Temperature and CO2 Correlations Found in Ice Core Records

by Emmanuel Quiroz

Abstract

Antarctic ice core analysis has provided scientists with a history of climatic and atmospheric changes over the past quarter of a million years. A similar rise and fall of temperature and CO2 has been observed in all of the ice core records. Three main CO2 and temperature relations are suggested with supporting evidence, but uncertainties and outside factors need to be taken into account. These ambiguities make it unclear whether CO2 is a forcing factor on climate.

Introduction

In the 1950s, Roger Revelle recorded the carbon dioxide (CO2) concentrations and temperatures in the mid-Pacific to find a relation between the two. After analyzing the data, Revelle suggested that atmospheric CO2 concentrations are a forcing factor on temperature through the greenhouse effect¹, more commonly known as global warming. Al Gore pays tribute to Revelle's research in David Guggenheim's film An Inconvenient Truth (a documentary of Gore's attempt to inform the world about the moral challenge of global warming).² In this film Gore presents a graph of ice core data from the past 600,000 years that reflects Revelle's proposed relationship between CO2 and temperature. But, as Al Gore acknowledges in the film, "The relationship is actually very complicated." There is continuing debate among researchers about whether ice core records show carbon dioxide affecting temperature or vice versa.³ The following review will investigate this relationship by examining recent research on Antarctic ice cores.

Scientist began to drill ice cores in the 1970s in order to construct a record of the atmosphere's temperature and CO2 concentration over the past hundred thousand years. They drill out cylindrical cores of ice from the ice caps and analyze the molecular composition of the ice. When the snow compacts into a layer of ice, tiny air bubbles of the atmosphere are encapsulated in the ice. The air in these atmosphere time capsules allows scientists to see the history of atmospheric CO2 concentrations. Also, the water in the ice contains concentrations of oxygen and hydrogen isotopes that vary in enrichment as a function of temperature.⁴ Therefore, they are able to determine a history of the atmosphere's temperature through this relationship. Each year the annual snowfall compacts into a new layer of ice with the most recent near the surface. Scientists can then date the ice back by its depth. With this information scientists can figure out a correlation between CO2 and temperature over the past hundred thousand years.

The measurements taken from ice cores show that temperature and CO2 increase and decrease in phase. This trend is found in all ice core analysis, but whether the temperature affects CO2 concentration is ambiguous due to potentially flawed estimations and uncertainty in measurements. The following summary reviews the hypothesized relations between temperature and CO2 derived from ice core records and discusses the uncertainties in ice core analysis.

Ice Core History and Records

Ice core drilling in Antarctica began in the 1970s with the Vostok ice core and continued into the 1980s with others such as the Taylor Dome and Byrd ice core research projects. The ice cores have allowed scientist to construct a timeline of the atmospheric temperature and CO2 concentrations dating back 420,000 years.⁵

In 1974, the Vostok camp in east Antarctica drilled down to 950m where the ice dates back 60,000 years. By 1985, drilling reached depths of 2,083m to ice that dates back 160,000 years.⁴ Finally, in 1999, the ice cores were drilled at 3,300m dating back 420,000 years.⁵ Taylor Dome camp, the most recent drilling, reached depths of 554m in 1994 with a higher resolution of data from the past 11,000 years.⁶ With the Vostok, Taylor Dome (referred to now as TD) and other ice core records in Antarctica, researchers are able to study the correlation between temperature and CO2.

Currently there are three hypotheses that describe the relations between CO2 and temperature seen in the Antarctic iced core records. The suggestion that is of much interest in environmental policies and Earth Sciences is that the changes in CO2 concentrations control the observed change in the Antarctic climate by the greenhouse effect.

In the Vostok ice core data, temperature and CO2 concentrations in the Antarctic atmosphere correlate almost in phase throughout the last four climatic cycles.⁵ Temperature is measured in the average annual change/deviation, positive or negative, from a chosen normal temperature, and the CO2 concentrations are measured in parts per million by volume. The first climatic cycle began 420,000 years before the present (BP) with both the temperature change and CO2 concentration at their maximums of 2o C and 280 p.p.m.v. respectively. Then both data reach their minimums around 335,000 years BP with a temperature change of -8 ^oC and a CO2 concentration of 180 p.p.m.v. And again around 320,000 years BP, they both rapidly reached their maximums.

