To summarize this, the last ice age was caused by the currents in the atlantic ocean stopping.
Causes of the Younger Dryas
The prevailing theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic thermohaline circulation in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America; however, geological evidence for such an event is thus far lacking. The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. A recent alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, bringing more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation. Neither theory explains why South America cooled first.
Previous glacial terminations probably did not have Younger Dryas-like events, suggesting that its cause has a random component. Nevertheless, there is evidence that some previous glacial terminations had post glacial cooling periods somewhat similar to the Younger Dryas.
Alternatively, a hypothesized Younger Dryas impact event, interpreted to have occurred in North America around 12,900 years ago (10,900 BC), has been purported to have initiated the Younger Dryas cooling and population bottleneck or near extinction of the peoples responsible for the Clovis tool culture. Material reported as nanodiamonds resulting from an impact turns out to be a different form of carbon. 
How fast did this happen?- to summarize this, pretty quick.
Abrupt climate change
The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12,900–11,500 years before present (BP) in sharp contrast to the warming of the preceding interstadial deglaciation. It has been believed that the transitions each occurred over a period of a decade or so, but the onset may have been faster. Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was ~15°C colder during the Younger Dryas than today. In the UK, coleopteran fossil evidence (from beetles) suggests that mean annual temperature dropped to approximately 5°C, and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas. Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.
We have been going though a warm period, I suspect that the increase in CO2 in the atmosphere from the begining of the industrial revolution has put off another cooling period for the planet. (Global warming has put off an ice age, which was begining at the begining of the industrial revolution)
1776 wasn't so long ago, check out the ice on the Delaware River in the famous painting:
ICE /\ /\ /\ /\ /\
If you don't know your US geography, the Delaware River is right outside of DC, it very rarely snows in the DC/Delaware area, at least recently.
Wheat prices are spiking because Russia will not allow exports, Mozambique has food riots because of rising prices.
We are begining to see the results of the oil spill.