IT鈥橲 not just for the last century that humans have been messing up the climate. It may have been going on for thousands of years.
When hunters arrived in North America and drove mammoths and other large mammals to extinction, the methane balance of the atmosphere could have changed as a result, triggering the global cool spell that followed. The large grazing animals would have produced copious amounts of methane, a potent greenhouse gas, from their digestive systems. They vanished about 13,000 years ago.
at the University of New Mexico in Albuquerque has calculated that when these animals were around they would have emitted 9.6 megatonnes of methane annually. Ice core records show atmospheric methane levels plunged from about 700 parts per billion to just 500 ppb at the time of their extinction. Disappearance of methane emissions from the extinct species is a possible cause, Smith says (Nature Geoscience, ).
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鈥淚t is conceivable that this drop in methane contributed to the cooling episode,鈥� says Smith. This would mean humans have been changing global climate since well before the dawn of civilisation.