The observed increase and decrease are the same in both temperature and CO2. Both decrease slowly over ~100,000 years until they reach their minimums; then, they rapidly increase to a reoccurring maximum in ~20,000 years. The minimums in temperature and CO2 concentration associate with the glacial maximum (ice age) observed by the volume of ice formed each year⁵. This data suggest that temperature (T) and CO2 both increase and decrease in phase with each other in a ~120,000-year cycle. This cycle has also been observed in the periods analyzed in the Antarctic Taylor Dome ice core records⁶ and in the Arctic V 19-30 core for Uvigerina Senticosa.⁴

CO2 and Temperature Lag

Further analysis of the T data collected at Vostok and Taylor Dome not only shows a trend in temperature and CO2 concentration change over time but also shows a time lag between the times they reach their maximums or minimums. This is valuable evidence to determine whether CO2 is a forcing factor on the change in temperature. In most of the records one can see that CO2 lags temperature, but the measured lag period is insignificant when compared to the large time frame.

In 1999, Hubertus Fischer et al. from Scripps Institution of oceanography compiled the records of the Vostok, TD, and Byrd ice cores and pointed out this lag between CO2 and temperature over the last 270,000 years. ⁷ A glacial termination begins at a temporal minimum and ends at a temporal maximum. In termination III (from 270,000 years BP – 230,000 years BP) CO2 concentrations reached a maximum of over 300 p.p.m.v. 600 (+/-200) years after temperature had peaked at a change of ~2o C. Then again in termination II (160,000 years B.P. - 120,000 years B.P.), CO2 concentrations reach their maximum 400 (+/-200) years later than the recorded temperature peak. Other sources, such as Eric Monnin et al., Callion et al., and Petit et al., all estimate this CO2 lag to be ~800 (+/-200) years after temperature^8,3,5. However, they also give notice that the 800-year lag period is very short and insignificant compared to the 5,000-year period in which the lag occurs. This makes the lag insufficient evidence to rule out CO2 as a forcing factor on climate change.

There is also uncertainty in approximating lag period due to the fact that the air bubbles trapped in the ice cores are younger than the ice that they are taken from. When the layer of ice has formed on the surface it is very porous, allowing younger atmospheric air to diffuse in and out until trapped at a certain depth. The age difference is approximated to be ~1,000 years .⁵An overestimated gas-ice age difference devalues the evidence of CO2 lagging temperature debatable.

Proposed Correlations

Currently there are three hypotheses that describe the relations between CO2 and temperature seen in the Antarctic ice core records. The suggestion that is of much interest in environmental policies and Earth sciences is that the changes in CO2 concentrations control the observed change in the Antarctic climate by the greenhouse effect. Thus CO2 and its greenhouse warming might be involved in the control of the ice ages.⁴ According to Stauffer et al., quoted by Lorius et al., a 40% change in CO2 concentration over the last climatic cycle matches up with a temperature rise of 2-4.5o C. Therefore, it is possible that CO2 is a controlling factor in climate change. Further evidence is found in the Dome C ice core, where an increase in the CO2 concentration towards the end of the last ice age began before or simultaneously with temperature change in the Antarctic. But the lag in temperature changes that would support this hypothesis is not found in the Vostok records. Overall, there is little direct evidence to support this hypothesis beyond the fact the temperature and CO2 concentration rise in synchronicity.

The second hypothesis is that temperature controls the CO2 concentrations by affecting the net diffusion of CO2 between ocean surfaces and atmosphere.⁶ Research on ocean surface temperatures and solubility of CO2 cited in Indermuhle et al. indicates that a change of 1o C in ocean surface temperatures causes a change in the ocean surface's CO2 partial pressure by 4.2%. In other words, as the temperature of the oceans increases, gaseous solubility of the ocean surfaces decreases, causing a net diffusion of CO2 into the atmosphere, or the reverse if temperatures decrease. This affects the observed atmospheric CO2 concentrations. Therefore, an increase in atmospheric temperatures causes the ocean surface temperatures to rise, giving a net increase in CO2 concentrations in the atmosphere. This hypothesis is clearly supported by the CO2 lag that was previously pointed out. The observed lag is due to the amount of time it takes for the atmospheric temperature change to warm the ocean surface and diffuse CO2 into the atmosphere.³

There are many other factors that complicate the change in CO2 concentrations observed in the Antarctic ice core research. CO2 concentrations could be affected by a change in ocean surface solubility of gases.

The third hypothesis is that both CO2 and temperature are controlled by the amount of radiation put out by the sun (insolation).⁹ The changes in insolation over time measured at 80° S latitude and the climate changes in Antarctica show a similar cycle over the past 150,000 years.⁴ The Vostok CO2 concentrations also share a similar trend. One can study the graphs from the CO2, temperature, and insolation data and see that they rise and fall almost in harmony. An increase in the sun's radiation can directly affect the Earth's climate and indirectly increase the atmospheric CO2 concentration by warming the ocean surfaces. This hypothesis is logical since insolation is controlled solely by the sun's activity.

