December 24. Pleistocene Ice Ages

There have been at least five major glacial periods in earth’s history – two in the Precambrian, one at the end of the Ordovician, one during the Carboniferous, and now. “Now” is perhaps an overgenalization, but it’s not all that certain that the most recent ice age is over. It started about 2.6 million years ago, and occupied the Pleistocene Epoch of the Quaternary Period of the Cenozoic Era. It ended – if it has ended – about 12,000 years ago, which is taken to be the end of the Pleistocene and the start of the Holocene, the present-day epoch of geologic time. 

I say we don’t know if it’s over because the 2.6 million years saw at least four major and several minor pulses of glaciation that alternated with times called interglacials, when the ice was much less extensive. We may be in the early stages of an interglacial period, or the whole cycle may be over. 

Maximum glaciation (ice shown in black) by Hannes Grobe/AWI, used under Creative Commons license.   

At the peak, glacial ice reached as far south as the Missouri and Ohio Rivers and New York City in North America, and across the British Isles, central Germany, and most of the plains of western and central Russia. Continental ice sheets as much as 3 to 4 km thick (2 to 2½ miles) were prevalent in the northern hemisphere, which seems to have been affected more than the southern hemisphere, though the Antarctic Ice Sheet expanded. Mountain glaciers in southern South America, New Zealand, and in the northern hemisphere also grew significantly.

Because so much water was locked up in the ice, sea level fell about 120 meters, or 390 feet. Huge areas of what are now the continental shelves, under water, were dry land, including the Bering Sea off Alaska, which created the famous Bering Land Bridge between North America and Asia.

What caused it? Over the course of these podcasts, you’ve heard about ideas of global cooling related to volcanism that might have led to glacial periods, as well as ocean circulation, the rise of plants that changed greenhouse conditions, and other things as possible causes for the sporadic glacial times through earth history. The bottom line is, we really don’t know for certain. The modern glacial period, the past 2½ million years, may be just the culmination of cooling that dates to 34 million years ago, when the Antarctic Ice Sheet got going.

One factor that has been promoted as a cause for ice ages is the variations in earth’s orbit. Regular cycles in the tilt of the earth’s axis and in the shape of the obit around the sun produce regular variations in temperature – but that happens all the time, and there have only been a handful of glacial periods over billions of years, so there must be more to it.

Pleistocene ice extent (USGS)

Ocean currents and the arrangements of continents may have a role. I mentioned the opening of the Drake Passage, creating a truly circumpolar oceanic current around Antarctica, as a possible factor in the growth of ice there. In the modern Northern Hemisphere, the changing arrangements of continents has made the Arctic Ocean an almost landlocked sea, and combined with the growth of the Isthmus of Panama about 3 million years ago that drastically changed global circulation patterns, maybe that all created the tipping point that led to the onset of glaciation.

A decrease in carbon dioxide levels would reduce the greenhouse effect and global average temperatures would be cooler. Such changes can be observed in ice cores, and there was indeed a general, slow, but significant decrease in CO2 in the atmosphere through most of the Cenozoic until recently. That would still call for a threshold or tipping point because the onset of significant continental glaciation was pretty abrupt, about 2.6 million years ago, and it is hard to explain the changing CO2 levels in ways that would account for the rapid changes from glacial to interglacial periods. And even within predominantly glacial or non-glacial times, there was lots of variability. It’s best to think of it as “mostly glacial” periods that lasted on the order of 40,000 to 100,000 years, and shorter interglacial periods, maybe 20,000 to 30,000 years – but with oscillation and change throughout it all.

The last glacial maximum was about 22,000 years ago, and at that time there was more ice in North America than in present-day Antarctica. Most of the estimates of the alternating temperatures and ice volumes suggest a gradual build-up to an ice maximum, over a period of 40 to 60 thousand years, but a pretty abrupt drop into an interglacial, over a period of around 10,000 or 15,000 years. That’s pretty much how long it’s been today since there were glaciers in Ohio.

There are at least 9 professional journals devoted to the study of the Quaternary and the glacial period. I’m going to leave the question of the cause at what I’ve said in this episode, recognizing that there is plenty of research going on. The effects of the ice age are much easier to describe than the causes, and four of our next five episodes will relate to some of those.

—Richard I. Gibson

Impact on Gulf of Mexico
Maximum glaciation by Hannes Grobe/AWI, used under Creative Commons license.