Ice Cores

Another significant area by which climate proxy data are gathered is from ice cores. The most popular places for obtaining ice cores are in Greenland and Antarctica because they represent long histories of ice accumulation. Many ice cores are collected and then sent to and stored at the National Ice Core Laboratory in Denver, Colorado, where they are further analyzed. Ice cores contain a wealth of information about the climate. Ice cores can contain an uninterrupted, detailed climate record extending back hundreds of thousands of years. Valuable information can be obtained from them, such as temperature, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity, and other climate indicators. It is the simultaneity of these properties recorded in the ice that makes ice cores such a powerful tool in paleoclimate research. In order to obtain the ice cores, a sharpened pipe rotating on a long cable is drilled into the ice. Once the core is drilled, it is extracted and brought to the surface.

Each year, new layers of snow collect on the ice sheets, and each layer is different in chemistry and texture. Summer snow is different from winter snow. For instance, in the summer, it is daylight 24 hours a day in the polar regions. Because of this, the top layer of snow has a different texture (it does not melt, but it differs in texture from the layers beneath it). When it turns cold and dark again, an increase in snowfall occurs. This layer looks much different from the layer that formed during the summer months. Similar to the case with sediment from ocean and lake bottoms, ice cores take on a characteristic banded appearance that differentiates the seasonal accumulations into distinct layers. Over hundreds of thousands of years, this snow, then ice, collects and traps a record of past climate within it. As scientists retrieve these cores—some from depths of more than 11,000 feet (3,500 m)—they begin unraveling the ancient climate history of an area.

Sometimes scientists dig snow pits in the field, wherein the snow layers are even easier to distinguish. By digging two pits adjacent to each other separated by only a thin wall of ice, it is possible to catch the light shining through the ice and illuminate the individual layers deposited within the ice.

Ice cores have been drilled from ice sheets since the 1960s. Some of the ice cores have contained data as far back as 750,000 years ago. These cores provide an annual record of temperature, precipitation, what the atmosphere was like at the time, any volcanic activity that may have been nearby, and what the prevailing winds were. The thickness of each layer supplies information as to how much snow accumulated. When cores are taken from the same area but show differences where the snow drifted from one place to another, it gives scientists a good idea of what the wind and atmospheric circulation patterns were like.

When scientists analyze the ice in the cores themselves, they look at the oxygen isotopes because that is where clues to past temperatures

These are the annual accumulation layers of the Quelccaya ice cap, each accumulation layer about 2.5 feet (0.75 m) thick. The light layers are the snow accumulation layers in the winter. The dry season is differentiated with the darker dust bands. (Lonnie Thompson, Byrd Polar Research Center, Ohio State University, NOAA Paleoclimatology Program)

These are the annual accumulation layers of the Quelccaya ice cap, each accumulation layer about 2.5 feet (0.75 m) thick. The light layers are the snow accumulation layers in the winter. The dry season is differentiated with the darker dust bands. (Lonnie Thompson, Byrd Polar Research Center, Ohio State University, NOAA Paleoclimatology Program)

are hidden. Specifically, the ratio of oxygen isotopes relates to how cold the air was where the snow was deposited. Colder temperatures have higher concentrations of light oxygen.

Oxygen is a key factor in understanding past climate. Oxygen exists as a heavy and light isotope. Oxygen-16 is considered the light isotope, and oxygen-18 is the heavy one. Oxygen-16 is the more common of the two. The ratio of the two isotopes of oxygen in water changes with the climate. In determining the ratio of heavy and light oxygen in ice cores and marine sediments of fossils, scientists learn how the climate varied over time. Because the heavier oxygen falls out, this leaves the light oxygen frozen in the ice caps, signifying cooler temperatures during this time period.

Thickness of an ice sheet is another important factor because it makes the ice sheet's temperature more resistant to change. Scientists measure the temperature of an ice sheet by lowering a thermometer into a borehole drilled to retrieve an ice core. The ice cap has an insulating quality that keeps temperature of the snow and ice preserved at the general temperature of the atmosphere when the layer was initially deposited. Near the Earth's bedrock surface, however, the lowest layers are warmed by heat from the Earth. Knowing the Earth's temperature, scientists use this information to calibrate the temperature record they retrieve from the oxygen isotopes. Thus, they can accurately detect temperature variations that have occurred since the ice age.

Some of the most valuable information obtainable from ice cores is data about the atmosphere. When snow initially forms, it crystallizes around tiny particles of smoke, dust, volcanic ash, pollen, and other matter suspended in the air. These particulates become trapped in the snow and can provide clues about the environment from which they came. When snow settles on the ground, air fills the spaces between the ice crystals. Then, as new layers of snow fall, the older layers are buried. Layer upon layer accumulates, and the space between the crystals is sealed off, trapping small pockets of the atmosphere in the ice. These small bubbles remain intact, and when an ice core is removed, the air bubbles are analyzed to see what the atmospheric conditions, and hence the climate, were like at the time of deposition.

Scientists must extrude (extract) the ice core from its barrel with extreme care. The cloudy layers visible in this core section were formed when dust fell onto the ice sheet and was trapped in the ice. (Mark Twickler, University of New Hampshire, NOAA Paleoclimatology Program)

According to scientists at NASA, records of methane in samples indicate that a climate conducive to wetlands once covered the Earth. According to Gavin Schmidt at NASA GISS (Goddard Institute for Space Studies), methane has oscillated in response to rapid climate changes, such as the Younger Dryas cold interval and rapidly increases in a warming climate, with a small lag behind temperature. This means that not only does methane affect climate through greenhouse effects, it can also be directly affected by climate. Natural methane emissions depend on the extent of organic decomposition in very wet conditions. For an individual wetland, an increase in the water table and/or an increase in temperature will lead to greater emissions on a very short time scale. Over longer periods of time, wetlands come and go as a function of rainfall patterns. While methane was not considered a very important greenhouse gas in the past, its role is better understood today, and it is now acknowledged to be an important greenhouse gas thanks to emerging mathematical models. Wetlands support a wide variety of life-forms that encourage the growth of anaerobic bacteria, which release methane as they decompose. Analyzing ice cores also allows scientists to correlate the amount of CO2 in the atmosphere with climate change. Being able to track the levels of CO2 throughout Earth's history helps scientists better understand the issues surrounding global warming today.

In fact, when levels of methane and CO2 found in ice cores are compared with those found today, scientists have determined that the levels are higher now than they have been in the past 220,000 years. Activities such as industrialization, agricultural practices, and deforestation are currently blamed for this situation. Ice core analysis also reveals that during the past 220,000 years, when the atmosphere contained low levels of CO2 and methane, climate was cool. Conversely, when these levels were high, climate was warmer. Scientists have determined there is a strong correlation between the Earth's temperature and greenhouse gas levels.

When wind-blown dust is analyzed in ice cores, it can be chemically analyzed to determine where it came from. The amount and provenance of the dust provide important information for unraveling the past because it sheds light on wind patterns and strength. Volcanic ash is another particulate that can provide climate change information because volcanic eruptions themselves sometimes contribute to climate change. Significant eruptions can put enormous amounts of ash into the atmosphere, cooling the climate for a period of time. If the layers of volcanic ash can be dated in the ice cores, then a correct geologic time line can be constructed from the layers.

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