Paleoclimatology 101 - The Great Meltdown.

Figure One - South of the Ice. 

In my first article on the ice age, I discussed the orbital theory (Milankovitch Theory) behind the creation of Pleistocene ice ages. In my second article, I explored North America during the height of the Wisconsin Ice Age, at the Last Glacial Maximum (LGM) around 18,000 years ago. In this article, I discuss what happened in North America when the ice sheets began to melt.

The Cordilleran and Laurentide ice sheets altered both land and sea geography and profoundly affected the climate and environment in North America. 

Figure Two - A map depicted the area in North America covered 
by the Cordilleran and Laurentide ice sheets 
and the new coastlines at the LGM.  

With four hundred feet of water from the oceans locked up in ice sheets on land and sea, there was more coastline for humans and animals to explore and inhabit around North America. Unfortunately, there wasn't much interior land in Canada or the northern US that wasn't buried under ice. Figure two depicts North America and where scientists believe the ice sheets were located at the height of the LGM around 18,000 years ago. Note the borderline of the United States now and how far land extended out onto the continental shelf during the LGM.   

In my second article, I explored the Last Glacial Maximum (LGM), or what scientists believe was the height of the Wisconsin Ice Age, around 18,000 years ago. It was not long after the LGM, perhaps around 17,000 years ago, that the Cordilleran and Laurentide ice sheets started to melt. It was a slow melt and scientists believe it took around twelve thousand years to melt most of the two ice sheets. After all, these were not your average, normal-sized, run-of-the-mill glaciers. The two ice sheets covered over a million square miles! Scientists estimate the thickness of the ice sheets from around a half a mile thick along the southern edges to over two miles thick in the middle and northern part of the ice sheets. The ice sheets were so heavy that they compacted the Earth's crust down around one thousand feet! 

Figure Three - My first book and my latest book. The Second Edition 

of my most popular book Shadows on the Trail, the introduction to my 

Paleoindian book series.  CLICK to ORDER BOOK 

According to the Milankovitch or orbital theory, the melting began because of the orbital cycle that allowed higher volumes of summer insolation (radiation from the sun) in the northern hemisphere. In my first article, I explained that for ice sheets to grow, they require less summer insolation which allowed ice from previous winters to survive and not melt during summer heat. By adding more ice year after year, decade after decade, century after century; the ICE SHEETS CAN GREW!    

By 13,000 years ago, the Laurentide ice sheet had decreased in  area by at least twenty-five percent and by volume by at least fifty percent. During this time, most of the meltwater from the Laurentide ice sheet was flowing down the Mississippi River drainage system causing flooding along river banks and eroding sediment as a massive flow of water advanced toward the ocean. Once the sediment-laden water reached the calmer sea, sediment settled out and formed a massive delta along the coastline. At the same time sea levels were rising, flooding river valleys with water and more sediment. Once the weight of the ice sheets was removed from the coastlines of Alaska and British Columbia, the land decompressed and rose in elevation.

One question often asked; how fast did sea levels rise? One estimate stated that sea levels across the globe rose 110 meters (361 feet), or 1.1 meter per century (3.6 feet per century) between 16,000 to 6,000 years ago. This was at a slow enough rate for humans and animals to adapt to the new environment. As a comparison, sea levels today are estimated to be rising 10 to 20 centimeters (.33 to .66 feet) per century. 

During the great meltdown, water was everywhere, including the parched desert basins of the American west. In the Great Basin of Nevada, California, Idaho, Oregon, Utah, and parts of Wyoming, there were over 100 freshwater lakes. Diversion of the jet stream to the south of the ice sheets in Canada, caused heavy rainfall from the Pacific on the deserts. Add in the meltwater from the ice sheets and lakes formed all over the Great Basin. With no outlets and low evaporation rates, some of the lakes grew gigantic enormous. 

One ice age lake example was Lake Bonneville which covered a good portion of Utah. Lake Bonneville was 19,000 square

Figure Four - Lake Bonneville compared to
size of Utah. 

miles and had a depth of over 1000 feet (figure four). Death Valley even carried water during the ice age meltdown. Further east, wind squalls roaring off the ice sheets, sandblasting the land. Since cold air is denser than warm air, the squalls blew down the slopes of the ice sheets creating gusts of wind up to 100 miles per hour. During the winter when the soil near the ice sheets was dry, the winds carried sand and silt in great dust storms across the great plains, creating 35,000 square miles of sand hills in north central Nebraska and South Dakota. Today, it is estimated that silt from the ice ages covers thirty per cent of the United States, now mostly hidden under forests and grasslands.

One of the great floods of all time happened during the meltdown of the ice sheets. A lobe of ice from the Cordilleran ice sheet served as an ice dam for Lake Missoula, a 3000 square mile ice age lake in what is now the Clark Fork Valley in Idaho (figure five). When the Cordilleran ice sheet melted, so did the lobe of ice that held the waters back at Lake Missoula. When the rising level of meltwater breached the weakening ice dam, it failed and a wall of water and ice, as much as 600 feet high, flooded Idaho and eastern Washington on its way down the Columbia River to the Pacific Ocean. The flood lasted for a month and created the scarred Channeled Scablands in eastern Washington. On the other side of Canada, Lake McConnell and Lake Agassiz raised their own havoc.    

Figure Five - a depiction of what the ice dam at Lake Missoula might
have looked like.  

Even after the great meltdown, the ice age had one more gasp. Around 12,900 years ago in a period called the Younger Dryas, a sudden and dramatic drop in temperature occurred in North America, returning us to the climactic conditions of the Last Glacial Maximum. The great plains returned to the cool, wet glacial conditions, reminiscent of the Wisconsin Ice Age. This cold period lasted for over one thousand years! 

The Younger Dryas is important for another reason; it marks the end of what some archaeologists believe to be the Clovis Paleoindian culture in North America. Some investigators believe that the Clovis culture was wiped out by an extraterrestrial event which triggered the Younger Dryas while others believe the Clovis culture ended in a more traditional manner. I will explore this in my next article. 

Figure Six - Author John Bradford Branney. See all of his
books at John Bradford Branney Books

If you missed my first two paleoclimatology articles, here are the links.  
Hey, and check out my books, OK?  
Milankovitch Ice Age Theory
Last Glacial Maximum

     

Paleoclimatology 101-Part Two-Last Glacial Maximum

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! 

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.  

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.   

                                                                                                                                                                             

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.

Figure Four - a Mastodon in a spruce forest in a midwestern state in the United States 

during the Last Glacial Maximum.    

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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