A family consumes 400 kWh/month of electricity and wishes to buy sequestration offsets from an organization that plants trees in Africa. The organization charges $19.00 per tree, and estimates that each tree will on average absorb 730 kg CO_2 over its lifetime. The electricity consumed emits CO_2 at the 2003 New York State average rate, based on the energy mix given in the table below.
The organization allows consumers to purchase fractions of trees. (a) How much must the family spend to offset emissions for one year? (b) Suppose each tree is planted in a stand of trees where it covers a cross-sectional area with 6 m diameter, and takes 20 years to mature. What is the rate of CO_2 sequestration in kg/m^2·year?
Data on emissions by source:
| Source | Share | Tonnes CO_2/MWh |
| Nuclear | 28% | — |
| Hydro | 18% | — |
| Gas | 19% | 0.42 |
| Other fossil | 33% | 0.77 |
| Other | 2% | — |
Notes: Emissions include only end-use emissions from energy conversion, and do not include other stages of life cycle, e.g., embodied energy in facilities.
“Other” includes primarily biofuels and wind. Since New York State is a net importer of electricity, each share allocation includes an estimated proportion of the net imports from outside of New York State to each source. Total consumption amounts to 158.0 TWh; of this amount, 13% are imported.
