Just imagine what the first human beings experienced when they migrated across an ice and land bridge from Asia to Alaska almost 11,000 years ago. Ice from the last ice age was retreating from northern United States. Woolly mammoths roamed the countryside. Several weeks ago a friend encouraged me to attend a lecture at the Roger Tory Peterson Institute which I did, but became distracted and drawn to the unrelated exhibit displaying the skull and tusks of the Randolph, N.Y., mammoth. This skull, not a fossil but a real bone, was discovered in 1934 during excavation for the existing fish hatchery. The exhibit provided a timeline of Earth's history describing a series of ice ages over the last 2.6 million years that covered much of North America and Europe with ice a mile thick. The most recent ice age lasted 12,000 to 25,000 years. Geologists found evidence that ice ages occurred at regular long intervals over Earth's history and, more specifically, over 10 ice ages developed in the last 2 million years. I set out to discover how drastic climate changes could occur.
Almost 150 years ago, the Swiss naturalist Louis Agassiz noted scrapes and gouges high on cliffs overlooking nearby valleys that he thought must have been caused by moving ice in glaciers thousands of years ago. Contemporary scientists disagreed when he theorized correctly; northern Europe was covered with ice a mile thick.
Scientists pondered this natural periodic drastic climate change. In 1935, the Yugoslavian Milankovitch, after 30 years of research, proposed three astronomical variables to explain the repeating cycles of ice ages. The first reason was the slight change in the axis of Earth's daily rotation which occurs over a 42,000-year cycle. The second was due to the changing shape of the Earth's orbit around the sun from a near circle today to an ellipse and back again to a circle over a 100,000-year cycle. The third reason was due to the wobble of the earth on its axis which takes over 22,000 years to complete. When the position of the Earth is just right with the northern hemisphere tipped away from the sun and the Earth is furthest from the sun while in an elliptical orbit, cooling takes place for 25,000 years to generate an ice age in the northern hemisphere. The book ''Ice Age - Solving the Mystery'' by John Imbrie and Katherine Palmer Imbrie explained how ice ages occurred with diagrams that prove, ''a picture is worth a thousand words.'' I hope to present the three reasons without discouraging readers, so please struggle on for a short time and you may be amazed.
This photograph from an exhibit map at the Roger Tory Peterson Institute demonstrates ice covering North America during the last ice age, 25,000 years ago and the appearance today on the right.
Photo by Robert M. Ungerer
Earth rotates on an axis spinning around completely in 24 hours. When the axis changes to more upright, the suns rays are less direct in the summer causing cooler summers therefore less melting of ice accumulation.
Today, the Earth is in a circular orbit, so it is the same distance from the sun during all four seasons. Heat from the sun is equal in the northern and southern hemispheres over the year. When the Earth's orbit changes from circular to an ellipse and the Earth is at the far end of the ellipse, heat from the sun is less intense so the seasons will be cooler. When this happens year after year for thousands of years during the 100,000-year cycle, the Earth cools enough to allow snow and ice to accumulate, creating an ice age.
Lastly, while spinning on its axis, the Earth also wobbles like a top slowing down. During one complete wobble in 22,000 years, the Earth's axis changes direction so that today's position in relation to the sun is summer, but in 11,000 years it will be winter.
Fortunately, we are between ice ages and predictions suggest the next ice age may come in 18,000 years. By then, if humans are still around, we will have learned how to live like one big family in harmony with our neighbors around the world, able to cope and adjust to an ice age.