Ice ages are not uniformly cold. There can be colder and warmer periods during the overall ice-age period. Colder periods lead to more extensive areas of continental ice sheets, valley glaciers and sea ice, while warmer periods lead to reduced areas of ice. An ice age ends when the Earth warms enough for the ice cover to recede, or disappear completely.
The regions on the fringes of extensive ice sheets and glaciers experience a cooling to the point that a consistently cold environment forms. Usually, the ground is frozen for much of the year, growing seasons are short, and only the hardiest of flora and fauna survive. The Russian tundra is an example of this landscape. Temperate and tropical zones become restricted to the lower equatorial latitudes.
A question that follows on from the definition of an ice age is: how cold does Earth have to become to produce one? Analysis of proxy temperature data e. There is no official minimum period of time for an ice age. Some colder periods in historical times are termed little ice ages, including between the 13th and 18th centuries. This period was characterised by longer and colder winters, and shorter, cooler summers.
Other mammals that thrived during this period include moonrats, tenrecs hedgehog-like creatures and macrauchenia similar to a llamas and camels. Although many vertebrates became extinct during this period, mammals that are familiar to us today — including apes, cattle, deer, rabbits, kangaroos, wallabies, bears, and members of the canine and feline families — could be found during this time.
Other than a few birds that were classified as dinosaurs, most notably the Titanis , there were no dinosaurs during the Pleistocene Epoch. They had become extinct at the end of the Cretaceous Period , more than 60 million years before the Pleistocene Epoch began.
Birds flourished during this period, including members of the duck, geese, hawk and eagle families. There were also some flightless birds such as ostriches, rheas and moas. The flightless birds did not fare as well, as they had to compete with mammals and other creatures for limited supplies of food and water, as a good portion of the water was frozen. As for vegetation, it was fairly limited in many areas. There were some scattered conifers, including pines, cypress and yews, along with some broadleaf trees such as beeches and oaks.
On the ground, there were prairie grasses as well as members of the lilly, orchid and rose families. About 13, years ago, more than three-fourths of the large Ice Age animals, including woolly mammoths , mastodons, saber-toothed tigers and giant bears, died out. It may be subtle, small changes each time, but there is a progressive change.
The next glaciation occurred in the period we call the Ordovician about million years ago. And then there was a very major ice age called the Permo-Carboniferous Glaciation which occurred on the southern continents, often called Gondwanaland, when they were arranged around the South Pole. Those continents were Antarctica, Australia, South America, India and Africa, and there are glacial deposits covering extensive regions which date from that period, about million years ago.
The most recent glaciation is the one we are living in now, but to say when it began is a little bit problematic. In addition, the structure of each ice age appears to be periodic. There are short periods of relatively warm conditions and then longer periods during which we seem to have glaciation.
And the cold periods are not just a result of glaciation, of course. Glaciation is the most spectacular consequence of these cold periods, but there are also substantial intervals which are not glacial but still very cold.
He looked at glaciers in the Alps and realized that they were once much more extensive than they are today. There were others in Switzerland, someone called Jean de Charpentier had already noticed this as well. Though there was a lot of skepticism at first, people began to realize that he was right. As geological mapping began, particularly in northern Europe, it was realized that glaciers had once been very extensive indeed.
So by the beginning of the 20th century it was realized that there were multiple glaciations, not just one, and then, subsequently everything has developed from there. There is geological evidence of ice ages. They can be found in the form of thick sequences of characteristic sediments called diamicton. They are just glacial deposits, but glacial deposits include not only sediments directly from the glacier, but also from meltwater sediments formed by meltwater streams or glacial lakes, or from where the glaciers were floating in the sea.
There are various forms of glacial lakes but, in principle, the basic difference is that they are water bodies which are dammed by ice, so, for example, if you have a glacier which advances up a river valley, it blocks the valley rather in the way that you put a cork in a bottle.
And of course when the ice blocks the valley, the river will still flow, and the water level will rise until it spills over. So a glacial lake is formed by being indirectly in contact with the ice, and there are characteristic sediments which such lakes contain and we can identify.
Because of the way that glaciers melt, which is controlled mostly by seasonal changes in temperature, you get annual discharge from the ice, and that gives you annual inputs of fine sediments, derived from under the ice, into the lakes. If we then look at the lakes we get what are called laminated sediments, or rhythmically laminated sediments, which are also called by the Swedish word varve, which means annual deposition.
So we can actually see annual laminated sediments in the glacial lakes, and we can even count those varves, and figure out how long the lake existed. So we can, in fact, do a lot with such materials. Now, if you think about Antarctica, you have large ice shelves where the ice goes off the land and onto the sea. And, of course, the ice is buoyant so it floats, and as it floats it carries with it pebbles and finer sediment material. But because of the heating effects of the water the ice melts and, therefore, drops that material.
That forms so-called ice-rafted detritus which is dropped into the oceans. So when we see the fossil record of these periods we can learn, by looking at these sediments and their associated landforms, where the glaciers were, how far they extended, and so on. So, for example, equatorial regions where the Sun is almost vertically overhead are the warmest areas, and the polar regions where the Sun is at an extreme angle to the surface are the coldest.
That means this differential in heating of the surface drives the ocean- atmosphere machine, which is trying all the time to transfer heat from the equatorial regions to the poles. If the Earth were a simple globe the transfer would be very efficient and there would be a very small contrast between the equator and the poles.
But because we have continents, they get in the way of this circulation and, accordingly, the circulation patterns become very complicated. The simple patterns become inhibited and modified by the land and, in particular, the mountains, which causes the pattern of circulation which we see today, driving the trade winds or the ocean currents.
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