A comment on the quantitative significance of aerobic methane release by plants

A recent study by Keppler et al. (2006; Nature 439, 187-191) demonstrated CH₄ emission from living and dead plant tissues under aerobic conditions. This work included some calculations to extrapolate the findings from the laboratory to the global scale and led various commentators to question the value of planting trees as a greenhouse mitigation option. The experimental work of Keppler et al. (2006) appears to be largely sound, although some concerns remain about the quantification of emission rates. However, whilst accepting their basic findings, we are critical of the method used for extrapolating results to a global scale. Using the same basic information, we present alternative calculations to estimate global aerobic plant CH₄ emissions as 10-60 Mt CH₄ year^-1. This estimate is much smaller than the 62-236 Mt CH₄ year^-1 reported in the original study and can be more readily reconciled within the uncertainties in the established sources and sinks in the global CH₄ budget. We also assessed their findings in terms of their possible relevance for planting trees as a greenhouse mitigation option. We conclude that consideration of aerobic CH₄ emissions from plants would reduce the benefit of planting trees by between 0 and 4.4%. Hence, any offset from CH₄ emission is small in comparison to the significant benefit from carbon sequestration. However, much critical information is still lacking about aerobic CH₄ emission from plants. For example, we do not yet know the underlying mechanism for aerobic CH₄ emission, how CH₄ emissions change with light, temperature and the physiological state of leaves, whether emissions change over time under constant conditions, whether they are related to photosynthesis and how they relate to the chemical composition of biomass. Therefore, the present calculations must be seen as a preliminary attempt to assess the global significance from a basis of limited information and are likely to be revised as further information becomes available.