I think the earth is constantly changing, and has been both warmer and colder than it is now. One of the definitions of an "ice age" is polar ice sheets, which we have now; we are then, in an ice age.
The bad thing, it seems, is when this fluctuating temperature occurs faster than animals and plants are able to adapt. This is what is occurring now, with one example in a rattlesnake study.
"We find that, over the next 90 years, at best these species' ranges will change more than 100 times faster than they have during the past 320,000 years"The result is potential mass die-offs.