It's a pretty well established and undeniable trend. It has what I would call momentum or inertia. It should be the obvious starting point. 90% of the past 400,000 years has been 5 to 8C colder than today. That's just a fact. The planet is uniquely configured for colder temperatures. That too is a fact.Who knows?
Maybe artificial global warming is offsetting a natural cooling period?
As I said, who knows?