635m. years ago – research indictates methane release tipping point.
31/5/2008 New Scientist Report published in Nature, vol 453, p. 642Research at University of Californian indicated that 635m years ago a “snowball earth” (ice sheet to the equator) was melted by methane discharge either tiggered by instability in the ice sheets or local warming starting a positive feedback or tipping point prcess whereby each increase in warmed cause by methane release resulted in more warming, more release etc in a runaway process.