Expedition for an Ice Core
What the ice gets, the ice keeps," said polar explorer Ernest Shackleton in July 1915. Shackleton himself was almost a keeper: He was moored for 15 months on an ice floe in Antarctica's blizzard-ridden Weddell Sea. His ship was en route to the continent, which Shackleton intended to cross via dogsled, when pack ice enveloped it. Pressure from the merging ice blocks crushed the ship, yet Shackleton and his entire 28-person crew survived. It is one of history's most harrowing and inspiring tales of endurance and rescue.
Glaciologists, our modern-day ice explorers, voyage to the globe’s frostiest, most unforgiving locales to retrieve what glaciers keep in their coffers--compacted snow and trapped air from ancient atmospheres, ash from long-quiet volcanoes, dust, insects, and pollen. By analyzing these relics, scientists can reconstruct hundreds of thousands of years of Earth’s climate. This frozen-in-time history is crucial in understanding how the climate system works today and how it may change.
“It is cold, it is windy, it is dangerous, it is an extraordinarily awkward environment to work in,” gasps Keith Mountain cheerfully as he stands atop the oxygen-poor Quelccaya ice cap 5,760 meters high in Peru’s southern Andes. “With such a bright surface, the possibility of radiation burns is extraordinary. Within a short time you can be in blizzard conditions, where you can’t see in front of you to take a measurement.” He steadies a hand-operated ice core drill with expedition-weight mittens. The drill is ready to bite into the 168-meter-thick frigid flooring that has existed on this peak for tens of thousands of years. With temperatures regularly below freezing, high-altitude spots like the Andes and high-latitude land such as Antarctica and Greenland preserve snowflakes. As the snow is compressed by the layers accumulating above, it gradually hardens atop the bedrock into a thick coating of glacier ice. It’s called an ice cap if it radially coats a peak, or an ice sheet if it spans a significant part of a continent and has submerged the underlying topography.
“You face a challenge between how long you can sustain in these conditions versus what you’re trying to get in terms of science,” says Mountain. The University of Louisville geoscientist is trying to extract one-meter lengths of a vertical cylinder of stratified ice 11 centimeters in diameter and several meters long. The coreand the six-day expeditionare short ones for Mountain and his colleague Lonnie Thompson, a glaciologist from Ohio State University. Thompson pioneered tropical ice coring and has participated in 47 major expeditions in 15 countries since 1974. The short core updates the 168-meter core pulled from Quelccaya’s summit in 2003, which required hauling six tons of equipment, including a solar-powered automatic drill, up the peak for a one-month stay.
After drilling is complete, local porters sled the cores to the edge of the ice cap, pack them in insulated boxes, and strap them to horses, which tote them 30 kilometers to a freezer truck waiting at the closest access road. The cores are driven across the Andes to Lima, sit in cold storage until approved by customs, and are air-cargoed to “30 degree C freezers at Ohio State University to await analysis.
Ghosts of Climates Past
Scientists compile data from ice cores around the globe to chart Earth’s overall temperature, precipitation, atmospheric composition, and climate patterns for hundreds of thousands of years. Quelccaya’s complete core provides a high-resolution “time capsule” back to 200 B.C. A new, lower-resolution 3,270-meter core pulled from Antarctica last December may extend the ice-core record back to 900,000 years. (To learn more about this core, click here.)
Ice cores reveal:
Timelines: In most tropical ice cores, the boundaries between layers of annual or seasonal snow are visible enough to count the core’s age. By analyzing the climatic signatures of each layer, researchers can date changes and events precisely. Bands of particles deposited from ancient volcanic eruptions of known age help delineate years or seasons. Frozen insects and other organic material swept into the snowfall by wind are carbon-dated, which can confirm dates obtained by counting layers.
Precipitation: Snow is compressed as it builds up into glacier ice. Thus, newer layers tend to be thicker, while older, deeper layers are thinned. After core depth, accumulation rates, ice temperatures, and ice flow are taken into account, the layer’s thickness roughly corresponds to how much snow fell at the coring site that year or season.
Atmosphere: As snow turns to ice, it traps bubbles of air. The air and frozen water provide chemical records of the composition of Earth’s atmosphere at different times. ”We can tell when lead was put into gasoline. We can see when legislation was passed to take the lead out of gasoline,” says Thompson. Polar ice cores have revealed preindustrial atmospheric concentrations of greenhouse gasescarbon dioxide, methane, nitrous oxide, and other gases that absorb infrared radiation emitted by the Earth’s surface, and trap heat in the atmosphere.
Temperature: By analyzing the oxygen atoms in the frozen water of an ice core, scientists can determine the average temperature of the air above the core site for that period. The technique is based on the fact that not all oxygen atoms have the same number of neutrons. These variations, which differ in mass but not chemical behavior, are called isotopes. Oxygen has three isotopes: oxygen 16, oxygen 17, and oxygen 18.
Here's how the technique works: As water vapor in a cloud condenses to form precipitation, slightly more oxygen 18 goes into the precipitation, leaving the remaining vapor enriched in oxygen 16. During glacial periods, the colder temperature of the atmosphere means that the moisture-laden air will lose more water to precipitation before it arrives at the glacier site. Thus the oxygen 18 to oxygen 16 ratio will be less in the snow deposited on the glacier during colder periods, providing a proxy for paleotemperature.
In Quelccaya cores, says Thompson, “you can see the medieval warm period when the Vikings settled in Greenland. You can see the onset of the Little Ice Age in the early 1500’s that contributed to the demise of the Vikings. And you can see the warming in the 20th century.”
Our Warming World
Ice core records, when combined with data from other paleoclimate proxies such as ocean sediment cores and tree rings, reveal that our climate has drastically changed many times over the last 100,000 years. It has switched from glacial periods, when ice sheets blanketed nonpolar areas, to warmer interglacial periods, and back again. These switches occurred slowly (taking hundreds of years) or abruptly (in as little as a decade).
During the last 100 years, global surface temperature has risen between 0.5 °C and 0.8 °C. This rise may have resulted from increases in greenhouse gas concentrations caused by increased burning of oil, gas, coal, and vegetation. Global warming has possible worrisome consequences, among them melting glaciers, an associated rise in sea level, regional drought, and more severe climate events. Most worrisome, however, is the possibility of changes that we cannot foresee with our current understanding of how the climate system works.
Thompson notes that the real challenge glaciologists face is not the tough expeditions, but ensuring that the information they obtain is used effectively. “If all we do is collect these records, we're just historians,” he says. “If we cannot take that data and bring about meaningful policy changes that make for a better world for the future, in some ways I think we may be wasting our time.”
AMNH's The Endurance: Shackleton's Legendary Antarctic Expedition
The accompanying website to AMNH's 1999 exhibit on Shackleton's ordeal.
Ice Man: Lonnie Thompson Scales the Peaks for Science
Learn more about Thompson's work as a scientist-explorer.
EPICA: European Project for Ice Core Drilling in Antarctica
Explore the science team that drilled the world's oldest ice core.
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Supplement a study of geology with a classroom activity drawn from this Science Bulletin essay.
- Ask students what they know about ice cores. What types of things are trapped in these ice timelines? What can they tell us about Earth's past?
- Have students read the essay (either online or a printed copy).
- As a class, discuss what students learned. Were they surprised to learn that ice cores reveal climate change? Why or why not?