Analysis of Ice Cores: Re-creation and Extrapolation of Carbon Dioxide Concentrations

Although past and present changes in climate can be related and traced back to several different sources, scientists wishing to determine the past and present state of the climate, as well as future estimates of climate change, often analyze the amount, or concentration, of carbon dioxide (CO2) present in our planet’s atmosphere. Current concentrations are relatively easy to measure, requiring only a sample of the air that surrounds us everyday. To obtain older concentrations from the past, however, scientists must be able to analyze previous states of the atmosphere through pristine air samples located in ice cores. By studying air trapped in these cores, an accurate picture of past CO2 levels can be reconstructed and compared to current levels, helping to establish a link between atmospheric CO2 variance and climate change.

The trapping of air within an ice sheet is a relatively slow process that occurs when air is allowed to circulate between the layers of snow which accumulate at the high elevations of an ice sheet, locations known as ice domes. As additional snow accumulates on the ice dome annually, the underlying layers from previous years are gradually buried and compressed, slowly shutting off the flow of air to subsurface layers. Increased pressure from the weight of the overlying layers eventually turns the snow into layers of ice, and at about 50 meters (165 feet) air from the outside atmosphere is no longer able to circulate among these layers. This lack of circulation, combined with increased compression, causes air which had been slowly diffusing to subsurface layers to be sealed off and trapped as small bubbles in a process known as sintering. The small amount of air contained within each bubble is never again exposed to the outside environment, thus forming a permanent record of the atmosphere at the time in which sintering occurred. Over time, each layer of ice and its associated air bubbles are buried deeper and deeper within the sheet. Scientists are then able to “reclaim” the air for study by drilling from the highest point of the ice dome and extracting a core of ice which can contain numerous annual layers of ice, some dating back hundreds of thousands of years. Sintered air within each layer can then be extracted and concentrations of key atmospheric constituents, such as CO2, can be measured and determined. By studying such concentrations, scientists can re-create past climatic aspects, such as global temperature, while determining the impact that natural and human factors have had on global climate.

Repeated evaluation of various ice cores shows, with high confidence, that CO2 levels reached a concentration of 280 parts per million (ppm) following the last glacial maximum and remained at this level for thousands of years. Around the turn of the 19th Century, however, concentrations began an accelerated increase, reaching 365 ppm by the end of the millennium, with nearly 70% of this increase occurring after 1950. This phenomenon, which has been labeled as the anthropogenic CO2 increase, can be linked to two major sources: the burning of fossil fuels and clearing of forested land. In this manner, carbon dioxide is being added to the atmosphere at a much greater rate than occurs naturally, causing an increase in global temperature and changing of global climate through the enhanced greenhouse effect.

In this activity, students will examine two different carbon dioxide sources: recent measurements from air samples collected at the Mauna Loa Observatory in Hawaii and older concentrations from an actual ice core drilled in 1975 at the Law Ice Dome in Antarctica. Analysis of both types of values will allow students to re-create concentrations of CO2 since 1700, determine the rate at which CO2 concentrations have changed since the 18th Century, and estimate future concentrations.

CO2 Activity Questions

1. Using the Law Dome ice core record, determine the average CO2 concentration (in ppm) for each century and compare these values. What can you infer from these averages?

2. Using Microsoft Excel, plot a graph of the Law Dome CO2 concentration versus time, treating each century as a separate series. What trends do you notice?

3. Use your graph to determine, on average, the amount that CO2 concentrations increased during each century. Are there any noticeable trends in these averages?

4. Consider the more recent CO2 data collected at the Mauna Loa Observatory in Hawaii. Construct a graph of the actual measured concentration versus time. What trends are noticeable with this data? Are these similar to those from the Law Dome data?

5. Make another plot of the average annual concentration at the Mauna Loa site from the data provided in the gray inset. Combine this data with the CO2 concentrations collected from the Law Dome by placing each set of data on the same graph.  Do these measurements reasonably align? What does this tell us about the accuracy of each collection method?

6. Based on the rate of increase shown in the Law Dome record from 1900-1975, what should CO2 levels be in 2007? How do these levels compare with the actual level measured at Mauna Loa? What does this tell you about the rate of increase of CO2 concentrations?

7. Based on your graphs and calculations, will CO2 concentrations continue to rise in the future? Briefly explain why or why not.

CO2 Activity Key

1. Average concentrations are: 

1700s—278.08 ppm
1800s—286.75 ppm
1900s—309.77 ppm

There is a dramatic increase in CO2 concentration averages from century to century, especially from the 19th to 20th Centuries. On average, concentrations increased by about 8 ppm from the 1700s to the 1800s, and by about 23 ppm from the 1800s to 1900s. This implies that a significant increase in concentrations occurred over this 100 year period.

2. Students should recognize a fairly consistent trend in CO2 concentrations in the 1700s and early 1800s before levels begin to drastically increase around 1850 (shortly after the industrial revolution). Concentrations then continue rising at a relatively rapid rate through the 20th Century.

3. Average rates of increase, determined from applying a linear trendline to the graph, are:       
1700s—0.051 ppm/year
1800s—0.113 ppm/year
1900s—0.390 ppm/year

The average rate of CO2 increase varies significantly between each century, from 0.051 ppm in the 18th Century to 0.390 ppm in the 20th Century. This increase suggests that the rate of CO2 release to the atmosphere has risen.

4. Students should notice that although some variance exists, the Mauna Loa plot shows a continual rise in CO2 concentrations since the mid-20th Century. This rise in concentrations is similar to the Law Dome trend, which shows an increase in concentrations during the period leading up to the time in which the first observations were conducted at Mauna Loa.

5. Both sets of data are extremely similar and, therefore, show very good alignment over the time period in which they overlap. This coincidence of values implies that each set of data has a certain degree of accuracy associated with its collection and analysis methods, and that each “paint” an accurate picture of past and current CO2 concentrations.

6. If CO2 concentrations had continued to increase at an average rate of 0.389 ppm, a value of 341.8 ppm would be expected for 2007. However, this extrapolated value is about 42 ppm lower than the actual value measured at Mauna Loa, suggesting the rate of CO2 release to the atmosphere has increased since 1975.

7. Concentrations will continue to increase in the future due to the rising rate at which CO2 is being added to the atmosphere. This upward trend can be seen graphically in the exponential increase shown in the mid and late 20th Century and by comparing both the average concentration and average rate of increase values for each century.