With this view in mind in seems clear that the value of carbon stored within a forestry project IS DIFFERENT than carbon emitted by fossil fuel combustion and land-use changes. The problem is how to evaluate these two so that they are comparable.

Solution

The proposed solution requires knowledge of three time constants. The first two have been identified previously. They are storage duration, t_s, and average lifetime of carbon in the atmosphere, t_c. The third, as suggested by the citation above, is the time-horizon, t_h. This is the time over which the storage matters. It might be the time to a critical CO₂ saturation level in the atmosphere or the time at which the new technology should arrive.

Emissions without storage
Let us assume that quantity of carbon, C_o, is emitted in year 0. Then the amount of carbon in the atmosphere due to emissions, C_e, in year t is given by (figure 1);

(1)

Figure 1: Emissions without storage

The average amount of this carbon to the year t_h is;

(2)

Storage
Now lets assume that we store the same amount of carbon, Co, now and release it again at year ts. Then the amount of carbon in the atmosphere due to emissions with storage in year t is given by (figure 2);

(3)

Figure 2: Emissions with storage

The average value of this carbon to the year t_h is;

(4)

The relative value of storage
The reduction in atmospheric carbon due to storage is given by;

	(5)
	(6)

Thus carbon storage has the same effect as emission reduction weighted by a constant that depends on the three time constants, t_s, t_c, and t_h.

Figure 3: The relative value of storage

a) th = 100 years, tc = 100 years

b) tc = 100 years, ts = 25 years

c) th = 100 years, ts = 25 years

Figure 3 shows the value of this equation versus each parameter holding the other two constant Note that the value of storage is dependent on all three variables. Each graph shows interesting relationships. Figure;

1. shows that the value of carbon storage increases with storage duration, t_s;
2. displays that as the time-horizon, t_h, decreases the value of carbon storage increases and
3. shows that the value of carbon storage increases with the average lifetime of carbon, t_c in the atmosphere.

Conclusions and Discussion

The effect of storage duration
This analysis suggests that carbon stored DOES HAVE a different value than emissions saved either from fossil fuel emissions or land-use changes. When discussing carbon storage it IS important to consider carbon-years.

Given our current understanding of the carbon cycle, t_c = 100 years, and if the time-horizon is also 100 years, then 1 tC-yr ~ 0.006 tC emitted. Note that the value is not linear in that 1 tC stored for 25 years IS NOT equivalent to 25 tC stored for one year. This especially true for longer storage periods.

For ease of computation it would be nice to have a constant value for tC-yr. The Best fit curve in figure 3a represents this constant. 1 tC-yr = 0.0088 tC emission saved. Note that it overestimates the relative value of carbon storage for periods less than 75 years, and underestimates the relative value for longer periods.

The effect of time-horizon
As noted earlier the longer the time-horizon the less valuable is carbon storage and the more valuable are emission reductions. In the limit, if there atmospheric CO₂ levels do not matter or there is no technological breakthrough, then storage has no effect, and only emission reductions matter.

On the other hand, if we have a target date that we must meet, then the value of carbon storage will increase with time as this date approaches.

The choice of time-horizon is one that must be chosen by the international community. At this time, there is no given deadline. It is difficult to predict technological breakthroughs, but perhaps given our current emission levels a critical date can be chosen. The author has chosen 100 years for demonstration purposes only.

The effect of average lifetime of atmospheric carbon
The average lifetime of atmospheric carbon is still under investigation. Fung (1996) states values between 50-100 years. Within this range there is a doubling of the value of carbon storage.

There may be a damping effect inherent in the system. If one assumes that the oceanic uptake of CO2 is roughly a constant, and as the amount of forest area continues to decrease, then as the amount of CO2 in the atmosphere increases the average lifetime should also increase. Given this result the value of carbon storage will increase with time.

General Comments
This analysis shows that carbon storage has small value compared to emission reductions. It suggests that the following carbon sequestration projects types are important:

• protection of existing forest
• sequestration and long-term protection of new forests
• creation of biofuels for fossil fuel replacement and
• creation of woodproducts for indirect emission reductions

References

1. Breuer, G (1979) Can forest policy contribute to solving the CO2 problem? Environ. Int., V2, 449-451.
2. Fung I (1996) The global carbon cycle and the atmospheric record: “The problem definition”. In: M. Apps and D. Price (ed.) Forest Ecosystems, Forest Management and the Global Carbon Cycle, proceedings of the NATO Advanced Research Workshop “The Role of Global Forest Ecosystems and Forest Resource Management in the Global Cycle” (Banff, Alberta, 1994), NATO ASI Series, Springer. 452 pp.

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