Sensitivity analysis: Impact of Static Impact Assessment to the

In Papers I, II, III and IV the climate impact of peat and biomass derived energy and fuels is calculated based on the carbon balance of different scenarios and by using the GWP method with a 100-year time horizon (GWP(100)) for the impact assessment, when the timing of the emission is not considered as long as the emission occurs during the 100 year time frame. This means that the climate impact has been taken into account during the 100-years impact time after the emission/sink takes place. Due to the fact that GHG emissions and sinks in biomass utilization scenarios are formed more or less in the future the use of the calculation method with GWP(100) values leads to temporally un-explicit results. For example, if the emission or sink actualize in the year 80, with the GWP(100) method the climate impact is calculated for the next 100 years until the year 180. In this case, part of the accounted climate impact is taking place after 100 years from the starting point of the assessment.

The temporally-explicit climate impact for biogenic and fossil carbon flows can be cal-culated with methods that use for example cumulative radiative forcing as a base. With calculation procedures presented in IPCC (2007c) the GWP values can be calculated as a function of the impact time, and this way, the time-explicit GWP and numerator AGWP can be determined.

In this chapter, the impact of the dynamic LCA approach on the results is assessed by calculating the AGWP (Wm-2, also called Radiative Forcing) of the emissions and sinks as a function of time. Emission profiles and the impact of the time-explicit method for four case examples are presented, and the climate impact is calculated with three methods: first, with time averaged biomass carbon stock and cumulative AGWP(100) values as in Papers I, III and IV; second, with the total biomass carbon stock and cumu-lative AGWP(100) as in Paper II and finally, with the total biomass carbon stock and cumulative AGWP(t) values where the impact assessment accounts only for the climate impact of the first 100 years starting from the beginning of the assessment.

The emission profile for forestry-drained, myrtillus type (Mtkg) peatland and peat utili-zation when compared to the unutiliutili-zation of peat and coal combustion is presented in Figure 35. The cumulative result is presented in Figure 36. In peatland assessment the timing of the impact assessment has a significant impact on the results. When the AGWP(t) method is used, the climatic impact is 47,6% higher than when the AGWP(100) method is used. Difference increases when the time horizon increases be-ing 5,5% with time horizon of 15 years and 22% with the time horizon of 50 years. Fig-ure 35 shows that the emission reductions when coal is changed to peat in this case re-sults evenly during the 100-year reference time. When the GWP(100) values are used in impact assessment with this shape of the balance curve of emission profile of the first 100 years, around a half of the impact can be discovered to reach over the assessment period.

Figure 35. An emission profile of peat utilization: Comparison of GHG emissions of the myrtil-lus type (Mtkg) peatland with peat utilization with Mtkg-type peatland without peat utilization and using coal for CHP production. Net balance between the systems is presented with the blue line.

Figure 36. Time dependent, cumulative radiative forcing of peat utilization from the myrtillus type (Mtkg) peatland when compared to the coal use in CHP production.

The emission profile for forest residue utilization and the cumulative AGWP results for forest residue and forest stand utilization are presented in Figures 37, 38 and 39. The

emission profiles for these two different types of forest biomass sources are significant-ly different: when the forest residues emit carbon exponentialsignificant-ly during a decomposition when left in the forest, forest stand accumulates carbon during a regrowth phase con-stantly until felling is repeated. In Paper III, the carbon stock of forest stand and forest residues is accounted by using a time averaged value for carbon stock. Figure 38 shows that the simple carbon balance method with the time average approach is an approxima-tion that gives a similar result as the AGWP(t) method. The difference between these is 7%. The difference in the climate impact of forest stand utilization is also relatively small, 11%, when time averaged values are used (Figure 39).

Figure 37. An emission profile of residue utilization. Comparison of GHG emissions of residue combustion to the emissions from residue decomposition in site.

Figure 38. Time dependent, cumulative radiative forcing of forest residue utilization.

Figure 39. Time dependent, cumulative radiative forcing of forest stand utilization.

The emission profile of LUC of mineral forest soil to palm oil cultivation is presented in Figure 40 and 41 and the cumulative AGWP results in Figure 42. In Paper IV, the time averaged values for palm oil AGB carbon stock were used, and the LUC emissions were assumed to come true during the first 20 years. It can be seen from Figure 42 that the

difference between the dynamic and time averaged static method results is 10.6%. If oil palm cultivation is assessed with a 25-year lifetime of oil palm stand and continuous cultivation (see Figure 42), the difference between AGWP(t) and AGWP(100) is 8.8%.

Figure 40. An emission profile of LUC emissions of land clearing and cultivation of Oil palm.

Figure 41. An emission profile of land clearing and cultivation of oil palm when the 25-year continuous cultivation cycle is used.

Figure 42. Time dependent, absolute and cumulative radiative forcing of converting mineral forest to palm oil cultivation. Average, cumulative AGWP(100) with single harvest, others with continuous harvest scenario.

The difference between the climate impact results of GWP(100) and time-explicit GWP(t) depends on the shape of the GHG emission profile of the studied biomass chain. The more there are emissions in the beginning of the reference time, the less the difference between these two methods affects the result. Consistently, the more there are emissions taking place at the end of the reference period, the more the results will differ.

The difference is significant in the peat fuel chain because the reference peatland causes significant emissions during the entire reference period. If the GWP(100) method is used, nearly half of the global warming impact will actualize after a 100-year reference time (until year 199). Time averaged values for forest carbon stock seem to be an ac-ceptable approximation because the difference to the GWP(t) method was only around 10%.