Ice-core record of atmospheric methane changes: relevance to climatic changes and possible gas hydrate sources
Résumé
The Antarctic and Greenland ice contains an almost direct record of past atmospheric CH4. The record over the last 200 years reveals a spectacular 150% increase of the CH4 atmospheric mixing ratio since pre-industrial times. At the scale of a glacial-interglacial cycle the record shows a remarkable correlation with climatic changes, with high (low) CH4 levels during warm (cold) periods. A striking feature of the glacial-interglacial CH4 record is the presence of large and abrupt (at the scale of a century or less) changes during the last glaciation and glacial-interglacial transition. The classical interpretation for the origin of CH4 changes prior to the industrial era involves mainly the wetland source. In the context of gas hydrates the question is to know whether the past ice-core record contains fingerprints of catastrophic hydrate release (CHR). We currently conclude that the available record shows no evidence for CHR but additional ice-core analyses are necessary to reach a more definitive conclusion.