Baseline time accounting--reply to the letter to the editor of Martin [Int J Life Cycle Assess (2013) 18(7):1279

Abstract

After becoming aware of a letter to the editor from Martin (2013), we realize there is a need for further elaboration of the baseline time accounting method laid out in Kloverpris and Mueller (2013). In the following, we addressMartin's comments. Martin states that our method “does not eliminate the need to choose a timeframe .” This is correct and inherent to any life cycle assessment (LCA) when it comes to the use of global warming potentials (GWPs). Later, Martin states that we assert that our “approach removes the need to consider arbitrary timeframes .” We are puzzled by this statement since Sect. 2.3 of our paper explicitly acknowledges that the use of the common 100-year GWP accounting period in LCA is indeed arbitrary. While we cannot avoid this circumstance, our method does eliminate the need to consider an arbitrary production period for biofuels (under the conditions described). We will later get back to the fundamental difference between the GWP accounting period and the assumed biofuels production period since the missing distinction between these two time perspectives appears to be the primary basis of Martin's critique. Martin states that the value of our insights “is diminished by the failure to include a full consideration of the importance of the timeframe of the analysis .” While we (and the three reviewers) did not find it relevant to include results other than GWP100 in our paper, the baseline time accounting concept is not restricted to this metric. To accommodate Martin, Table 1 shows results for GWP20 and (the more unusual) GWP30. As shown in the table, the applied GWP accounting period is certainly of significance for the type of indirect land use change (ILUC) called accelerated expansion. Meanwhile, the sensitivity to the choice of GWP accounting period is not unique as indicated by the GWPs for methane, also included in Table 1. For the type of ILUC called delayed reversion, the GWP accounting period is without significance. We acknowledge that this is due to the simplified assumption of constant carbon sequestration over the GWP accounting period. In reality, there will be some sensitivity to the choice of GWP accounting period for delayed reversion but not to the same extent as for accelerated expansion. For additional discussion, we refer to Sect. 4.1.2 in Electronic supplementary material 1 of Kloverpris and Mueller (2013), particularly. Martin states that “The baseline time accounting approach uses a 100-year timeframe ,” but, as mentioned above, there is no conceptual or methodological restriction on the chosen GWP accounting period. More correctly stated, baseline time accounting converts a time shift in land use emissions from a single batch of biofuels into an ILUC factor that is consistent with the GWP concept, and the results presented by Kloverpris and Mueller (2013) are based on a GWP accounting period of 100 years. The chosen GWP accounting period ensures consistency with the common global warming metric used for greenhouse gas (GHG) emissions. Since the ILUC factor is meant to be added to the GHG emissions from the biofuels supply chain, consistency between addends is necessary. Martin states that “It should come as no great surprise that changing the denominator of a fraction from 30 to 100