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Figure One - North American Paleoindians surviving after the Last Glacial Maximum. |
In my first article on paleoclimatology, I discussed a concept called the Milankovitch Ice Age Theory which explains why ice ages occur and how often their cycles happen. In my second article, I write about what North America looked like during the Last Glacial Maximum (LGM) of the Wisconsin
Ice Age.
If you missed my first article, here is the link, but be sure to come back after you read it; Paleoclimatology 101 - Milankovitch Ice Age Theory
According to most scientists, the Wisconsin Ice Age reached
its Last Glacial Maximum (LGM) sometime between 20,000 and 18,000 years ago. To keep
things simple, I am using 18,000 years ago for the LGM in this article. There were probably several
advances and retreats of the ice sheets during the Wisconsin glaciation, but since ice sheets and glaciers are very destructive to landscapes, they wipe out a lot of the evidence from previous events.
As I mentioned in the first paragraph, my focus is on North America, but readers should be aware that the last ice age impacted many other countries and continents in the northern hemisphere. Figure two is an excellent illustration of the vastness of the last ice age. The map is looking down from the North Pole, and the areas in blue are the estimated extent of the ice sheets on both land and sea. There were places in the northern hemisphere where the ice sheets were as much as three kilometers thick!
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Figure Two - Looking down from the North Pole and showing in blue the
land mass covered in ice and snow during the last ice age.
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Contrary to social media, fake news, and popular belief, Earth's climate has always been in a state of flux throughout our multi-billion-year history. The climate was heating up and cooling off a long time before humans stepped on the planet. Geologic evidence indicates that the Pleistocene, the geologic period of the last ice ages, was a particularly volatile time. Although there have been several events in the geologic past which caused catastrophic climate change, much
of the climatic cycles are related to how the Earth rotates around the sun, and that is the case for the ice ages.
During the LGM, thick ice sheets covered most of Canada and portions of the northern United States (figure three). The massive ice sheets altered geography, climate, and the living environment on both land and sea. Scientists named the two largest ice sheets covering much of North America, Cordilleran on the west and Laurentide on the east. The Cordilleran and Laurentide ice sheets had a tremendous impact on North America's climate. The water for these ice sheets came from the oceans. Scientists believe that to accommodate the estimated size of the ice sheets, global sea levels had to drop approximately 120 meters or 400 feet. This sea-level drop exposed the continental shelf around North America and created a landmass northwest of Alaska called Beringia. New landmasses along the continental shelf and in Beringea became available for habitation by both animals and humans. This makes me wonder how many "Prehistoric Atlantis" colonies exist underwater along the continental shelf now that sea levels have risen.
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Figure Three - Key elements of North America during the Last
Glacial Maximum (LGM). |
What was it like in North America during the Last Glacial Maximum? One, it was much colder! That makes sense. Scientists estimate that the presence of the ice sheets may have caused global temperatures to drop nine to twelve degrees Fahrenheit. And though not all scientists agree with the effect on the tropics, some scientists propose that temperatures may have dropped an average of five to nine degrees Fahrenheit in the warmer climates of the Earth. Of course, the closer to the ice sheets, the more uncomfortable the temperature drop. I have read temperature estimates of eighteen to twenty-two degrees Fahrenheit lower than today along the front of the ice sheets and thirty-seven to forty-one degrees Fahrenheit lower on top of the ice sheets. We can quibble about whether the temperature was X degrees or Y degrees, but bottom line it was colder. The ice sheets were so massive that the jet stream split and went around them. The ice sheets created a high-pressure atmospheric zone above them where anticyclonic winds circulating clockwise. These winds were probably fierce and destructive.
The terrain along the southern margins of the ice sheets was most likely tundra-covered periglacial land resulting from seasonal thawing of snow in areas of permafrost where the runoff, refroze into ice wedges and other structures. Further south from the ice sheets, scientists believe that there were vast spruce forests from the Rocky Mountains to the East Coast of the United States with interspersed loess and sandhills (figure four).
Around 17,000 years ago, the ice sheets started to melt. The northern hemisphere received more summertime insolation from the sun causing an overall reduction in ice sheet thickness and expansion (read my article on Milankovitch Theory). In North America, when the Cordilleran and Laurentide ice sheets melted, it created a mess. Huge lakes formed and oceans received a huge influx of icy freshwater and icebergs, affecting the circulation patterns in the oceans. By 15,500 years
ago, the ice sheets had melted enough to raise sea levels high enough to create the Bering Strait, but not enough melting to open the ice-free corridor between the Cordilleran and Laurentide ice sheets. It would be hundreds of years later before humans and animals could traverse the flooded and boggy passageway between the two ice sheets. Survival for humans in the ice-free corridor required food, clothing, and firewood availability.
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Figure Four - a Mastodon in a spruce forest in a midwestern state in the United States
during the Last Glacial Maximum.
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For decades, scientists believed that the first humans into America migrated through the ice-free corridor between the Cordilleran and Laurentide ice sheets at around 14,500 years ago. However, evidence gathered in the last two decades, indicates there were humans south of the ice sheets, perhaps as early or earlier than the Last Glacial Maximum. So, where did these humans come from? I will cover that story on another day.
For my final article on Paleoclimatology, I will discuss the Younger Dryas, a period of rapid cooling in the late Pleistocene from 12,800 to 11,500 calendar years ago. It followed closely on the heels of dramatic and abrupt warming that brought the last Ice Age to a close around 17,000 calendar years ago. In the meantime, check out my prehistoric adventures, you will be glad you did!
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