Ice Cores That Tell the Past

Climate Change and GISP2
During the past few decades, researchers have established the existence of a climate system on Earth that is characterized by complex integration and feedback. The sun and all the parts of the Earth - the oceans, the atmosphere, the land masses, the snow and ice masses, all life, and the inner earth - are parts of this system. Changes in any one part of the system affect all the others and ultimately result in climate change. Climate change is actually a continuous process, but in the past the changes have ranged from the slow and gradual to the surprisingly fast and dramatic. This much we have learned about the climate system; but beyond this we are less knowledgeable. How do the parts of the system interact? How will specific changes in one part affect the others and, ultimately, the climate? What patterns of processes occur to produce the changes we have observed such as the cycle of glacial advances and retreats? What climate changes will occur as a result of our activities?

Humanity's production of CO2, nitrous oxide, sulfuric and nitric acids, CFC's and other "greenhouse" gases, as well as our direct impact on large ecosystems, makes understanding the climate system an imperative. From what we do know now, our activities could raise the average temperature a few to several degrees centigrade over he next few decades in addition to altering weather patterns. If so, the potential exists for severe, or catastrophic, disruption of the EarthÕs living and climate systems.

Ice cores, cylinders of ice drilled out of glaciers and polar ice sheets, have played an important role in revealing what we know so far about the history of climate. Today, United States scientists are embarking on a new ice coring project in Greenland with a wide range of state of the art analyses in the hopes of resolving questions about how the climate system functions. Drilling for The Greenland Ice Sheet Project Two* (GISP2) began in 1989. When they reach the bottom of the ice sheet, 3000+ meters thick, in 1992 they will have recovered the longest, most detailed, continuous record of climate available from the northern hemisphere stretching back 200,000 years or more through two glacial/interglacial (cold/warm) cycles.

While long ice core records exist from Antarctica, fewer comparable records exist from the northern hemisphere. The newest theories1 on the transitions between glacial (cold) and interglacial (warm) periods involve changing circulation patterns in the North Atlantic Ocean. The modern ocean circulation pattern includes the well known Gulf Stream. This flow of water brings significant amounts of heat, in the form of warm water, from the tropics to the North Atlantic. When it reaches the North Atlantic and cools, by giving its heat off to the atmosphere, it becomes dense enough to sink to the bottom. This water flows slowly through the depths of the Atlantic to the Indian and Pacific oceans and eventually returns, approximately 1000 years later, to the North Atlantic. It is surmised that during glacial periods this circulation was shut off, changing temperatures in the polar regions. Because CO2 is soluble in seawater, the cessation of this circulation also has important implications for CO2 in the atmosphere. Since the North Atlantic is where this circulation is driven - the cooling, sinking water sets the circulation in motion - it has become a region of significant interest and means that GISP2 will provide a unique record for evaluating these theories.

As an example of the changes to be investigated, the GISP I core (a predecessor to GISP2) has revealed evidence of extremely rapid rises in temperature on the order of several degrees centigrade in several decades2. While the location of the GISP I core precludes further illumination of this event, if such rapid changes can be investigated during GISP2, the mechanisms by which such a change could occur can be understood. Furthermore, our understanding of the relationship between changes in the "greenhouse" gases (such as CO2 and methane (CH4)) and temperature can be investigated by examining the timing and magnitude of their change during different climatic periods.

How Ice Cores Record a History of Climate

How can a history of climate be reconstructed from an ice core? When snow falls it carries with it the compounds that are in the air at the time, compounds ranging from sulfate, nitrate and other ions, to dust, radioactive fallout, and trace metals. When snow falls in a place where temperatures above freezing are rare (there is only a hint of any melting at the GISP2 site in the 750 year record recovered to date), such as in polar regions or at high altitude, the snow from one year falls on top of the previous year without melting.

