For more than a century, two families of ice age theories have been proposed: insolation based theories proposed by Adhemar, and atmospheric CO_2 ones proposed by Tyndall. The major technique advance of deep-sea sediment drilling, as well as new interpretations of stable oxygen isotopic composition of the deep-sea fossil foraminifera, which is regarded as a proxy index of continental ice volume and sea water temperature, established the now well recognized glacial-interglacial variations in climate over the late Pliocene and Pleistocene, e.g. ~3 Ma. A landmark progress came from Hays et al. (1976) who unambiguously demonstrated that the change in insolation induced by the Earths axial tilt (obliquity), the wobble of this tilt (precession), and the degree of circularity of the Earths orbit around the sun (eccentricity) brings on ice ages (glaciations) every hundred thousand years or so during the Pleistocene. Many subsequent observations, from the deep-sea to continental records, confirmed the co-variation between insolation and the alternation of glacial ice age phases, and warmer interglacial phases, at least in terms of frequency domains. However, these findings cannot explain the so called ~100 ka conundrum, where the size of 100 ka insolation forcing is relatively small yet the apparent ice sheet response is large, as well as the bi-polar symmetry in climate changes at precessional frequency of ~23 ka, despite the fact that the precession cycle drives insolation changes in antiphase between the two hemispheres. As such, other factors may be considered in the driving of Northern Hemisphere Glaciation (NHG) global glacial-interglacial variations. Several hypotheses have been put forward, such as: (1) the gradual decreasing CO_2 drove global cooling through to a threshold value where insolation changes may force the ice age cycles; (2) the effect of local insolation that modulated the East Antarctica Ice Sheet after the gradual CO_2 decline that drove its growth to maximal extent; (3) the Southern Ocean marine biological pump changes forced by the ocean water stratification and current changes that modulated the atmospheric CO_2 thus triggered ice ages; (4) the clearing of regolith under the North American ice sheets which modulate ice sheet thickness and extent changes which allow the ice sheets to grow further south and be influenced more by precessional cycles, as well as strengthened the fresh bedrock weathering and drawdown more atmospheric CO_2; and (5) because ice mass balance depends on whether the temperature is above or below the freezing point, a physically more relevant parameter to measure insolation forcing should be the insolation integrated over a given threshold that allows for ice melting, and thus triggering the glaciations. Unfortunately, we cannot determine which one is right or several of them acting together so far. The most distinct features of the ice age climate are both the onset of the NHG at ~2.7 Ma and the transition of ~41 ka based glacial-interglacial cycles to cycles of a ~100 ka dominated frequency at ~1.0 Ma, namely the Early Middle Pleistocene Transition.