In the early days of climatology, a disturbing possibility reared its head whenever people ran computer models of Earth's climate — suddenly, everything would freeze over. It's called the White Earth climate, and this is why it hasn't happened.
The White Earth climate is a kind of end state for the world's weather, as once the cold starts taking over, it's very hard to tip the scales back. As ice slowly covers the globe, the sunlight coming in is reflected back into space. Very little of it is absorbed, so the cold Earth stays cold. Of course there would be other effects; the lowest layer of the atmosphere would thin, so we'd have a bit more trouble breathing. On the other hand, with so little pressure and temperature variation to work with, storms would be unimpressive.
The world couldn't become a White Earth without a big kick, a dimming of the sun or other catastrophic event, but it's a surprisingly easy trough to fall into, at least when it comes to simulations. It's not uncommon to find it invoked on sites that deny climate change — or say that we should be pumping out carbon to keep the globe from freezing over. And actual scientists have wondered why the world isn't frozen. One theory is that the Earth has a kind of thermostat; when carbon dioxide is in the air, it combines with water to form the famous acid rain. This rain comes down and dissolves silicate rocks, the water and the dissolved rocks rush to rivers, and then to the ocean, where creatures use the minerals to build their shells. The creatures die and sink to the bottom of the ocean, taking the carbon out of the atmosphere.
As the temperature goes up, the warmth helps the rain dissolve the rocks more easily, and increases the rain to certain wet regions. More carbon goes into the ocean, and the temperature comes down. If the climate gets cold, the reactions that dissolve the rock slow down, less carbon leaves the atmosphere, and the temperature goes back up. This built-in buffer may be one of the reasons we're not all wearing parkas right now.