Ice Cores

Introduction

Ice cores are long cylinders of ice drilled from glaciers and ice sheets. They provide a detailed picture of how global and regional climates and levels of certain pollutants have changed over long periods of time, in some cases up to 800,000 years before the present. When snow falls, it captures samples of the atmosphere, along with its current concentrations of dust, ions, and important global climate forcing agents such as the greenhouse gases carbon dioxide (CO₂) and methane. At high latitudes and altitudes where it is too cold for frequent melting, snow compresses into layer upon layer of ice, preserving these air bubbles for centuries to hundreds of millennia.

The mix of isotopes (atoms of the same element that have different atomic weights) of oxygen and hydrogen in this ice indicates past temperatures and how they varied over long periods of time. Thus, ice holds relatively detailed records of past climate fluctuations—clueing scientists in on changes in local precipitation, temperature, wind strength, moisture, and atmospheric composition. Combined with sea sediment cores and other measures that more roughly indicate climate shifts over the past few million years, ice core research helps provide a context in which to view human-caused climate change.

Historical Background and Scientific Foundations

The most thoroughly studied ice cores, which also hold the longest climate records, come from the ice sheets of Antarctica and Greenland. One of most famous of these is the Vostok ice core, drilled by Soviet engineers in the early 1980s. This revealed that long-term cycles of global warming and cooling brought on by changes in Earth’s orbital parameters have been significantly amplified by changing atmospheric concentrations of CO₂ and methane over the last 160,000 years.

In 1998, the Vostok project yielded one of the deepest ice cores ever recovered, reaching to a depth of 11,886 ft (3,623 m). It confirmed that greenhouse gases rose and fell with temperature over 420,000 years and over four regular, roughly 100,000-year natural cycles between cold glacial periods known as ice ages and warmer periods known as interglacials. An Antarctic ice core drilled in the area known as Dome C in 2004 extended the record back over 740,000 years, including 7 glacial-interglacial cycles, and confirmed the Vostok data.

Although ice cores demonstrate that long-term warming and cooling of Earth’s climate, coupled with increases and decreases of greenhouse gases, are natural phenomena, they also show that current levels of atmospheric CO₂ (about 360 parts per million) and methane (about 1,700 parts per billion) are unprecedented over the last several hundred thousand years. These inflated greenhouse-gas concentrations—largely boosted by human alterations of the landscape and the burning of fossil fuels—may thus lead to a warming that is unprecedented in ice core records.

Impacts and Issues

Ice records from Greenland, where scientists have been drilling cores since the early 1960s, suggest that global climate has occasionally shifted abruptly within the framework of 100,000-year glacial-interglacial cycles. A Greenland ice core completed in 1992 showed that the first 8,000 years of the Eemian Period—an unusually warm interglacial cycle that ended 114,000 years ago—were characterized by wild swings in temperature, likely spurred by heat-driven changes in air and ocean currents such as the North Atlantic current and the global ther-mohaline. These temperature swings, with drops and spikes spanning up to 18°F (10°C), occurred over time periods as short as 10 to 30 years. Because the Eemian was just a few degrees warmer than our current climate, scientists believe its enormous instability may hint at conditions under our warmed future climate if human emissions of greenhouse gases go unchecked.

Meanwhile, ice cores from ice sheets and stable glaciers at mid- to low-latitudes, where most people live, provide evidence that certain heavily populated regional climates are already warming. These cores also provide context for this change, showing how regional climates have fluctuated over the centuries before humans began keeping relevant records. For example, ice cores from the Upper Fremont Glacier, located 10,170 ft (3,100 m) above sea-level in Wyoming’s Wind River mountain range, reveal that colder temperatures dominated the region from the mid-1700s to the mid-1800s during a relatively short cooling period known as the Little Ice Age. After that, ice cores show that the region’s temperature climbed abruptly. Ice cores taken from glaciers on the Tibetan Plateau in Central Asia, meanwhile, show persistent warming since 1800, with the warmest temperatures occurring after 1950. They also reveal temperature variations related to short-term natural climate cycles.

Scientists are concerned that many of these important mid- and low-latitude ice records may be lost within the next century, as they are vulnerable to melting. Indeed, many glaciers and ice sheets outside Antarctica and Greenland have been in a state of rapid retreat since the mid-1800s.

WORDS TO KNOW

FOSSIL FUELS: Non-renewable fuels formed by biological processes and transformed into solid or fluid minerals over geologic time. Fossil fuels include coal, petroleum, and natural gas.

GREENHOUSE GAS: A gas whose accumulation in the atmosphere increases heat retention.

ICE AGE: Period of glacial advance.

ICE SHEET: Glacial ice that covers at least 19,500 square mi (50,000 square km) of land and that flows in all directions, covering and obscuring the landscape below it.

INTERGLACIAL PERIOD: A geological time period between glacial periods, which are periods when ice masses grow in the polar regions and at high elevations.

SEA SEDIMENT CORE: A cylindrical, solid sample of a layered deposit of sediment on the ocean floor that can provide information about past climate changes.

THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters; also termed meridional overturning circulation.

See Also Climate Change; Global Warming; Greenhouse Gases

IN CONTEXT: ICE CORE STUDIES LINK CO₂ AND GLOBAL WARMING

A 2005 ice-core study by National Aeronautics and Space Administration (NASA) scientists found that atmospheric carbon dioxide was already 27% higher than the highest CO₂ level found during the last 650,000 years. All data confirmed a step-wise correlation between atmospheric carbon dioxide concentrations and the warming of the global climate.

BIBLIOGRAPHY

Books

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Barnola, J. M., et al. “Vostok Ice Core Provides 160,000-year Record of Atmospheric CO₂.” Nature 329 (1987): 408–413.

EPICA Community Members. “Eight Glacial Cycles from an Antarctic Ice Core.” Nature 429 (2004): 623–628.

Etheridge, D. M., et al. “Natural and Anthropogenic Changes in Atmospheric CO2 Over the Last 1,000 Years from Air in Antarctic Ice and Firn.” Journal of Geophysical Research101 (1996): 4115–4128.

Jouzel, J., et al. “Vostok Ice Core: A Continuous Isotope Temperature Record Over the Last Climatic Cycle (160,000 years).” Nature 329(1987): 403–408.

Nielsen, Rolf Haugaard. “The Climate Conundrum: Chill Warnings from Greenland.” New Scientist (August 28, 1993).

Thompson, Lonnie G., et al. “Tropical Glacier and Ice Core Evidence of Climate Change on Annual to Millennial Time Scales.” Climate Change 59 (2003): 137-155.

Web Sites

Greenland Ice Sheet Project 2. “Ice Cores that Tell the Past.” http://www.gisp2.sr.unh.edu/MoreInfo/Ice_Cores_Past.html (accessed March 24, 2008).

Sarah Gilman