Global warming potential factors and warming payback time as climate indicators of forest biomass use |
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Authors: | Kim Pingoud Tommi Ekholm Ilkka Savolainen |
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Institution: | (1) VTT Technical Research Centre of Finland, Tekniikantie 2, Espoo, P.O. Box 1000, FI-02044, VTT, Finland |
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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, GWPbio and GWPbiouse, 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. |
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