Marginal yield, technological advances, and emissions timing in corn ethanol’s carbon payback time

Previous estimates of carbon payback time (CPT) of corn ethanol expansion assumed that marginal yields of newly converted lands are the same as the average corn yield, whereas reported marginal yields are generally lower than the average yield (47–83 % of average yield). Furthermore, these estimates assumed that the productivity of corn ethanol system and climate change impacts per unit greenhouse gas (GHG) emissions remain the same over decades to a century. The objective of this study is to re-examine CPT of corn ethanol expansion considering three aspects: (1) yields of newly converted lands (i.e., marginal yield), (2) technology improvements over time within the corn ethanol system, and (3) temporal sensitivity of climate change impacts.

A new approach to CPT calculation is proposed, where changes in productivity of ethanol conversion process and corn yield are taken into account. The approach also allows the use of dynamic characterization approach to GHGs emitted in different times, as an option. Data are collected to derive historical trends of bioethanol conversion efficiency and corn yield, which inform the development of the scenarios for future biofuel conversion efficiency and corn yield. Corn ethanol’s CPTs are estimated and compared for various marginal-to-average (MtA) yield ratios with and without considering technology improvements and time-dependent climate change impacts.

The results show that CPT estimates are highly sensitive to both MtA yield ratio and productivity of ethanol system. Without technological advances, our CPT estimates for corn ethanol from newly converted Conservation Reserve Program (CRP) land exceed 100 years for all MtA yield ratios tested except for the case where MtA yield ratio is 100 %. When the productivity improvements within corn ethanol systems since previous CPT estimates and their future projections are considered, our CPT estimates fall into the range of 15 years (100 % MtA yield ratio) to 56 years (50 % MtA yield ratio), assuming land conversion takes place in early 2000s. Incorporating diminishing sensitivity of GHG emissions to future emissions year by year, however, increases the CPT estimates by 57 to 13 % (from 17 years for 100 % MtA yield ratio to 88 years for 50 % MtA yield ratio). For 60 MtA yield ratio, CPT is estimated to be 43 years, which is relatively close to previous CPT estimates (i.e., 40 to 48 years) but with very different underlying reasons.

This study highlights the importance of considering technological advances in understanding the climate change implications of land conversion for corn ethanol. Without the productivity improvements in corn ethanol system, the prospect of paying off carbon debts from land conversion within 100 years becomes unlikely. Even with the ongoing productivity improvements, the yield of newly converted land can significantly affect the CPT. The results reinforce the importance of considering marginal technologies and technology change in prospective life cycle assessment.