Could a snowball earth occur again?
The steadily increasing Solar luminosity and the present continental configuration conspire against it, but a large asteroidal or cometary impact could possibly trigger a snowball earth given the present cold ocean.

This is a fair question given that the global climate has cooled dramatically over the last 50 million years and 20 thousand years ago (the Last Glacial Maximum) the ice extent was as great as at any time since the last snowball earth. On the other hand, the Sun is nearly 6% more luminous now than during the Marinoan snowball earth, when lowering greenhouse gases to present levels triggers a snowball earth in most climate models. As Solar luminosity will only rise in future, a snowball earth becomes a progressively less likely outcome.

If a preponderance of tropical continents made the globe cold (see What caused the snowball earths?), the present geography with its enlarged boreal and subtropical land areas should have made it relatively warm. Before the ongoing intervention by our own species, this this was not the case. A likely explanation is that the rate of global CO2 emission from volcanoes has declined. Dan Schrag at Harvard University in Cambridge, USA suggests that this is because there is little carbonate sediment on the floor of the Pacific Ocean and therefore little CO2 release from volcanoes of the Pacific "ring of fire". He predicts that when subduction switches to the carbonate-rich Atlantic Ocean, the globe will warm again. Schrag also thinks that the present large boreal land areas acts as a "safety switch" preventing extreme lowering of CO2. When it gets cold, these land areas are covered by ice sheets and silicate weathering is diminished. When the snowball earths occurred there was little high-latitude continental area. Large polar sea-ice caps developed that reflected Solar radiation but did not cover much land area. According to this reasoning, a snowball earth is unlikely without a major redistribution of the continents.

On the other hand, a climate model predicted that if the 10-km-diameter asteroid that hit the Earth 65 million years ago extinguishing the dinosaurs and many marine lineages hit instead today, a snowball earth would result. This is because the present cold ocean is more susceptible to surface freezing than the warm Cretaceous ocean (when a snowball earth did not occur) during the decade of reduced Solar forcing due to dust thrown up by the impact.