As each yearÕs snowfall is buried by successive years' snowfall, the constituents contained in the snow are buried along with it. By drilling down from the surface of an ice sheet and analyzing snow from greater and greater depths, a history of the compounds in the air can be obtained. Further, snow that is deeper than 80 meters at the GISP2 site turns into ice from the weight of the snow above it, and trapped in the ice are small bubbles of air. Thus, in addition to trapping compounds from the air, an ice sheet traps a small sample of the air itself. This trapped air is also analyzed and provides information about the composition of the atmosphere at the time the ice formed.

Like ice cores, deep sea cores have also provided information about climate, but from accumulated sediments on the ocean floor. Unlike ice cores, which provide direct climate information, sediment cores provide indirect information. An example of this indirect evidence is the method for determining temperature. When sediment cores are analyzed researchers painstakingly sort out plankton shells which twist in different directions depending on the temperature of the water they grew in. By counting the number of shells that twist each way the temperature of the surface water at the time that they grew can be determined. Understanding the behavior of these plankton in the modern world is necessary to produce a historical record of temperature for the ocean.

Sediments also accumulate very slowly relative to snow on an ice sheet. This results in much longer records from sediment cores, but a much reduced ability to resolve short term changes. While periods of hundreds to thousands of years might be resolved in a sediment core, annual and even seasonal resolutions are possible with ice cores. On the other hand, sediment cores can provide records which are as long as several million years compared with the several hundred thousand years of ice cores. Because of these differences, sediment cores and ice cores provide complimentary climate information; ices cores provide high resolution, direct information and sediment cores lower resolution, less direct records, but from much longer time periods.

GISP2 is located at the highest point on the Greenland ice sheet (38¡ 28' W, 72¡ 35'N, 3208 meters above sea level) on the "ice divide" of central Greenland. Ice on the west of the roughly north south divide passing through the "summit" flows to the west and ice to the east flows east. Here the ice moves almost vertically down to the bottom and the annual layers get progressively thinner and thinner as they get deeper. This is the optimal place for drilling. It will yield the longest continuous record available from the ice sheet and each yearÕs snowfall will be thinned the least providing the greatest detail.

GISP2 Research

As already indicated, ice cores provide a more direct record of climate than sediment cores. They also provide a staggering breadth of information. One of the cornerstones of ice core research is the d18O (delta-O-18) isotopic record (16O and 18O are isotopes of oxygen; they are the same chemically, but have slightly different weights). Water in the oceans contains primarily oxygen with an atomic weight of 16 (16O, oxygen-16). A small fraction however is 18O, 12% heavier than "typical" oxygen. Water molecules with 18O are the same as regular water in most respects except that because it is heavier, it does not evaporate as readily and condenses slightly more easily than water with 16O. Depending on the temperature of evaporation and how far the water has had to travel before it fell as snow on the summit of Greenland, the ratio of 18O to 16O will vary. This ratio, known as d18O, can be measured very accurately using a mass spectrometer. Over short time scales the change in temperature from summer to winter produces a very clear oscillation in the 18O/16O ratio. This oscillation is used to determine the age of the core at different depths, simply by counting the oscillations. Over longer time periods, this ratio indicates the average temperature of the regions between the evaporation site and the coring site. GISP2 investigators are also analyzing for the ratio of 1H/2H (Hydrogen to deuterium) which will allow even finer detail about source temperature and condensation history to be obtained. Dr Pieter Grootes of the University of Washington and Dr. Jim White of the University of Colorado, Boulder are working on these isotope measurements for GISP2.

The major ions found in snow also have annual signals. Some ions such as sodium (Na+) and chloride (Cl-) are principally derived from the sea. Others such as sulfate (SO4=) come from human, biological, and volcanic activity as well as from the sea. The burning of fossil fuels in the northern hemisphere produces sulfate and nitrate (NO3-) and can be seen as high levels of these compounds in the ion record from previously drilled, shallow Greenland ice cores3. The ion record from an ice core reveals important information about the source of the air to the drill site (which is critical for interpreting other measurements), volcanoes (which produce sulfate and chloride), and changes in the activities that produce the ions such as fossil fuel combustion. Dr. Paul Mayewski (Chief Scientist for GISP2), Dr. Wm. Berry Lyons, and Dr. Mary Jo Spencer of the University of New Hampshire and Dr. Eric Saltzman of the University of Miami produce the ion record for GISP2.

