The operative word is could.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.
"It is conceivable that this drop in methane contributed to the Younger Dryas cooling episode," says Smith.
The operative word is conceivable. Is this the sort of crap that science has become? The reason it's called prehistory is because there was no one at the time around capable of recording what wass going on -- lack of writing, etc. Which of course leaves it open to all sorts of speculation, including this sort. Editorial standards need to be raised. I know, I sound petulant. But this kind of stuff is really becoming tiresome. Or maybe just ts being taken seriously is.