Glacial-interglacial water cycle, global monsoon and atmospheric methane changes

The causes of atmospheric methane (CH4) changes are still a major contention, in particular with regards to the relative contributions of glacial-interglacial cycles, monsoons in both hemispheres and the late Holocene human intervention. Here, we explore the CH4 signals in the Antarctic EPICA Dome C and Vostok ice records using the methods of timeseries analyses and correlate them with insolation and geological records to address these issues. The results parse out three distinct groups of CH4 signals attributable to different drivers. The first group (~80% variance), well tracking the marine δ18O record, is attributable to glacial-interglacial modulation on the global water cycle with the effects shared by wetlands at all latitudes, from monsoonal and non-monsoonal regions in both hemispheres. The second group (~15% variance), centered at the ~10-kyr semi-precession frequency, is linkable with insolation-driven tropical monsoon changes in both hemispheres. The third group (~5% variance), marked by millennial frequencies, is seemingly related with the combined effect of ice-volume and bi-hemispheric insolation changes at the precession bands. These results indicate that bi-hemispheric monsoon changes have been a constant driver of atmospheric CH4. This mechanism also partially explains the Holocene CH4 reversal since ~5 kyr BP besides the human intervention. In the light of these results, we propose that global monsoon can be regarded as a system consisting of two main integrated components, one primarily driven by the oscillations of Inter-Tropical Convergence Zone (ITCZ) in response to the low-latitude summer insolation changes, anti-phase between the two hemispheres (i.e. the ITCZ monsoon component); and another modulated by the glacial-interglacial cycles, mostly synchronous at the global scale (i.e. the glacial-interglacial monsoon component). Although atmospheric CH4 record integrates all wetland processes, including significant non-monsoonal contributions, it is the only and probably the best proxy available to reflect the past changes of global monsoon. However, the utility of CH4 as a proxy of monsoon changes at any specific location is compromised by its bi-hemispheric nature.