Methane: 40,000 BC to 1995
Atmospheric concentrations of methane (swamp gas) during the Holocene (last 10,000 years) are related to the extent of wetlands especially at low (tropical) latitudes. The lowest methane concentration in the Holocene is at 5200 yrs BP, with values of a little less than 600 ppbv (parts per billion, volume) apparently corresponding to a great drought in which previously abundant tropical wetlands mostly disappeared from such areas as North Africa and Tibet. Some time within a century of this drought (i.e. at about 3150 BC) there was a sudden jump in atmospheric of 40 ppbv, corresponding to either an abrupt increase in extent of wetlands or perhaps sudden outgassing due to rapid melting of ice in permafrost regions. Either theory suggests a sudden climatic event comparable to other sudden shifts such as at 8200 yrs BP or at 12000 years ago, the time of the "Younger Dryas" climatic shift.
It appears that the era of extensive rich tropical wetlands gave way to a severe drought in the late sixth millennium BC, followed by an abrupt event that involved sudden melting of large areas of permafrost, or flooding of arid lands, or both. All of this is in accord with the ideas of ancient thinkers including the Greeks (especially Aristotle), the Sumerians (eg the epic of Gilgamesh) and of course the Bible.
Blunier, et al, "Variations in atmospheric methane concentration during the Holocene epoch" Nature v 374 2 March 1995
rlm / dak