All three of these hypotheses account for experimental data from specific patterns observed in certain ice core records but are unable to give a general summary that takes all of the different patterns into account. Therefore, there is a continuing debate as to which hypothesis holds true.

Other Forcing Factors

There are many other factors that complicate the change in CO2 concentrations observed in the Antarctic ice core records. CO2 concentrations could be affected by a change in ocean surface solubility of gasses. Depending on the ocean surface saturation and chemical makeup, it will act as either a sink or source of atmospheric CO2 .⁶

Living and dead biological material (biomasses) also acts as CO2 sources or sinks that influence the atmospheric CO2 concentration. As vegetation grows, it uses up the CO2 in the air, and when any living matter dies it releases CO2. During the Holocene, a period when CO2 was generally increasing, there was a random drop of ~10 p.p.m.v. CO2.⁷ This drop may be attributable to an increase in terrestrial vegetation growth that consumed CO2 from the atmosphere. Therefore, the net carbon release by biomasses is another factor that affects atmospheric CO2 concentration.

Flaws in Data

In ice core analysis, researchers run into uncertainties and possible errors that obscure the recorded data. Mentioned before, the age of the trapped CO2 is found by the age of the ice it is enclosed in. There is an age difference between the age of the ice and the age of the CO2 but the difference varies by glacial period. Monnin points out in his article that in the Holocene the age difference is ~2,000 years and in the Last Glacial Maximum (20,000 years BP) the age difference is ~5,500 years.⁸ The estimated uncertainty in ice-gas age is ~10%. He also points out that the uncertainty of the age of the ice increases in older ice cores. The estimated uncertainty of ice is +/-200 years around 10,000 years BP and +/-2000 years around 41,000 years BP. Older ice has a lower data resolution.

In analyzing ice cores, both Monnin et al. and Indermuhle et al. report that chemical impurities in the ice need to be taken into account when measuring CO2 concentrations. High resolution Taylor Dome ice core measurements show that CO2 produced from chemical reactions between these impurities masks the actual atmospheric CO2 concentrations.⁸

Another flaw is noted by Veizer et al. reporting that it is unclear whether the relationships between CO2 and temperature found in ice cores reflect a global or local phenomenon.¹¹ Veizer found that oxygen isotopes measured in calcite and aragonite shells show oscillations of tropical sea surface temperatures in phase with the ice core climate records from Antarctica, thus providing evidence to support the idea of climate variability as a global phenomenon. But, this data is at odds with temperature models that depend on CO2 as a forcing factor. Therefore, whether climate change is global or local depends on whether CO2 is a forcing factor on temperature. This further complicates any hypothesis taken from ice core analysis.

Discussion

Records taken from ice core analysis in the Antarctic have provided scientists with a history of the Earth's past climatic and atmospheric conditions. Ice core records show CO2 and temperature rising and falling in unison. This correlation is evident in all ice core analysis, but an 800 (+/-200) year time lag between CO2 and temperature is observed in some of the records but not all. And within these specific records the lag is only found during certain periods of ~5,000 years. The CO2 lag supports a hypothesized relation between temperature and the ocean's net release or intake of atmospheric CO2. The opposing hypothesis is that CO2 is a forcing factor on temperature and is proposed by many scientists but has little numerical evidence from ice core records. The third proposal is that the Sun's radiation has an effect on both temperature and CO2. These hypotheses all seem logical and have supporting evidence, but uncertainties in ice core analysis and other contributing factors complicate any relations observed in Antarctic ice core records. Scientists are finding better methods of measuring CO2 and temperature in ice cores by using more reliable isotopes. Also, the Taylor Dome ice core is currently providing a more accurate record. The Antarctic ice cores have provided a large amount of evidence but the relation between temperature and CO2 remains unclear.

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Writing this Essay

I was inspired to write this piece after I watched An Inconvenient Truth for a critical analysis assignment in my writing class. When Al Gore stated that global warming has been proven in ice core research I decided to do my review article, the next project in the class, on ice core analysis so that I could educate myself and others on how ice core research proves global warming is correct.

I would like to give a special thanks to my Introduction to Scientific and Technical Comunication instructor, Cynthia Taft. She was extremely helpful in guiding me with research and constructive feedback on my writing. Writing this essay was a stressful struggle but Ms. Taft helped me get through hard times of writer's block in order to complete my essay.

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