The case for human causes of increased atmospheric CH4

We propose that humans significantly altered atmospheric CH4 levels after 5 000 years BP and that anthropogenic inputs just prior to the industrial rev olution accounted for up to 25% of the CH4 level of 725 ppb (parts per bill ion). We base this hypothesis on three arguments: (1) the 100 ppb increase in atmospheric CH4 that occurred after 5000 years BP follows a pattern unpr ecedented 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 pro ducing anomalously large CH4 inputs to the atmosphere prior to the industri al revolution. If the areas flooded for farming harbored abundant CH4-produ cing weeds, disproportionately large amounts of CH4 Would have been produce d in feeding relatively small pre-industrial populations. (C) 2001 Elsevier Science Ltd. All rights reserved.