Well, scientists have such a time machine. It's called an ice core. Scientists collect ice cores by driving a hollow tube deep into the miles-thick ice sheets of Antarctica and Greenland (and in glaciers elsewhere). The long cylinders of ancient ice that they retrieve provide a dazzlingly detailed record of what was happening in the world over the past several ice ages. That's because each layer of ice in a core corresponds to a single year--or sometimes even a single season--and most everything that fell in the snow that year remains behind, including wind-blown dust, ash, atmospheric gases, even radioactivity.

Indeed, fallout from the Chernobyl nuclear accident has turned up in ice cores, as has dust from violent desert storms countless millennia ago. Collectively, these frozen archives give scientists unprecedented views of global climate over the eons. More important, the records allow researchers to predict the impact of significant events--from volcanic eruptions to global warming--that could strike us today.

Special thanks to Mark Twickler, University of New Hampshire

In April 1986, Russia's nuclear power station at Chernobyl exploded, killing 250 people and sending radioactive fallout around the world. Less than two years later, as the graph indicates, scientists detected Chernobyl radioactivity in snow at the South Pole--a graphic reminder of how small our planet is. In cores from Antarctica and Greenland, researchers have pinpointed the beginning of atomic-bomb testing in the mid-1950s. They have also identified a spike representing fallout from stepped-up atmospheric testing that took place just prior to the 1963 Test Ban Treaty, which allowed for underground tests only. In the years following 1965, by which time some 90 countries had signed the treaty, Antarctic snow revealed a sharp drop in radioactive fallout.

Graph modified from: Dibb, J., Mayewski, P.A., Buck, C.F. and Drummey, S.M., 1990, Beta radiation from snow, Nature, 344, 6270, 25

Gases trapped in ice cores show the dramatic impact that human activities have had on the planet since the Industrial Revolution. The first graph reveals how atmospheric carbon dioxide, methane, and nitrous oxides from coal- and oil-burning power plants, cars, and other fossil-fuel-burning sources have climbed along with the world population, with as yet unknown effects on the climate system.

The second graph displays similar results with sulfates and nitrates. Sulfates, which originate primarily in coal-fired power plants, started rising around 1900. (This rise is partially attributed to increased volcanic activity in the Caribbean around the turn of the century; other volcanic eruptions--represented by large spikes in the graph--can be seen at numbers 1, 2, and 3.) Nitrates didn't begin to climb significantly until after 1950, when cars and oil-powered plants appeared in a big way. Scientists credit the leveling off in sulfates and nitrates at the graph's far right--that is, the most recent period--to a less-polluted atmosphere after the 1972 U.S. Clean Air Act went into effect.

Data in gases graph from:
Etheridge, D.M., Pearman, G.I., and Fraser, P.J., 1992, Changes in tropospheric methane between 1841 and 1978 from a high accumulation rate Antarctic ice core, Tellus, Ser. B, 44, 282-294. (CO₂ and CH₄)

Keeling, C.K., Adams, J.A., Ekdahl, C.A., and Guenther, P.R., 1976, Atmospheric carbon dioxide variations at the South Pole, Tellus, 28, 552-564. (direct measurements)

Machida, T., Nakazawa, T., Fujii, Y., Aoke, S. and Watanabe, O., 1995, Increase in atmospheric nitrous oxide concentrations during the last 250 years, Geophys. Res. Lett., 22, 2921-2924. (N₂0)

McEvedy, C. and Jones, R., 1978, Atlas of world population history, Penguin. (world population)

Data in particulates graph from:
Mayewski, P.A., Lyons, W.B., Spencer, M.J., Twickler, M.S., Buck, C.F. and Whitlow, S., 1990, An ice core record of atmospheric response to anthropogenic sulphate and nitrate, Nature 346(6284), 554-556.

Viking colonies in Greenland abruptly vanished toward the end of the 14th century. Why? One clue comes from ice cores. This graph, which combines results from cores taken in both Antarctica and Greenland, tracks sodium levels over the past 1,200 years. In colder periods, seas become stormier because of the greater contrast in temperatures between the tropics and the poles, and so more sodium--an indicator of seasalt--winds up on the ice caps. About 1400 AD, the cores at both poles clearly show a sharp rise in sodium, which some scientists say marks the onset of the Little Ice Age, a period of much cooler temperatures that lasted into the 19th century. For the Vikings, a series of abnormally cold winters in the late 1300s spelled doom.

Graph modified from:
Kreutz, K.J., Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S.I. and Pittalwala, I.I., in press 1997, Bipolar changes in atmospheric circulation during the Little Ice Age, Science.

