The Evolution Of An Ice Age
Throughout the past 40 million years our Earth has been in a continuous ice age. This period is considered an ice age because of the ongoing presence of an ice sheet that currently covers Antarctica. An ice age can be thought of as a period of time where the Earth’s climate is constantly reducing in temperature, which causes the formation of ice sheets and glaciers. This cooling of the Earth doesn’t happen all at once, it goes through cycles of warm and cold weather. The colder cycles are referred to as glacial periods and the warmer cycles are referred to as interglacial periods. These periods occur on intervals of 40,000 and 100,000 years. During these cycles ice sheets and glaciers advance and retreat with respect to the climatic conditions. The last glacial period, the Wisconsin Glaciation, ended approximately 11,000 years ago. The deglaciation of ice sheets and glaciers, which happens when we enter a interglacial warming period, are important because they have an immense impact on the land structure and vegetation throughout our continent, as well as the rest of the world. The deglaciation of the Wisconsin Glacier caused the formation of the Great Lakes as well as Niagara Falls in New York. This melting of ice sheets also causes sea levels to rise which covers landforms. The water submerges bridges and passageways between continents that were used by animals which migrated from continent to continent. This prevents certain species of mammals and plants from returning back to their native land, which drastically changes the vegetation and habitat of certain areas. This paper will look at proposed factors that caused previous ice ages and glaciation periods: atmospheric composition, Milankovitch cycles (Earth‘s orbit around the sun), the movement of tectonic plates and changing continental positions.
Our Earth’s atmosphere is made up of such gases as nitrogen, oxygen, methane and carbon dioxide. The purpose of our atmosphere is to protect our earth by absorbing ultraviolet rays from the sun and controlling temperatures. This comes into play during the shift from day to night; it helps control the drastic reduction in temperature. The important role the atmosphere plays in glaciation is with a process called the greenhouse effect. The greenhouse effect is where the atmosphere warms the Earth’s surface. This occurs when there is an excess of CO2 (carbon dioxide) and methane, which are considered greenhouse gases, in the air. This excess causes the planet to warm and ice sheets to melt. It is thought that the reduction of CO2 and methane in the atmosphere was a critical factor in the formation of the last ice age. This reduction would cause a period of steady cooling throughout the Earth, which in the end led to the formation of glaciers and ice sheets. There are many theories of how greenhouses gases formed to help deglaciate huge ice masses, one of which is volcanism. When volcanoes erupt they release methane and other greenhouse gases into the atmosphere, this accumulation of gases causes a global warming effect on earth.
Another important factor associated with the formation of ice ages is an idea developed by a Serbian mathematician Milutin Milankovic, called Milankovitch cycles. This theory is uses the eccentricity, obliquity and precession of the Earth’s orbit around the sun to explain changes in climate on 100,000 and 40,000 year time scales. When these three factors fluctuate it affects the amount of solar radiation, not to be confused with solar heat, that reaches the Earth. This controls the length and severity of the seasons, which has a significant effect on glaciation. Glaciation occurs when there are cooler summer and winter temperatures. These conditions cause a buildup of snow from previous winters that never had a chance to melt due to colder weather. This snow and ice eventually accumulates forming glaciers and ice sheets.
The term eccentricity describes the shape of the Earth’s orbit around the sun. The shape of the orbit fluctuates over time; it goes from being circular to being elliptical. This changing cycle lasts for about 100,000 years, which corresponds with the glacial and interglacial periods that have been studied, which was the basis for this theory. The eccentricity affects the amounts of sun radiation received by Earth; this controls the distinction between seasons in each hemisphere. Depending on the shape of the orbit one hemisphere might have large differences in temperature between seasons, whereas in the other hemisphere the temperature change might be mild. These variations in temperature and solar radiation, even though they seem small, play a significant role in the creation and thawing of ice sheets.
The next important aspect of Milankovitch’s theory is obliquity; this explains the tilt of the Earth’s axis. The change in obliquity or tilt of the Earth’s axis causes the creation of seasons. This change happens gradually over time, the tilt of the axis causes one pole to be tilted away from the sun and eventually as the earth orbits the sun it will become tilted towards the sun. The Earth’s orbital period of change takes about 41,000 years. This transformation also influences the severity of seasons because it controls how much solar radiation is reaching our planet. The greater the axis is tilted, the stronger the change in seasons.vi The changes in seasons only apply to higher latitudes, where ice sheets tend to form. Obliquity has very little effect on lower latitudes since the way the axis tilts controls the distribution of solar radiation at each pole.v Lower obliquity triggers less solar radiation at the poles, which in turn makes it very cold. These cold conditions produce an ideal setting for the formation of glaciers and ice sheets.
The last major aspect of this theory of climate change is the precession or the wobble of the Earth. This controls when the Equinoxes, the days where the sun is directly over the equator, occur. Currently these days are March 21 and September 21, on these days each place on Earth experiences 12 hours of light and 12 hours of darkness. The precession of the Earth also controls to location of the Earth’s aphelion, the point that is farthest from the sun, and the perihelion, the point that is closest to the sun. These two points play extensive role in climate change. They determine the seasonal balance of each hemisphere by influences the amount of solar radiation that reaches the Earth’s surface.v This, like the change in obliquity and eccentricity, can lead to ideal conditions for the formation of ice sheets.
