There is a lot of speculation about why the Earth has not entered into a new ice age, following on from the pattern of glacial cycles recorded in the polar ice cores. It is a possibility that the Earth's tilt towards the Sun actually increased in the recent past. I know this sounds slightly crazy, but it is a possibility. This would have the effect of increasing the summer temperatures of the high latitudes, which is a vital factor in determining whether there is snow accumulation which leads to glaciation. Here is an article which validates this idea:article.