Scientists know that small-scale ice ages occur every 20,000 to 40,000 years and that massive ones happen every 100,000 years or so. They just don’t know why. The current working theory - first proposed in 1920 by Serbian engineer Milutin Milankovitch - is that irregularities in Earth’s orbit change how much solar energy it absorbs, resulting in sudden (well, geologically speaking) cooling. While this neatly fits the timing of short-term events, there’s still a big problem. Over the past few decades, studies have shown that orbital fluctuations affect solar energy by 1 percent or less - far too little to produce massive climate shifts on their own. "The mystery is, what is the amplification factor?" says University of Michigan geologist and climatologist Henry Pollack. "What takes a small amount of solar energy change and produces a large amount of glaciation?"
The position of the continents does have some influence too. When seacurrents over the north and southpole are blocked (like they are at this time - by the circumference of america and asia in the north and by antarctica in the south) an icecap builds up on the poles and reflects a lot of solar energy back in space. So, in these periods the Milankovitch-cycles are amplified and ice-ages occur, while at other times they do not.
Studies of ice and seabed cores reveal that temperature rise and fall is heavily correlated with changes in greenhouse-gas concentrations. But it’s a chicken-and-egg problem. Are CO2 rises and falls a cause of climate change or an effect? If they are a cause, what initiates the change? Figuring this out could tell us a great deal about the current global warming problem and how it might be solved. But as Matthew Saltzman, a geologist at Ohio State puts it, "We need to know why greenhouse gases fluctuated in prehuman times, and we just don’t."
I like one of the responses on to the article:"What causes ice ages?" may not be the right question to ask. The proper question is, "what causes interglaciation?" Almost all of the past forty million years has been an ice age, with brief periods where the world warmed up somewhat. We are apparently towards the end of one such period.
Article
