The Ice Ages Cometh
There have been several major periods in Earth's history when ice sheets spread across the northern and southern continents from the poles. The most recent glacial period in the northern hemisphere began 2 to 4 million years ago. This was around the time that the Isthmus of Panama closed. Prior to closure, the surface currents in the Atlantic Ocean connected with the Pacific between the Americas. After closure, the Atlantic circulation stayed in the Atlantic; the Gulf Stream intensified bringing more warm, equatorial water into the north Atlantic. This change in ocean circulation apparently caused the onset of northern hemisphere glaciation.
Exactly how the change in ocean circulation caused glaciation is a matter of continuing debate (and research). One hypothesis holds that prior to the closure of the Isthmus of Panama the Gulf Stream carried warm water into the Arctic Ocean thereby keeping the Arctic relatively warm. After closure, the Gulf Stream was saltier and denser and as it cooled it became dense enough to sink in the North Atlantic forming a deep ocean current (North Atlantic Deep Water), thus depriving the Arctic of a source of heat. Another hypothesis holds that an intensified Gulf Stream carried warm water (producing warm, moist air over it) to high latitude. This warmer, moister air would cause an increase in precipitation (snowfall) which could result in the growth of glaciers.
Continental ice sheets have advanced and retreated across the northern continents many times during the past 2 to 4 million years. These advances and retreats can be correlated with variations in the Earth's orbit around the sun. The amount of solar radiation that the northern hemisphere receives in the summer (to melt the winter's snow) varies with changes in the eccentricity of the Earth's orbit from more round to more oval (100,000 year cycles), the tilt of the axis from 21.5° to 24.5° (41,000 year cycles), and the precession of the Earth's axis like the wobbling of a child's spinning top (23,000 year cycles).
With each increase in the polar ice caps comes a decrease in sea level. With each reduction of the ice caps, as the ice melts, sea level rises. During the maximum advances of the ice sheets the entire continental shelves were exposed, including the Bearing land bridge between North America and Asia. Sea level rose rapidly for several thousand years following the breakup of the glaciers around 10,000 years ago. Sea level rose very slowly for several more thousand years but is now rising at a moderate rate of about 2 mm per year. The rate of sea level rise is expected to increase as a result of the ongoing warming of the Earth.
Today, we are in an interglacial period. The glaciers should come back. In fact, based on the present state of the Earth's orbital cycles, we should be on the return to the next glacial advance. However, global warming, presumably caused by human activities, may be preventing the next glacial epoch.
It is interesting to note that humans evolved during the ice ages. Humans first came to the Americas over the Bearing land bridge before the latest continental ice sheets broke up around 10,000 years ago. Great civilizations arose in the Middle East, India, the Far East and the Americas during the current interglacial period.