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Not to mention that a sample of even 25, 50, 100, or 500 years really means nothing when we are thousands of years past the last ice age.
The case for human causes of increased atmospheric CH4 over the last 5000 years
"We propose that humans significantly altered atmospheric CH4 levels after 5000 years BP and that anthropogenic inputs just prior to the industrial revolution accounted for up to 25% of the CH4 level of 725 ppb (parts per billion). We base this hypothesis on three arguments: (1) the 100 ppb increase in atmospheric CH4 that occurred after 5000 years BP follows a pattern unprecedented in any prior orbitally driven change in the ice-core record; (2) non-anthropogenic explanations for this increase (expansion of boreal peat lands or tropical wetlands) are inconsistent with existing evidence; and (3) inefficient early rice farming is a quantitatively plausible means of producing anomalously large CH4 inputs to the atmosphere prior to the industrial revolution. If the areas flooded for farming harbored abundant CH4-producing weeds, disproportionately large amounts of CH4 would have been produced in feeding relatively small pre-industrial populations."
"MORE EVIDENCE that the Earth is warmer than at any time in the past 1,000 years has come from ice cores in a glacier on the "roof" of the world.
Himalayan ice cores provide convincing evidence that the past 50 years - and the 1990s in particular - have been the warmest of the past millennium, says a study published today in the journal Science.
An international team of scientists drilled three cores each of about 150 metres (500ft) into the Dasuopu glacier, an ice field on the flank of Xixabangma, a peak that rises to 26,293ft on the southern rim of the Tibetan plateau."
Ice cores from a Himalayan glacier confirm global warming - The Independent (London, England) | Encyclopedia.com