LAYERS UPON LAYERS

Glaciers and ice sheets form on land when snow builds year after year, and where the amount of snow that accumulates in a year is greater than the amount of snow that melts or moves downslope. The snow that started out as a fluffy covering begins to compact under its own weight and the weight of overlying new snow. Eventually the snow compacts to the point where it becomes ice, and the accumulated weight causes the ice to flow. Continent-sized bodies of this slow-moving ice, called ice sheets, are found in Antarctica and Greenland.

Ice sheets are so large that they flow outward in all directions from the center, and flow faster in areas where there is less resistance (downslope and between mountain ranges). Smaller bodies of moving ice, though still quite large to the human eye, are known as glaciers. Glaciers may be part of an ice sheet where it is moving through a passage or they can be moving ice in an ice field at the top of or between high mountains.

The snow that creates glaciers and ice sheets falls in seasonal cycles, which are apparent as layers in the ice. A thick layer of snow is added during the wet, snowy season; a thinner (sometimes dusty) layer is added during the dry season. Glaciers tend to have approximately 1,000 to 10,000 layers, while parts of the Antarctic ice sheet have up to 800,000 annual layers.

Annual layers in Peru’s Quelccaya glacier in 1977. The thick layers show the wet season while thinner layers show the dry season. Photo courtesy of Lonnie Thompson.

OBTAINING ICE CORES

Ice cores are cylinders of ice that are drilled from the surface of a glacier or an ice sheet down to the bedrock underneath. By drilling through the entire glacier, scientists obtain ice from every layer, allowing them to learn more about the conditions every year at that location. For this reason, they choose areas where the ice is not moving very much. The oldest ice is at the bottom of the glacier. It is important to drill down to the contact between the ice and the bedrock in order to obtain a continuous record to the oldest ice possible at that site.

A lightweight, portable drill recovering ice cores from the Greenland ice sheet. Photo courtesy of Natalie Kehrwald.

Obtaining ice cores is an adventure! Scientists must travel to the coldest and most remote places in the world to collect them. Scientists live for weeks or months at places where it is below freezing all the time. They have to melt snow to have liquid water for drinking and cooking. When drilling ice cores in Antarctica or Greenland, they are flown to the middle of the ice sheet. The planes land on the ice, dropping off the scientists and all of their necessary equipment, food, and other supplies. Coring an ice sheet takes a few years. Camps that can be re-used during the next summer drilling season are set up.

A plane arriving to pick up ice core scientists on the Greenland ice sheet. Photo courtesy of Natalie Kehrwald.

Once the plane takes off, research teams can look around in all directions and see nothing but a flat, white, windy plain. Temperatures range from 20 degrees Fahrenheit to minus 50 degrees Fahrenheit, and winds can reach up to 100 miles per hour. Drilling camps tend to have between 5 to 50 people depending on the type of project. The scientists live in tents. Sometimes they cook and eat in snow caves that they have hollowed out to provide shelter from the wind.

Natalie and Lonnie Thompson enjoy a hot meal in the mess tent. Photo courtesy of Carol Landis.

An ice sheet, such as the West Antarctic Ice Sheet, can be thousands of meters thick. The drill is able to retrieve up to three meters at a time, which takes several hours. Drilling a very deep ice core can take years because the work can only be done during the polar summer.

Glaciologist Gifford Wong examining ice cores in Antarctica. The ice cores are in metal trays and green plastic wrapping to protect them from breaking. Photo courtesy of Natalie Kehrwald.

MOUNTAIN GLACIERS BY TRUCKS AND YAKS

Drilling ice cores on mountain glaciers is different from drilling ice cores on ice sheets. Mountain glaciers are thinner than the deep ice sheets. Therefore, the scientists can usually drill all the way to bedrock in one season. Often aircraft is not practical for traveling to mountain glaciers, so scientists drive as close to the drill site as possible. They then hire porters to transport the equipment and supplies up to a base camp, where the scientists get acclimated to thinner air.

Scientists typically spend about a week at base camp, allowing their bodies to build more red blood cells to gather and carry oxygen from the thin air. Because they allow for this physiological response, scientists don’t need to use oxygen tanks at the high elevations. Oxygen tanks would add considerable weight and risk to the already difficult expedition. While the scientists are becoming acclimated, the porters continue to move the equipment and supplies up to the drilling site, which is often at an elevation between 19,000 and 21,000 feet.

Keep in mind that all of the necessary equipment for drilling ice cores in the mountains weighs up to six tons. Once the ice has been drilled and packaged in protective tubes and boxes, the combined weight of equipment and ice being brought down the mountain is about ten tons.

