Ice cores
Ice cores are cylindrical samples of ice, typically around 10 centimeters in diameter, drilled from glaciers, which contain a historical record of Earth's atmosphere. As snow accumulates over time, it compresses into ice, trapping air bubbles that reflect the composition of the atmosphere when the snow fell. This layered structure allows scientists to analyze changes in temperature and trace gases, providing insights into both natural and human-induced atmospheric changes. Ice cores have been instrumental in reconstructing climate history, revealing evidence of volcanic eruptions and pollution dating back thousands of years, including impacts from ancient Roman activities and modern nuclear events.
Notable ice core drilling sites include Vostok in Antarctica, which has provided a record extending back 420,000 years, and Greenland, where ice cores offer a shorter but consistent climate history. Researchers continue to explore deeper ice deposits, such as at Dome C, which has revealed climatic data spanning over 800,000 years. The study of these cores is crucial for understanding the relationship between greenhouse gas concentrations and temperature fluctuations, contributing to ongoing discussions about climate change and pollution. As methodologies improve and longer records are retrieved, ice cores promise to enhance our understanding of Earth's climatic past and inform future sustainability efforts.
Ice cores
Ice cores contain evidence of an orderly sequence of changes that occurred in the Earth’s atmosphere, including changes in temperature and trace gases. They therefore provide a history of the Earth’s climates.
Background
An ice core is a cylinder of ice measuring 10 centimeters in diameter and obtained by drilling vertically into a glacier. Glacial ice is produced by the natural, gradual transformation of snow into ice. Snow is made of fragile ice crystals surrounded by air. In the process that transforms snow into ice, this air is trapped in tiny bubbles that are identical in composition to the atmosphere that produced the snowfall. As snow falls year after year and is compacted into ice, each layer can be identified visually or electronically. In this way, glaciers preserve a record of atmospheric changes that have occurred over time, and ice cores can be drilled to retrieve that record.
Variation in the Atmosphere’s Composition
Because they contain a chronologically ordered record of atmospheric composition, ice cores can be used to reconstruct the history of both natural and anthropogenic pollution. The chemical composition of Earth’s atmosphere varies over time. It reacts to volcanic eruptions, for instance, which add carbon monoxide, carbon dioxide (CO2) and sulfur dioxide to its normal content of nitrogen, oxygen, argon, CO2, and methane. Thus, changes in the chemical composition of the air trapped in ice cores make it possible to date past volcanic eruptions.
In the same way, some human activities can be identified and dated. Analysis of ice cores from the Alps and Greenland has identified the pollution resulting from the smelting of lead ore by the Romans more than two thousand years ago. More recently, traces of chemicals linked to nuclear explosions or to the Chernobyl nuclear reactor meltdown have been detected in glacial ice. In contemporary times, when issues of climate change and pollution are uppermost in the minds of many, glaciologists have sought to retrieve the longest possible ice cores in order to produce a substantial historical record to contextualize current atmospheric trends.
The Longest Ice Cores
The first giant ice core was drilled in Vostok, the Russian base in Antarctica. The region of Antarctica in which Vostok is located is extremely cold and dry. The advantage of low precipitation in glaciology is that, because each year produces less ice, many years are recorded relative to core length. The disadvantage of such low precipitation is that, because each annual layer of ice is extremely thin, it can be very difficult to produce an accurate count. When all analyses were complete, Vostok glaciologists concluded that the 3,623-meter core drilled in 1996 represented the last 420,000 years of the Earth’s history.
The normal analysis of an ice core consists of analyzing the content of the ice for the presence of deuterium (an isotope of hydrogen) and the presence of oxygen 18 (O18; an isotope of oxygen), because both are reliable indicators of the Earth’s temperature at the time of the snowfall. The temperature records obtained thanks to the Vostok ice core compare well with cores obtained in the center of Greenland. Since the Greenland is smaller and thinner (1,600 meters deep on average) than the Antarctic ice sheet (2,400 meters deep on average), the paleoclimatic record derivable from Greenland ice cores is shorter. However, that record is consistent with the Vostok ice core record. Greenland also receives more precipitation than does Vostok, resulting in thicker annual layers of ice; a layer as old as 100,000 years can still be as much as 1 centimeter thick. Greenland ice cores therefore are outstanding at establishing a chronology for the last glacial period for which the estimated error does not exceed 2 percent for the first forty thousand years.
In an effort to increase the length of the paleoclimatic record, scientists from Europe have drilled in two other regions of the East Antarctic Ice Sheet under the European Project for Ice Coring in Antarctic (EPICA). A site on Dome C located at 75°06′ south latitude and 123°21′ east was selected. There, where the ice sheet is thicker while both the ice and the air remain extremely cold, a permanent research base, Concordia, was built by France and Italy (partly financed by the European Union). Dome C is ideal for the thickness of its ice and its low precipitation; it initially provided a 3,140-meter ice core, revealing 740,000 years of climatic history. Since then, even deeper ice has been retrieved, extending knowledge of climatic history to over 800,000 years.
Testing Theories of Climate Change
In order to analyze the ice core samples that have been retrieved, scientists crush the ice in a vacuum, releasing the air trapped within the sample without allowing it to mix with the modern atmosphere. They can then analyze these air samples, compute the proportion of such as CO2 and methane in the air, and compare their proportions to the temperature record derived from the deuterium and O18 analyses. The goal is to see if an increase of GHGs in ice always leads to an increase in the Earth’s temperature or, conversely, if an increase in the Earth’s temperature releases CO2 and methane into the atmosphere.
Context
Scientists continue to search for deeper deposits of ice from which even longer historical sequences can be identified. The Dome C site holds the promise of ice cores that will extend the scientific record to more than one million years into the past. Meanwhile, research also continues on ways to recover more, and more precise, information from ice cores. Analysis of core samples containing volcanic dust taken at Dome C, for instance, has demonstrated that the magnetic polarity of the Earth reversed about 780,000 years ago. In 2021, analysis of European ice cores confirmed a composition that closely corresponded with major events in climate change, such as the Little Ice Age and known volcanic eruptions. As ice cores get longer and scientists find ways to learn more from them, the historical record will become more detailed over longer periods of time.
A 2024 study at Dartmouth College showed that air pollution from the burning of fossil fuels could alter the atmospheric chemistry thousands of miles away. Researchers studying ice cores from Alaska and Greenland discovered the long reach of fossil fuel emissions. They measured the decrease in an airborne by-product of marine phytoplankton activity called methane sulfonic acid (MSA). The researchers believed that using sustainable energy sources instead of fossil fuels could reverse the effect.
Key Concepts
- Dome C: the Antarctic Plateau location of the Concordia Research Station
- glacial ice: ice created by the compression of snow into glaciers
- paleoclimatology: the study of past climates
- Vostok: a Russian Antarctic research station built in 1957 during the First Geophysical Year
Bibliography
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