Ice Ages and the Greenhouse effect

The Earths most recent cooling period

I had to go back nearly 60 million years to research the beginning of our most recent cooling period. Between 52 and 57 million years ago, the Earth was relatively warm. Tropical conditions actually extended all the way into the mid-latitudes (northern Spain and central United States), polar regions experienced temperate climates and the difference between the polar and equator in terms of temperature was much less than it is today. It was so warm that trees were able to grow in both the Arctic and the Antarctic. But this warm period, known as the Eocene (See Figure below) was soon followed by a cooling trend.

Between 52 and 36 million years ago, ice caps developed in East Antarctica and spread towards sea level. The water temperature began to drop to between 5 and 8 degrees Celsius. Between 36 and 20 million years ago a continental-scale temperate ice sheet developed in East Antarctica. In North America the annual air temperature dropped by approximately 12 degrees Celsius.

By 7 million years ago southeastern Greenland was completely covered with glaciers, and by 5 million years ago these glaciers were creeping into Scandinavia and the northern Pacific region. Between 3 and 5 million years ago, the Earth began warming up and the Oceans around Antarctica and Greenland were much warmer than even today.

However, this interglaciation was soon over and 18,000 years ago marked the most recent coldest period of our Earths history as ice caps again covered the polar regions and extended through the mid-latitude North American continent and through Europe.

What are the causes of the last 60 million years of cooling?

Well, there are many suggestions and perceptions on the issue, including: - a worldwide climate change - the placement of the continents - the greenhouse affect - changes in the earths orbit - and changes in the Suns energy among others. I am going to concentrate on The Greenhouse Effect and how it possibly caused our last Ice Age.

Changes in the concentration of carbon dioxide in the atmosphere is a strong candidate to explain why the overall climate of Earth changes. As you are aware, carbon dioxide influences the mean global temperature through the greenhouse effect. Solar radiation entering the Earths atmosphere is predominately short wave, while the heat radiating form the earth is long wave. As we have learned in class, water vapor, methane, carbon dioxide, along with traces of other gases in our atmosphere absorb this long wave radiation. Because the atmosphere of the earth doesnt allow this long wave radiation to leave, the solar energy is then trapped and forces the temperature of the earth to increase. If not for the presence of our atmosphere, earths temperatures would be well below the freezing point for water.

Throughout a million year period, the average amount of carbon dioxide in our atmosphere is affected by four primary fluxes. Flux of carbon dioxide due to (a) metamorphic degassing, (b) weathering of organic carbon, ( c) weathering of silicates, and (d) burial of organic carbon. The degassing reactions as in volcanic activity and the combing of organic carbon with oxygen release carbon dioxide into the atmosphere. While the burial of organic carbon found in soils and mud from swamps removes the carbon dioxide from our atmosphere. This organic carbon is buried before it is able to decay.

Plate tectonics disrupt these carbon fluxes as well, and in a variety of ways. Some tending to elevate and others tending to lower the atmospheric carbon dioxide level. Some experts believe that the early warm trend 55 million years ago was caused by elevated atmospheric carbon dioxide and that a subsequent decrease in atmospheric carbon dioxide led to the cooling trend over the past 52 million years. One theory proposed as the cause of this decrease in carbon dioxide levels in our atmosphere is that mountain uplift lead to enhance weathering of silicate rocks, and thus the removal of carbon dioxide from the atmosphere.
 

Why are we so sure that the composition of the atmosphere influences climate?

The most convincing evidence comes from the glacial caps themselves. In Antarctica and in Greenland researchers have drilled deep holes into the ice and extracted cores of ice that are several thousand meters long. Glacial ice accumulates just as sedimentary rock does- with the oldest ice on the bottom and growing younger as you move up the extraction. The ice forms in layers because there is a lot of snow fall in winter and hardly any in the summer months. These annual layers are easily distinguishable in the ice core and can be counted to determine any part of the core. The Vostok site contains records that extend 160,000 years into the past.

These ice cores preserve several kinds of information to study the paleoclimate. The most important being the small air bubbles that were trapped in the glacier as the snow fell and became compressed to form ice. These bubbles are actual samples of ancient air and can tell us the composition of the atmosphere at that time.

Another form of information that these cores tell us is derived from the oxygen atoms that form the H20 molecules in the ice. Some oxygen molecules are heavier than others, from this you can infer the temperature of the atmosphere during this time period. If the temperature was colder, then a larger ratio of light oxygens were present. If the air was warmer, there are proportionally more heavy oxygens.
 

Physical Evidence of Ice Ages = Glaciers

Modern glaciers can be used as a guide in studying Ice Ages. They can demonstrate how the piled-up snow changed to sandlike grains near the surface and to solid ice below. The ice became about 150 feet thick and pushed out at the edges. This creeping ice rubbed away small hills and carried the hills gravel, sand, and clay into the valleys and created deposits called glacial tills. Streams from the melting edges of the glaciers scattered sand and gravel in long, crooked mounds called eskers. The deposits that formed under the ice, today appear as small hills called drumlins.

During these flows, ice carried boulders and soil southward until it reached a climate warm enough to melt it completely each summer. As it melted, it left a heap of rubbish, called a moraine, along the line of melting. Today these can be seen as gently sloping mounds across the landscape. Whenever this ice retreated (melted), the water from the melting ice had to find new channels, because the old river valleys were filled with deposits. Large amounts of the flow were caught behind the moraines, and the waters spread out to form lakes. The levels of the lakes rose and overflowed at low places, and as this happened they often joined one another in long chains connected by small streams. The lakes formed from this melting range in sizes from small pools to the Great Lakes.