I think they say that the small shift towards warmer weather caused a cascading effect. In other words, the small amount of warmer weather in turn caused this beetle to flourish which in turn drives the weather up even more.
I may have beech front property within my life time yet!
Mountain pine beetle is a native species, but climate change (warmer winters) has resulted in a population explosion and this beetle has been devastating western North American pine forests. It is an increasingly common example of a native species that has become invasive due to climate change. It is marching eastward and could eventually devastate pine forests across the continent.