Excellent point.
But how long does it take for an ice age to manifest itself? Decades? Centuries? And it is not inconceivable that an ice age could be precipitated by global warming, speculations about ice melt disrupting the Gulf Stream have been proposed, and although just speculation, there may be many other similar processes at work we don't even know about. We don't really know, of course, but we do know from the geological record these changes occur naturally. We know the earth's climate is stable in the long term and highly variable in the short term. In between, who knows? So it is not unreasonable to suspect climate transients could be unwittingly initiated by the activities of a brand new and rambunctious Type I civilization like ours.