“Today the total mass of ice on the planet is continuing to decline as global warming increases the melting. Sea level is rising in response.”
By Dr. John J. Hidore
February 23, 2018—-Planet Earth was formed from cosmic debris approximately 4.5 billion years ago. For almost its entire history the planet has been warmer than it is today. Over the past 540 million years relatively warm conditions have existed on our planet probably 90 percent of the time. Temperatures have averaged 5 degrees C (9°F) higher than at present. However, there have been times when the planet cooled enough so that massive amounts of ice formed on the land masses.
The Pleistocene Ice Age
Probably the most important single global environmental event since the human species has been on Earth was the Pleistocene geological epoch. Large areas of the land masses were covered by sheets of ice and mountain ranges supported many individual glaciers. Geologists have dated the Pleistocene Epoch as beginning about 2,588,000 years ago and continued until about 11,700 years ago.
The Pleistocene ice age was not a single glacial expansion followed by warming. It included several large advances and retreats. Each period of expansion included many minor advances and retreats. Significant fluctuations in the last million years averaged about 100,000 years in length. Of the 100,000 year periods, extreme cold prevailed about 90% of the time. The warm periods, or interglacials, were relatively short, averaging approximately 10,000 years.
Areas Covered by Ice
During this epoch large ice sheets formed in both hemispheres on land masses near the poles. At the maximum some 30% of earth’s land surface was covered by ice. The ice sheets reached a thickness of 1500 to 3000 meters (4950-9900 ft). The impact of the ice sheets extended well out from the edge of the ice. Permafrost (permanently frozen soil) extended outward several hundred kilometers in both North America and Eurasia. In North America the present path of the Ohio and Missouri Rivers mark the approximate southern limit of the ice sheet. These two rivers were formed by the melted water at the edge of the ice.
Mountain ranges developed glaciers that extended into the surrounding lowlands and in many cases into the nearest ocean. Pack ice covered the polar seas, and icebergs floated far into the tropical oceans being carried by cold ocean currents.
During the time when ice was most extensive over Earth, temperatures averaged 4°C ( 7.2°F) less than at present. In the northern hemisphere it was perhaps 8 to 12°C (14 – 25º F) lower than the current mean temperature. At the edge of the ice the temperature was about -6°C ( -21º F). At the edge of the permafrost is was 0°C (32º F).
The Wisconsin Glacial Advance
The most recent major glacial advance in North America is known as the Wisconsin. It has other names in other geographical regions. It took place about 30,000 to 12,000 years ago. The peak of the advance was about 18,000 years ago. Huge ice sheets extended as far south as 50º N in Scandinavia and to 40º N in North America. Polar ocean currents extended in the North Atlantic as far south as 45º N. The ice mass was great enough to lower sea level approximately 125 to 140 meters (413 to 462 ft). As the ice accumulated the continental margins changed and the landmasses became more extensive. It was during this time that there may have been a land bridge between Asia and North America, allowing humans to cross into North America.
The Pleistocene ice age had a tremendous effect on Earth. The ice sheets and mountain glaciers changed the surface of the land over large areas. There are still visible features on the continents resulting from the moving ice. There was also a mass extinction of species. Large numbers of species of plants and animals disappeared. The melting of the ice continues to impact life including our species.
The ice has been retreating irregularly since its peak. Over the last 6000 years, sea level has been within a meter (3.3 ft) of that of the present. Today the total mass of ice on the planet is continuing to decline as global warming increases the melting and sea levels is rising in response.