Global warming potential factors and warming payback time as climate indicators of forest biomass use

Abstract

A method is presented for estimating the global warming impact of forest biomass life cycles with respect to their functionally equivalent alternatives based on fossil fuels and non-renewable material sources. In the method, absolute global warming potentials (AGWP) of both the temporary carbon (C) debt of forest biomass stock and the C credit of the biomass use cycle displacing the fossil and non-renewable alternative are estimated as a function of the time frame of climate change mitigation. Dimensionless global warming potential (GWP) factors, GWP_bio and GWP_biouse, are derived. As numerical examples, 1) bioenergy from boreal forest harvest residues to displace fossil fuels and 2) the use of wood for material substitution are considered. The GWP-based indicator leads to longer payback times, i.e. the time frame needed for the biomass option to be superior to its fossil-based alternative, than when just the cumulative balance of biogenic and fossil C stocks is considered. The warming payback time increases substantially with the residue diameter and low displacement factor (DF) of fossil C emissions. For the 35-cm stumps, the payback time appears to be more than 100 years in the climate conditions of Southern Finland when DF is lower than 0.5 in instant use and lower than 0.6 in continuous stump use. Wood use for construction appears to be more beneficial because, in addition to displaced emissions due to by-product bioenergy and material substitution, a significant part of round wood is sequestered into wood products for a long period, and even a zero payback time would be attainable with reasonable DFs.

Appendices

Appendix 1. The REFUGE3 model

This appendix describes the REFUGE 3 model that was used to calculate the atmospheric concentrations and radiative forcing. The model calculates atmospheric concentrations and radiative forcing for CO₂, CH₄ and N₂O. The emissions discussed in the study are added to a background of global emission pathways that correspond to emission levels that would reach a 450 ppm-eq concentration target by 2100. The relative shares of CO₂, CH₄ and N₂O in this background pathway have been approximated from a number of emission-reduction scenarios.

After its release into the atmosphere, a CO₂ emission impulse is assumed to decay according to the impulse response function reported by IPCC (2007b, page 213), an approximation based on the Bern2.5CC carbon cycle model (Joos et al. 2001). An emission pulse, regardless of its origin, is mixed in the atmosphere and, gradually, the increase in atmospheric concentration is transferred partially to the ocean and the biosphere. The resulting increase in atmospheric CO₂ concentration S(t) is then defined by

$$ S(t) = \int\limits_{{{t_0}}}^t {E\left( \tau \right)\left( {{a_0} + \sum\limits_{{i = 1}}^3 {{a_i}{e^{{\left( {t - \tau } \right)/{b_i}}}}} } \right)d\tau } $$

(A1)

where E(t) is the annual emission level and the parameters of the impulse response function are a ₀  = 0.217, a ₁  = 0.259, a ₂  = 0.338, a ₃  = 0.186; and b ₁  = 172.9 years, b ₂  = 18.51 years and b ₃  = 1.186 years. To improve the computability of the convolution in (A1), a recursion formula of Pingoud and Wagner (2006) has been used.

In the calculations, the biogenic C emission pulse was followed by a sink due to the re-growth of biomass. This sink reduces the level of global emissions in the REFUGE3 model and, in principle, it is accounted for simply as a series of gradually declining negative emission impulses in (A1). Therefore, the total concentration impact of biomass use as energy is a combination of the emission and subsequent sinks, and the atmospheric CO₂ dynamics for both the emissions and sinks.