An additional important aspect when discussing the development of an ice age is the movement of tectonic plates. These plates make up our Earth’s lithosphere. There are seven large plates, essentially the continents, that all move in relation to one another. These movements are characterized by three different boundaries. These boundaries are transform, divergent and convergent. Transform boundaries occur when two plates slide past each other, resulting in faults. Divergent boundaries occur when two plates slide apart from each other, resulting in ridges. Lastly, convergent boundaries occur when two plates slide into each other, with colliding or one sliding underneath the other. This is a very slow process and is largely caused by friction and gravity. The movement of these plates is important when dealing with glaciation because they can cause climatic shifts. The repositioning of these large plates has a large impact on the distribution of land masses, mountain ranges and how the oceans connect.ii This allows tectonic plates to have a direct influence on global climate. This is due to the face that these landforms can change control temperature by blocking wind, rain and solar radiation. An example of this is the Himalayas which were formed by plate movements.ii This large mountain range has increased the Earth’s total precipitation. This leads to a decline in greenhouse gases as a result of the large amount of rain and precipitation ridding the atmosphere of CO2.
Plate movement also generates volcanic activity. When the plates move there are more volcanic eruptions which emit CO2 into the atmosphere and cause global temperatures to rise. In addition, underwater volcanic hot spots can melt ice sheets from underneath. Another result of these hot spots is the melting of ice in the oceans and releasing CO2 into the atmosphere. The repositioning of tectonic plates plays a considerable role in altering ocean currents as well. Ocean currents are movements of hot or cold water throughout the ocean that are modified by factors such as wind, temperature and planet rotation. These currents are capable of raising and lowering global temperature, which essentially aids in the forming or melting of ice sheets. At certain times the ocean currents around the equator were able to circulate around the earth which allowed more warming to occur due to the higher degree of ocean re-circulation.
One more important element with plate movement is the location of the plates or continents, this movement is referred to as continental drift. Continental drift comes from the idea that all the continents used to be connected as one large landmass and over time they moved to the arrangement they are in now. Most of the large ice sheets over time have been concentrated in higher latitudes as long with larger landmasses. This is because large landmasses make it easier for glaciers and ice sheets to form due to the fact that they cannot form over the ocean.ii Also, continents have to drift near the poles, or colder areas, to accumulate enough snow to form a glacier or ice sheet. Most modern glaciers are found in mountainous areas because there is usually a lot more moisture and colder conditions at higher altitudes. The central issue with the location of continents is when they are in a position to block the warm current from the equator to each pole. As continents move old seaways are block and new seaways are developed which create new ocean currents and change how the ocean transports cold and hot water. When the temperature of large bodies of water is altered it in turn modifies the global climate.
In addition to the formation of glaciers and ice sheets there are also factors that can intensify these periods of glaciation. Two major contributors to this are an increase in the Earth’s albedo and rapid deforestation. The Earth’s albedo deals with how much light it reflects and absorbs from the sun. This impacts global climate by influencing changes in temperature, winds, ocean currents and precipitation. When there is a lot of ice and snow covering the Earth the albedo tends to be higher due to it reflecting more light than it is absorbing. Lower air and land temperatures are a result of having a high albedo, which causes ice and snow accumulations to grow larger and last longer.
In summary, it is important to analyze the factors that have led to the formation of ice sheets and glaciers in the past. The atmospheric composition of the Earth, which is becoming a main topic of focus in our climate today, and amount of CO2 in the air is a major contributor to the development of interglacial and glacial periods. Also, the Earth’s eccentricity (orbital shape), obliquity (tilt) and precession (wobble), also know as Milankovitch cycles, have a huge effect on the climate and seasonal variations of our planet. Lastly, the movement of tectonic plates, which causes the changing of continental positions and also creates landmasses and shifts in ocean currents, is one of the most influential factors discussed in this paper. There are many different aspects that go into the formation of these glacial periods, even though only a few were mentioned in this paper. The importance of analyzing these factors is to plan ahead for the future. When the Wisconsin Glacier melted it brought about a lot of changes within our landscape, animal life and vegetation, as well as changing sea levels. It created completely new rivers and lakes while at the same time destroying or drowning existing lands. As for vegetation, the melting of the glacier killed off or relocated many species of plants and trees. Since the industrial revolution, humans have played an immense role in the emission of greenhouse gases and the warming of our planet. As we have begun to witness the excess amounts of greenhouse gases that are being emitted into our atmosphere have caused a vastly increasing global warming trend. This trend has led not only to an overall warming in temperature but also the melting of ice caps and shifts in ocean currents that have caused huge devastating tropical storms. If the warming we are experiencing continues, the remaining ice sheets could melt a lot sooner than predicted which would cause sea levels to rise and a shift in ocean currents, ultimately putting many of our ocean front communities under water. With the extensive amounts of CO2 and other greenhouse gases being pumped into our atmosphere and the immense global warming we are experiencing today, we need to look to history to make sure we understand the consequences of the current transformation of our climate.
Dawson A.G. 1992. Ice Age Earth. Routledge
Available from: http://museum.state.il.us/exhibits/ice_ages/why_4_cool_periods.html. Accessed 2008 Mar 19.
Available from: http://www.nasa.gov/. Accessed 2008 Mar 20.
Available from: http://www.ncdc.noaa.gov/paleo/globalwarming/what.html#greenhouse. Accessed 2008 Mar 20
Available from: http://www.museum.state.il.us/exhibits/ice_ages/why_glaciations1.html. Accessed 2008 Mar 19
Available from: http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_2.html. Accessed 2008 Mar 20.
Wright H. 1993. Global Climates Since the Last Glacial Maximum. Minnesota: U of Minnesota Press.
Available from: http://www.eoearth.org/article/Albedo. Accessed 2008 Mar 19.