Global Environmental Problems

Spring 2006

Problem Set #9

Due: 19 April

1.

A tax on carbon dioxide is often discussed as one way to reduce emissions. A typical value for the tax that would be required to comply with the Kyoto Protocol is $100 per metric ton of carbon emitted ($100/tC).

A.

Norway has adopted a tax of $50 per ton of carbon dioxide. Convert this to dollars per ton of carbon.

B.

How much would a $100/tC tax add to the price of coal? The average carbon content of U.S. coal is about 65 percent.

C.

What is the current price of coal in the United States? Data can be found at http://www.eia.doe.gov.

D.

In the United States, coal is used primarily to generate electricity. The average energy content of U.S. coal is about 25 MJ/kg. Existing coal-fired electric power plants are about 32 percent efficient (i.e., 100 J of heat energy is required to produce 32 J of electrical energy). How much would a $100/tC tax add to the price of electricity in dollars per kilowatt-hour? Compared this to the current cost of generating coal-fired electricity, about $0.05/kWh.

E.

Gasoline, like crude oil, is about 85 percent carbon. The density of gasoline is 0.75 kg/L. How much would a $100/tC tax add to the price of a gallon of gasoline? Compare this to the current price of gasoline.

2.

In his 1989 State of the Union address, President George H.W. Bush proposed planting 10 billion trees to absorb some of the carbon dioxide that the U.S. emits into the atmosphere through fossil-fuel burning. Forests are net absorbers of carbon only while they are growing; after reaching maturity, the flow of carbon into a forest is balanced by the flow out. The mature forest will store carbon as long as it stands.

A.

Estimate, very roughly, the land area required for 10 billion trees. Compare this to the existing areas of forest, grasslands, and cropland in the United States given by WRI.

HINT: Think about how closely mature trees are spaced, and how many such trees there are per acre or hectare. If you are really interested, you can find data on trees per hectare (or acre) on the web for different types of natural forest or forest plantations.

B.

Based on your answer to part A, roughly how much carbon would be stored in these trees when they are mature?

HINT: The new forest will replace an existing ecosystem--probably some combination of cropland and temperate grassland and woodland. Over a hundred or more years, the old ecosystem will be transformed into a stable new ecosystem, with corresponding amounts of both live and dead organic matter. Data on carbon storage in various types of ecosystems was given in the class notes for "nutrient cycles," and is also given in appendix XII in Spherical Cow.

D.

Suppose that land can be purchased for conversion to forest for $500 per acre (one could also pay landowners this amount to accept a covenant agreeing to permanent reforestation) and seedlings cost $1 each to purchase and plant. Very roughly, what is the cost of reforestation in dollars per metric ton of carbon sequestered?

HINT: Assume most of the roofs are existing single-family residences. The average existing single-family residence has a living area of about 1500 square feet. Remember that roof area is smaller than the living area in multi-story residences, and that not all roof area is usable. Useable roofs are those that are flat or tilted southeast to southwest, and are unshaded most of the day. Assume that the average usable roof surface receives 6 GJ of solar energy per square meter per year. Also assume that photovoltaic cells can convert solar energy into electricity with a overall efficiency of 10 percent.