The case for human causes of increased atmospheric CH4

We propose that humans significantly altered atmospheric CH4 levels after 5000 years BP and that anthropogenic inputs just prior to the industrial revolution accounted for up to 25% of the CH4 level of 725 ppb (parts per billion). We base this hypothesis on three arguments: (1) the 100 ppb increase in atmospheric CH4 that occurred after 5000 years BP follows a pattern unprecedented in any prior orbitally driven change in the ice-core record, (2) non-anthropogenic explanations for this increase (expansion of boreal peat lands or tropical wetlands) are inconsistent with existing evidenced and (3) inefficient early rice farming is a quantitatively plausible means of producing anomalously large CH4 inputs to the atmosphere prior to the industrial revolution. If the areas flooded for farming harbored abundant CH4-producing weeds, disproportionately large amounts of CH4 Would have been produced in feeding relatively small pre-industrial populations. (C) 2001 Elsevier Science Ltd. All rights reserved.