Another property of the core is being studied by Dr. Julie Palais of the University of New Hampshire and Dr. Michael Ram of University of New York at Buffalo. The amount of dust carried to Greenland varies with the amount of land where dust can be picked up by the wind, the strength of the wind, and also, with volcanic activity and fires. Like the isotopes and ions, there is an annual signal of dust in the core. A dust peak is often found in the spring section of an annual layer. Like isotopes these can be counted to determine the age of the core. Volcanoes can produce large quantities of particles and leave a record in the ice. Scanning electron micrographs of the particles from a particularly large dust peak in an ice core may reveal that it is from a known volcano and allow a firm date to be placed on that section of core. For prehistoric times, the dust record is a key tool for reconstructing a history of volcanic activity. Further at the end of a glacial advance there is often a period of dustiness as the glaciers retreat and leave large unvegetated land areas. These periods can be detected by high levels of dust in the core.

An important baseline measurement is electrical conductivity. This research is done by Dr. Ken Taylor of the Desert Research Institute in Reno, Nevada. Electrical conductivity measurements (ECM) of the core is a very rapid method to indicate how acidic the core is without the chemical detail of the ion analyses. The value of the measurement is that it can be done for the whole length of the core in high resolution and provide an immediate picture of the core and allow quick detection of interesting areas, such as a volcanic eruptions. Because it is a high resolution, continuous measurement it can be used, along with the other measurements, for time frequency analysis in order to identify cycles in the climate signal.

The relationship between CO2, other "greenhouse" gases and global warming is of great importance and much debated. The ice core record from Vostok, Antarctica shows a near perfect correlation between CO2 and temperature4. Several groups are analyzing the gas bubbles from the core. Dr. Martin Wahlen of the New York State Department of Health will be measuring CO2 concentrations in addition to methane (CH4), nitrous oxide (N2O), and other gases. Dr. Alex Wilson of the University of Arizona is determining CO2 concentrations and carbon-14 (14C) dating the CO2 in the ice to help determine the age of the core. This information will indicate, for example, not only how much CO2 was in the air at any given time but, based on its isotopic signature,what its source was. Dr. Michael Bender of the University of Rhode Island, in addition to the other gas investigators, will be making special analyses of the isotopic composition of the bubbles. These studies will reveal fine details about how gases might change in the ice with time.

Studies of the trace metal content of the core will help to resolve the debate about large meteor strikes on Earth by detecting iridium, a product of meteor strikes. Dr. Ed Boyle of MIT, whose work on sediment cores has been important in linking climate change and ocean circulation5, will be analyzing for iridium, an element found in abundance in extraterrestrial material, as well as a broad suite of other elements.

Physical characteristics of the core reveal annual layers resulting from the temperature difference between summer and winter as well as detecting any deformation of the glacier that could affect the record. Dr. Richard Alley of The Pennsylvania State University and Dr. Tony Gow and Dr. Deborah Meese of the Cold Regions Research and Engineering Lab in Hanover, New Hampshire produce the physical record of GISP2.

The Outlook for the Future

After four summers of drilling and the equivalent of decades of analyses all squashed into five years of intense work, what will GISP2 produce? A new view of Earth history will have evolved - the most detailed view of the last 200,000 years ever seen. This view will literally be a "time machine" that will tap a reservoir of information barely recognized as a resource two decades ago. Theories about climatic change will have been tested and perhaps even more importantly, new views of the history of climate will spawn that are perhaps not even possible to guess about today. We cannot look into the future yet, but we can look to the past. From the past we gain perspective, a different view of who we are, what we can do and the environment of which we are a part. By 1992 as the last of the 3000 meters of core come to the surface, a group of scientists, technicians, engineers, and students will have sampled the atmosphere as it was during the onset of the Industrial Revolution, at the earliest periods of modern civilization, and into the last major ice age, through to the last warm period and into the previous major ice age. Unknown natural cycles may become clear, new interactions between the atmosphere and climate may reveal themselves. From this view of the past, the magnitude of human involvement will be placed in sharper perspective and our view of the Earth system will be probed yet farther.

