As we try to unravel Earths history we look for signs in the landscape that provide indications of former episodes of extreme climates. In North America we recognize evidence of a recent ice age and wonder if it has ended or if we are just in a brief warm interlude before another cold interval. We have two learning objectives for this lesson. First to discuss the characteristics of ice ages and second to consider how the changing extent of glaciers and ice sheets is influenced by other components of the Earth system. We can identify several ice ages during the last billion years of Earths history. During these times thick glaciers and ice sheets covered large regions of Earth. Extensive glacial deposits provide evidence of massive ice sheets that may have extended almost to the equator near the end of the Proterozoic era creating a condition known as Snowball Earth.
Ice ages in the first half of the Phanerozoic era indicate times of cooler temperatures and lower greenhouse gas concentrations The most recent ice age, the Pliocene-Quaternary, began less than 3 million years ago and many scientist interpret data from this event to suggest that we are technically still in this ice age. During its maximum extent, a sheet of ice a couple of miles thick would have advanced southward out of Canada before retreating again as climate warmed. So why do we get ice ages? Ice ages last for millions of years and are generally related to the relative position of continents and oceans. As you might expect locating continents over a Pole allows ice to build up and may result in an ice age. This was the case for the events of the Karoo ice age when several pulses of glaciation occurred over a span of about 100 million years in the Paleozoic Era.
However, 3 million years ago the distribution of continents was not much different than today. With one significant exception, water was flowing freely between the Atlantic and Pacific Oceans through a narrow gap that separated North and South America. That gap soon closed and circulation patterns in the Atlantic Ocean changed leading to more humid air carried further north and resulting in an increase in precipitation and the development of ice sheets in northern latitudes. Contrary to what many people think, ice ages are not continuous times of extremely low temperature. For example, examine this graph to see how temperature varied over 450,000 years This includes the most recent part of the Pliocene Quaternary ice age As you can see an ice age is actually characterized by a series of shorter climate cycles composed of alternating pulses of warmer and colder temperatures. We can observe a series of longer cold intervals known as glacials that are interrupted by shorter warm intervals known as interglacials.
These alternating cold and warm intervals reflect the advance and retreat of glaciers. The interglacials last for 10,000 to 20,000 years. We may actually be in an interglacial now. Or maybe the ice age has ended. We'll just have to wait about 10,000 years to find out. If you look more closely at the graph you can see that even within cold glacials there are significant fluctuations of temperature resulting in colder or warmer intervals. All of these changes can be explained by small changes in the shape of the Earth orbit and/or the inclination of Earth's axis. Temperatures in the polar regions become colder if Earth's orbit takes it farther from the sun during winter. and if there is an increase in the tilt of Earths axis. Over the course of tens of thousands of years these types of changes have the potential to drastically change Earth's climate. When they all align in just the right way they can lead to substantial warming or cooling trends to generate glacials and interglacial cycles as well as shorter more frequent cycles within these intervals.
Okay, so now we know that several long lasting ice ages have been triggered in Earth's history due to changes in the relative positions of continents and oceans and that climate cycles through warm and cold intervals during these events. but what causes an ice age to be sustained or to end. We'll consider the factors to help sustain an ice age to be positive feedbacks and processes that lead to the end of an ice age as negative feedbacks. Perhaps the most significant positive feedback arises from the ice itself. Ice and snow make very reflective surfaces, the term albedo is used to describe the reflectivity of a surface. Bright, shiny or light colored surfaces have high albedo values and will reflect solar radiation. Reflection results in less solar energy being absorbed at the Earths surface and produces a cooling effect.
In contrast, dark surfaces like forests or oceans have low albedos and will absorb solar radiation and become warmer. Positive feedbacks in the climate system would result in larger ice sheets. Ice sheets reflect solar energy causing less to be absorbed and leading to a decrease in temperatures, and thus sustaining the growth of the ice sheets and prolonging the ice age. Further, the growth of ice sheets would diminish the area of the globe covered in by dark ocean waters and forests. Further reducing the solar energy capable of being absorbed. In contrast negative feedbacks represent processes that result in an increase in temperature that would encourage melting of the ice sheets and the expansion of the oceans and forests. This might result from gradual changes in the distribution of continents and oceans or from an increase in atmospheric carbon dioxide concentrations.
Carbon dioxide and other greenhouse gases would trap heat close to Earth's surface, accelerating the collapse of ice sheets. Such an influx of gases would result from volcanic eruptions or from the decrease in photosynthetic activity due to cooler temperatures. So for today we had two learning objectives How confident are you that you could successfully complete both these tasks?.