I used to think that, but the more I learn about boloids, megavolcanoes and the like I can also see other important factors. For example the Long Valley (CA) mega crater just over 700,000 years ago, the Yellowstone megaeruption around 640,000 years ago. Something triggered our most recent major ice age about 125,000 years ago, but the megavolcano Toba sent the temperature even more sharply downward about 74,000 years ago. I haven’t studied time lines on boloid events in the past million years, but bet there would be some interesting possibilities there also.
A volcano is a pimple on the ass compared with a bolide impact. And glaciation results from heat, not cold, which is why massive cloud cover won’t cover the Earth with miles of ice — the hydrologic cycle slows to the level seen well, on Antarctica, dryest of the continents (but not due to cloud cover). Energy is needed to evaporate all that water, and is delivered from above, by a big fast-moving chunk of space debris; the water vapor rises with the fireball; the heat in the vapor is shed at high altitude; the precipitation falls as ice, snow, sleet, freezing rain, and some rain.
Antartica had temperate flora 3 million years ago. The Eltanin impact happened 2.5 million years ago. The myth of the 30 million year old Antarctic ice cap is going to die hard — but it will die.