IT’S 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.
Felisa Smith at the University of New Mexico in Albuquerque has calculated that when…
