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Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years

2008; Nature Portfolio; Volume: 453; Issue: 7193 Linguagem: Inglês

10.1038/nature06950

1476-4687

L. Loulergue, A. Schilt, Renato Spahni, Valérie Masson‐Delmotte, Thomas Blunier, B. Lemieux-Dudon, Jean-Marc Barnola, Dominique Raynaud, Thomas F. Stocker, J. Chappellaz,

Cryospheric studies and observations

The air bubbles trapped in the Antarctic Vostok and EPICA Dome C ice cores provide composite records of levels of atmospheric carbon dioxide and methane covering the past 650,000 years. Now the record of atmospheric carbon dioxide and methane concentrations has been extended by two more complete glacial cycles to 800,000 years ago. The new data are from the lowest 200 metres of the Dome C core. This ice core went down to just a few metres above bedrock at a depth of 3,260 metres. Two papers report analyses of this deep ice, including the lowest carbon dioxide concentration so far measured in an ice core. Atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout the eight glacial cycles, but with significantly lower concentrations between 650,000 and 750,000 years before present. The cover shows a strip of ice core from an Antarctic ice core from Berkner Island, this slice from a depth of 120 metres. Photo by Chris Gilbert, British Antarctic Survey. Elsewhere in this issue, we move from climates past to future plans for climate prediction. A detailed atmospheric methane record from the EPICA Dome C ice core that extends the history of atmospheric methane to 800,000 years before present is detailed. Spectral analyses indicate that the long-term variability in atmospheric methane levels is dominated by ∼100,000 year glacial–interglacial cycles up to ∼400,000 years ago with an increasing contribution of the precessional component during the four more recent climatic cycles. Atmospheric methane is an important greenhouse gas and a sensitive indicator of climate change and millennial-scale temperature variability1. Its concentrations over the past 650,000 years have varied between ∼350 and ∼800 parts per 109 by volume (p.p.b.v.) during glacial and interglacial periods, respectively2. In comparison, present-day methane levels of ∼1,770 p.p.b.v. have been reported3. Insights into the external forcing factors and internal feedbacks controlling atmospheric methane are essential for predicting the methane budget in a warmer world3. Here we present a detailed atmospheric methane record from the EPICA Dome C ice core that extends the history of this greenhouse gas to 800,000 yr before present. The average time resolution of the new data is ∼380 yr and permits the identification of orbital and millennial-scale features. Spectral analyses indicate that the long-term variability in atmospheric methane levels is dominated by ∼100,000 yr glacial–interglacial cycles up to ∼400,000 yr ago with an increasing contribution of the precessional component during the four more recent climatic cycles. We suggest that changes in the strength of tropical methane sources and sinks (wetlands, atmospheric oxidation), possibly influenced by changes in monsoon systems and the position of the intertropical convergence zone, controlled the atmospheric methane budget, with an additional source input during major terminations as the retreat of the northern ice sheet allowed higher methane emissions from extending periglacial wetlands. Millennial-scale changes in methane levels identified in our record as being associated with Antarctic isotope maxima events1,4 are indicative of ubiquitous millennial-scale temperature variability during the past eight glacial cycles.