Ice Cores
Ice cores reveal information about the earth's climate history. The information from ice cores is both more precise and more compelling than from other sources. Accurate history of our earth's temperature and carbon dioxide levels can be obtained from the ice sheets of Antarctica and Greenland.
Drilling into the ice sheets on Greenland and Antarctica has revealed much about the history of our climate and atmosphere. Ice cores more than 2 kilometers long reach back 160,000 years into the earth's history. By examining the water and the trapped gases, scientists have discovered there have been temperature changes and atmospheric changes in the past that could account for the ice ages. These cores go back two ice ages ago and show that those ice ages lasted about 100,000 years and the interglacial periods (which we are now in) lasted about 10,000 years. They also show that the temperature and the ice ages go hand in hand with levels of greenhouse gases in the atmosphere--but they don't show which causes which. Do the greenhouse gases cause the ice ages or could the ice ages affect the level of greenhouse gases?
How Do Ice Cores Reveal Information About Our Past?
Temperatures
Information about temperatures can be determined by examining the type of water in the ice. Water can exist as normal water, or it can contain the isotopes, heavy oxygen (18O) or heavy hydrogen (2H). Normal water, because its vapor pressure differs and it has lighter mass, separates from heavy water (water that contains one of the isotopes of oxygen or hydrogen) through processes of evaporation or snow formation. This separating is called fractionation. Taking data from current snowfalls, an equation of temperature can be derived looking at this amount of fractionation. So scientists look at the fractionation that occurred in the snowfalls that created the ice in the past and they can determine what the temperature was when the snow fell! Pretty neat, huh! This creates a record of the temperatures through the years.
Carbon Dioxide
The makeup of ancient atmosphere can be determined by directly examining air that has been trapped in continental ice sheets. As snow falls and gradually turns into ice, air bubbles from the atmosphere-at-that-time become trapped and remain unaltered in the ice. To examine this air, ice cores can be removed from the ice pack. Ice must be removed carefully enough to prevent this ancient air from being contaminated with present day air.
The ice core taken in 1982 from the Vostock Research station in Antarctica is 2200-mile long and contains ice that dates back over 165,000 years ago. More recent ice cores from Greenland contain ice that dates back 250,000 years. From these cores, scientists have discovered that there is a close link between carbon dioxide levels and climate change. Atmospheric samples show two periods when carbon dioxide levels have risen above 250 ppm. Both of those correspond to interglacial periods where surface temperatures warmed to present day levels. The ice cores also reveal that during the last two ice ages carbon dioxide levels were between 180 and 240 ppm. In fact, the lowest carbon dioxide levels found in the ice cores (below 200 ppm) correspond with the lowest temperatures over the last 160,000 years.
Ice cores have also shown that trace gases have varied quite a bit in the past. It appears there is a link between these variations and the amount of solar radiation entering our atmosphere. Therefore, natural solar cycles may determine the concentration of trace gases, which will have an affect on global temperatures. So as we get more information from analyzing ice cores, we will get a clearer picture of how the atmosphere has changed and how these changes affect our global climate.
Additional Ice Core Information
NOVA OnlineNOVA Online - Cracking the Ice Age. Background information on Ice and Global Warming
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