Drilling Technology

Retrieving a 3000 meter core is not a simple task. Ice must be retrieved from great depths and pressures. Information about the drill's angle, depth, power consumption, must all be relayed to the surface during drilling, and slight flaws in the drill barrel can damage the core The Polar Ice Coring Office (PICO) of the University of Alaska, Fairbanks provides the state-of-the-art drilling technology for recovering this deep core and has spent several years developing a drill capable of working at these depths. The 20 meter long drill consists of a specialized drill head, a core barrel, chip catcher, motor, instrument package, and anti-torque knives all suspended from a 4000 meter cable. The drill is lowered to the bottom of the bore hole where a section of core between 2 and 6 meters long is cut, broken off at the base and winched back to the surface. This core is then sent to a "science trench", a large room cut out of the snow for processing and analyzing the core, while the drill goes down the hole for more ice.

Because the concentrations of the compounds being measured in the ice are very low it would be easy to contaminate the ice samples. The touch of a bare hand would hopelessly contaminate a final ice sample. Once the core sections are in the science trench they are passed through a "core processing line"; a series of stations where the core is processed, sampled and packed for shipment back to laboratories in the United States. The outside of the core is cut first for use in isotopic analyses which are not easily contaminated then the cleaner, inside pieces of the core are rinsed briefly in ultra-pure water under clean conditions and packaged for shipment. Scientists must wear special "clean suits" over their warm clothing to insure that they themselves do not contaminate the samples.

* The Greenland Ice Sheet Project Two (GISP2) is being carried out by scientists from: the U. S. Army Cold Regions Research and Engineering Laboratory in Hanover, NH, Carnegie Mellon University the Desert Research Institute in Reno, Nevada, The Massachusetts Institute of Technology, Ohio State University, Pennsylvania State University, the New York State Department of Health, the State University of New York at Albany, the State University of New York at Buffalo, Lamont-Doherty Geological Observatory of Columbia University, University of Arizona, University of Colorado, University of Miami, University of New Hampshire, University of Rhode Island, University of Washington, the U. S. Geological Survey in Tacoma, Washington, and the University of Wisconsin.

GISP2 is funded by the United States National Science Foundation Division of Polar Programs as a part of the Arctic System Science Initiative (ARCSS). The University of New Hampshire coordinates GISP2 scientific activities. Logistical and drilling support is provided by the Polar Ice Coring Office (PICO) at the University of Alaska in Fairbanks. Permission to work in Greenland is generously provided by The Commission for Scientific Research in Greenland and the governments of Denmark and Greenland and is gratefully acknowledged. The 109th TAG Air National Guard, Schenectady, NY, and the U. S. Air Force Military Airlift Command from McGuire Air Force Base in New Jersey provide air transport. Support at Sondrestrom Air Base in Greenland is provide by the U.S. Air Force Space Command.

  1. Broecker, W. S. and G. H. Denton. (1990). "What drives the glacial cycles?" Sci. Am. 262(1): 48 - 56.
  2. Dansgaard, W., J. W. C. White and S. J. Johnsen. (1989). "The abrupt termination of the Younger Dryas climate event." Nature. 339: 532-533.
  3. Mayewski, P. A., W. B. Lyons, M. J. Spencer, M. Twickler, W. Dansgaard, B. Koci, C. I. Davidson and R. E. Honrath. (1986). "Sulfate and nitrate concentrations from a south Greenland ice core." Science. 232: 975-977.
  4. Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N. I. Barkov, Y. S. Korotkevich and V. M. Kotlyakov. (1985). "A 150,000-year climatic record form Antarctic Ice." Nature. 316: 591-596.
  5. Boyle, E. A. (1986). "Paired carbon isotope and cadmium data for benthic foraminifera: Implication for changes in oceanic atmospheric carbon dioxide." Glob. Biogeochem. Cycles. 3(3): 21 - 239.