Ada beberapa teori tentang hal itu, mulai dari perubahan sirkulasi laut, naiknya ketinggian Rocky Mountains, perubahan orbit Bumi, dan perubahan konsentrasi gas rumah kaca di atmosfer secara alami.
There are several theories about that, ranging from changes in the ocean circulation, the increase in the height of Rocky Mountains, changes in earth?s orbit, and the natural changes in the concentration of green house gas in the atmospher.