tree height (h):

tree height (h) = tangent 30o x (33 m)

= (0.557) x (33 m)

= 19.05 m

add ~1.5 m (the level of the observer’s eyes); 19.05 + 1.5 = 20.5 m

2. Calculate tree carbon content. An estimate for the carbon mass per unit of a volume of wood is

249 kg of C / m3

This figure is derived using the mean sugar maple wood density and the molecular formula for cellulose. Using this value, the carbon content of the tree is obtained as

(tree volume) x (249 kg of C / m3) = kg of C in the tree

Note that this carbon was stored over the entire life of the tree. The age of the tree must be estimated to compare the auto’s yearly emission rate. Tree age can be determined by dendrochronology, aging by rings. We will use an increment borer or tree section to age this tree. I will demonstrate this for you.

3. Calculate the carbon produced by gasoline combustion. As an estimate of a person’s ‘typical’ contribution of carbon dioxide to the atmosphere in automobile exhaust, we will take a poll of the class. We will assume that the gasoline used is 89 octane. We will also assume 100% combustion of this fuel. This is not completely realistic. Some of the gasoline would be unburned, and some of the unburned portion would be trapped in the catalytic converter.

The carbon emissions of a given auto can be estimated by

(# of L of gasoline used) x (591 g of C/L of gasoline) = (# g of C)

(To convert gallons to liters (L), multiply by 3.785)

This exercise is modified from one created by Dr. Paul Weihe, Davis and ElkinsCollege: Tree Measurement and Carbon Cycling: A Laboratory Exercise, Bulleting of the Ecological Society of America 78 (2): 142-3.)

References and Links:

Dendrochronology:

  1. Grissino-Mayer, Henri D. Ultimate Tree-Ring Web Pages. Accessed March 5, 2004.

Cars:

  1. EPA’s Green Vehicle Guide: Auto Emissions. Accessed March 5, 2004.
  2. Find your car’s energy impact store.
  3. Green Cars. Is your car in the top 100?
  4. The Greenest and Meanest Vehicles. GreenerCars.com. Highlights of the Model Year.

Assignment:

  1. Make a comparison between the vehicle you most often drive and the greenest vehicle you can find that still meets your needs. Use to determine the estimated fuel consumption and cost per year. Also, compare emissions of both vehicles.
  2. Do a shopping comparison. Determine the car that you’d most like to buy and compare it to one of the new low emission cars, such as the Toyota Prius or Honda Civic Hybrid. Calculate the costs of your vehicle for the first five years. Be sure to include the list price (for models that are comparably equipped), as well as the cost of fuel and insurance. After 5 years, which has cost you more? Now compare the tailpipe emissions. After making these comparisons, will you include environmental considerations in the purchase of your next vehicle?

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