The atmospheric CO₂ concentration C(t) then induces radiative forcing (RF), with the expression Ramaswamy et al. (2001, p. 358)

$$ RF(t) = \alpha \ln \left( {C(t)/{C_0}} \right), $$

(A2)

where C ₀ corresponds to the atmospheric CO₂ concentration before 1850 (278 ppm assumed) and α = 5.35 W/m². An additional concentration pulse S(t) adds to the background concentration C _bg (t) so that C(t) = C _bg (t) + S(t), where C _bg (t) is an generic estimate for a global 450 ppm-eq concentration pathway. As the formula for RF is non-linear, the marginal increase in RF from a marginal emission impulse depends on when the emission is released, as the global background CO₂ concentration varies over time. However, this effect is only minor and the change in RF for an emission impulse released in 2000 (i.e. when the atmospheric concentration is the lowest) is roughly 10% higher than when the impulse is released during the latter part of the century when the concentration is highest. Therefore, our illustrative emission impulse, released in 2010, should be applicable throughout the time frame used with reasonable accuracy.

As an example, the cumulative radiative forcing (CRF) or AGWP calculations of the energy use of 5 cm diameter branches (related to Fig. 1) are shown in Fig. 8. Note that when the displacement factor DF = 0.6 and there are no fossil C emissions of the bioenergy system then the C pulse from the fossil fuel energy system, functionally equivalent to the bioenergy system, is 0.6 Mg C.

Fig. 8

Cumulative radiative forcing (CRF), i.e. AGWP, as a function of the timeframe (2010, t) for the unit pulse emission of 1 Mg fossil C at 2010 (AGWP_fos), 1 Mg of biomass C debt due to harvest of 5 cm branches at 2010 (AGWP_bio) and the displaced fossil C emissions at 2010 (AGWP_biouse) when DF = 0.6. Note that AGWP_biouse / AGWP_fos = −0.6. In the simple case when fossil C emissions of the bioenergy system are zero, the AGWP of the functionally equivalent fossil energy system is the dashed line and equal to -AGWP_biouse . The warming payback time is the time when the AGWPbio curve cuts the AGWP curve of the functionally equivalent fossil energy system

Appendix 2. Estimation of GWP_biouse in combined material and energy substitution

The estimation of the GWP_biouse of material substitution in this study is based on the data of the life cycle assessment (LCA) of Häkkinen and Wirtanen (2006) in which a modern wood office building, the research centre of the Finnish Forest Research Institute in Joensuu, was compared with a functionally equivalent (fictional) concrete building of a conventional design (Table 1). The numbers are thus case-specific but characterize the factors involved in material substitution.

Table 1 LCA data of the Finnish Forest Research Institute (Metla) office building and its functionally equivalent concrete alternative (Häkkinen and Wirtanen 2006)

The GWP_biouse is estimated as follows: assuming a C content of 0.5, 305 Mg C is sequestered in long-lived wood structures. By using wood as a construction material instead of concrete, 597 Mg CO₂eq emissions or 163 Mg C were saved. Assuming that 40% of the stem biomass is used in the long-lived structures and the rest, 60%, — the by-product flow – is used as bioenergy, it implies that the total stemwood consumption is equal to 763 Mg C, and the by-product flow 458 Mg C. Part of the latter goes to the energy required for processing the wood materials and the rest can be used to displace fossil fuels elsewhere. By assuming that all the renewable primary energy consumed in wood building is wood-based and that the average moisture content of the by-products (bark, chips, sawdust) is 50% with a specific heat value of 16.6 GJ/(Mg dry matter), 114 Mg dry matter wood or 57 Mg biomass C of by-products is consumed as the internal process energy of wood materials. Thus, 401 Mg biomass C can be used to displace fossil energy elsewhere. Assuming DF = 0.6 for this energy substitution, 240 Mg C fossil emissions could be displaced. Thus, the total reduction in C emissions at the year of harvest/construction would be equal to 708 Mg C of which 43% consists of biomass C sequestered into long-lived material, 23% of material substitution and 34% of energy substitution. The reduction would represent 93% of the biogenic C stock of the stemwood consumed.

For illustrative purposes it is assumed that the lifetime of the building would be (just) 50 years after which the demolished wood structures would be recycled into bioenergy. If DF at 2060 were equal to 0.6, 183 Mg C could be displaced so that the net emission would be 122 Mg C, or 16% of the biomass C of the stemwood.