Recently, an Ohio State University team went to western Tibet to drill ice cores. Their equipment was packed, inventoried, and shipped to western China, where it was loaded into trucks. The team of scientists and their Chinese colleagues and helpers drove for five days, sometimes across stretches where there were no roads. Eventually, they were no longer able to drive the trucks up the rugged slope. At that point, they transferred the cargo to a team of herders who used yaks to carry the equipment up the mountain.

A yak carries boxes containing 12 ice cores down to base camp. Photo courtesy of Mary Davis.

Yaks are very strong, but they cannot easily walk on large boulders or slippery ice. After a few days, the scientists were at the edge of the ice and the yaks could no longer be used. The drill, tents, cooking supplies, and all of the other material had to be brought up the mountain by humans, with backpacks weighing up to 60 pounds. This meant several trips were needed to get the equipment from the edge of the ice to the base camp, and again from the base camp to the drill site. The scientists lived and worked in tents at an elevation of about 20,000 feet for over two weeks.

A high-altitude ice core drilling camp in Tibet. Photo courtesy of Natalie Kehrwald.

In Tibet, the Ohio State University team was able to drill three cores of about 150 meters (400 feet) each. As the scientists bring each core to the surface, they examine and describe it before placing it in a plastic sleeve and then into a snow pit at the drill site for cold storage. When they have reached bedrock or are no longer able to get more samples from this site, they package the cores into cardboard tubes for transport. A team of porters arrives to carry the tubes down to the lowest edge of the ice, where pack animals are waiting.

Near the edge of the ice, the porters place six tubes inside a specially insulated box, add packs of frozen gel (much like those you might put in an ice chest), and add another layer of insulation. Then they secure the box lid in place. Each pack animal can carry one or two boxes (6 or 12 cores) down to the refrigeration truck that is still farther downslope. The ice can remain frozen for up to five days in the boxes. It is critical that the ice not be allowed to melt!

On another drilling expedition, in a warmer climate, a refrigeration truck broke down near a small town. While the truck was being repaired, the leader, well-known glaciologist Lonnie Thompson, bought all the ice cream in a grocery store and gave it to the villagers, making room in the store freezers to keep the ice cores frozen temporarily.

Half of the cores that were obtained in this recent expedition to Tibet were left in China for independent analyses. The others were flown to the United States, where they were trucked or flown to Columbus, Ohio. When cores arrive at the cold storage facility at the Byrd Polar Research Center, they are quickly unloaded and maintained from that point forward at a temperature of minus 30 degrees Fahrenheit.

Natalie saws an ice core at the Byrd Polar Research Center. Photo courtesy of Carol Landis.

The scientists may spend months to years analyzing the ice cores in their laboratory to gain information on past climate, such as temperatures, storm patterns, changes in atmospheric chemistry, and times of drought.

LEARNING ABOUT THE PAST

Ernest Shackleton, an early Antarctic explorer, once said, “What the ice gets, the ice keeps.” This means that once something is covered in ice, it and any associated information is preserved in the ice. Items can be large, such as animals that were on the glacier surface during a surprise snowstorm, or smaller, more common objects. Insects, parts of plants, volcanic ash, wind-blown dust, bacteria, and the ice itself provide information on past temperatures, accumulation, aridity, and wind patterns.

Ice cores can provide season-by-season or year-by-year details on past climate. Through investigating the contents of ice cores, scientists can learn what the climate was like during past ice ages and tell when the climate suddenly changed in the past. They are able to look at the warming or cooling trends of present times and determine if they are similar to what has happened at other times in the past.

From looking at ice cores and other environmental records, scientists have been able to see that the warming of the past 70 years is significantly different from any other known warming in the past. Once scientists are able to know what has happened in the past, they are better able to understand what is happening in the present.

RESOURCES

These two videos from Polar Palooza explore ice coring. Watch these and others at Polar-Palooza’s YouTube channel!

Reading Ice Cores

This Polar Palooza video follows a 2007 expedition in which Mary Albert (CRREL) and Jeff Severinghaus (Scripps) led a team of 9 researchers and 3 drillers in a 3-week project to drill down through nearly 125 meters of “firn” and ice close to NSF’s Summit Station, Greenland. It explores how scientists “read” ice cores and what the information means.

Ice Drillers are Hard Core

Deciphering the secrets of past climate hidden in ice cores depends on the technical skills and ingenuity under pressure of drillers from ICDS, the University of Wisconsin-Madison’s Ice Coring and Drilling Services. In this video, ICDS staffers Lou, Mike and Jay explain why they enjoy the life of drillers, braving extreme cold in some of the remotest regions of the globe.