Prominent multimillennial to millennial time-scale changes in the greenhouse gases CO2 and methane occurred during the pre-modern Holocene, as documented by measurements from Antarctic and Greenland ice cores (Figure 3-4). Blunier et al. (1995) suggested that changes in methane reflected changes in the hydrological cycle at low latitudes. This interpretation appears to be supported by evidence that the minimum in methane coincides with the time at which many tropical lakes dried up. The subsequent increase in methane is thought to reflect an increasing contribution from northern wetlands as these areas recovered from inhibited growth during earlier, colder temperatures. The large, abrupt decreases in methane in the early Holocene coincide with abrupt coolings in at least the North Atlantic and western European regions. It has been argued recently that interpolar methane gradient data from ice cores is evidence of an abrupt switching on of a major Northern Hemisphere methane source, probably in Siberia, in the early Holocene between about 9,000 and 11,500 years ago (Smith et al., 2004).
Indermühle et al. (1999) interpreted the changes in CO2 together with δ13C as evidence of changes in terrestrial biomass and sea surface temperatures. The decline in CO2 between about 7,000 and 11,000 years ago is