Annual layers of snowfall in ice cores can be counted as easily as tree rings, allowing precise dating of events such as volcanic eruptions. Distinct annual layers stand out because, in snow that falls in summer, crystals are larger and acidity higher than in winter snow. In some cases, scientists can even tell seasons apart, by using a laser to measure the concentration of dust particles. (Winds are generally stronger in springtime, meaning more dust gets blown into the atmosphere.) In this photograph of an ice core drilled in the Kunlun Mountains of western China, the thick, lighter bands indicate heavy snowfall during the monsoon season in the year 1167 AD, while the thinner, darker strips show layers of dust blown into the snowfield during the dry season.

Core photo courtesy of Lonnie G. Thompson, The Ohio University

(BP - Before Present)

Ice cores have revealed that global climate--long thought to change only very gradually--can shift with frightening speed, in some cases in a matter of years. As this graph shows, one such jump occurred about 12,000 years ago, as the last glacial period (the Pleistocene) was giving way to our current warm "interglacial" period (the Holocene). Suddenly, possibly in less than five years, average temperatures, which were slightly cooler than today's, plunged by about 27°F, returning the world to near-glacial conditions. (As the graph indicates, calcium levels tend to go up and snow accumulation down with temperature, which is estimated by comparing the ratio of oxygen isotopes in water--see "Temperature" in core at left.) The Younger Dryas, as this freak period is known, lasted about 1,300 years before it returned--just as abruptly--to the temperatures typical of the period immediately preceding it.

Data in graph taken from:
Alley, R.B., Meese, D., Shuman, C.A., Gow, A.J., Taylor, K., Ram, M., Waddington, E.D. and Mayewski, P.A., 1993, Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event, Nature 362, 527-529.

Grootes, P.M., Stuiver, M., White, J.W.C., Johnsen, S. and Jouzel, J., 1993, Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores, Nature 336, 552-554.

Mayewski, P.A., Meeker, L.D., Whitlow,S., Twickler, M.S., Morrison, M.C., Grootes, P.M., Bond, G.C., Alley, R.B., Meese, D.A., Gow, A.J., Taylor, K.C., Ram, M. and Wumkes, M., 1994, Changes in atmospheric circulation and ocean ice cover over the North Atlantic during the last 41,000 years, Science 263, 1747-1751.

Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S.I., Yang, Q. and Prentice, M., in press, 1997, Major Features and forcing of a high latitude Northern Hemisphere atmospheric circulation over the last 110,000 years, Journal of Geophysical Research.

Temperature has yo-yoed over the ages as wildly as it does through any single year. Like natural thermometers, ice cores have recorded these fluctuations, which scientists can "read" by examining isotopes of oxygen and hydrogen in water trapped in the ice. These isotopes come in two forms--"light" and "heavy." Light isotopes have regular hydrogen and oxygen, while heavy isotopes have either hydrogen with an extra neutron or oxygen with one or two additional neutrons. Since heavy isotopes precipitate out of the atmosphere more quickly than light ones, scientists can measure the ratio between the two isotopes to estimate the temperature at any given time. The data in this graph, gleaned from a core drilled in central Greenland, shows how temperatures have risen by more than 20°C (36°F) since the height of the Ice Age 25,000 years ago.

Graph modified from:
Cuffey, K.M., Clow, G.D., Alley, R.B., Stuiver, M., Waddington, E.D. and Saltus, R.W., 1995, Large Arctic-temperature change at the Wisconsin-Holocene transition, Science 270, 455-458.

Approximately 73,000 years ago, an Indonesian volcano known as Toba erupted with enough force to send more than 600 cubic miles of volcanic material into the atmosphere. Detected on this graph, which displays volcanic sulfate levels between 20,000 and 110,000 years ago, Toba was the largest eruption of the past 500,000 years. (The seemingly larger spike at about 53,000 years ago involved a series of smaller eruptions on Iceland, which is far closer than Toba is to Greenland, where this core was taken.) Such violent, so-called caldera eruptions can drastically alter global climate, by spewing so much ash and sulfur compounds into the atmosphere as to block sunlight and lower temperatures worldwide. Ice cores offer scientists the best means available to learn how past eruptions have affected climate--and thus to predict the impact that future ones might have. If an eruption on the order of Toba, which climatologists believe may have led to as much as several centuries of cold climatic conditions, were to occur today, it could seriously disrupt life on Earth.

Graph modified from:
Zielinski, G.A., P.A. Mayewski, L.D. Meeker, S. Whitlow, and M. Twickler, 1996a, A 110,000-year record of explosive volcanism from the GISP2 (Greenland) ice core, Quaternary Research, 45, 109-118.

Many scientists fear that rising levels of so-called "greenhouse gases" from the burning of fossil fuels and other human activities will cause global warming, with potentially grave consequences for human agriculture and society. One of the clearest signs that elevated levels of greenhouse gases can result in warming comes from an ice core taken near the Russian Vostok station in Antarctica. This graph tracks temperature and atmospheric levels of carbon dioxide (CO₂) and methane (CH₄) from the present back to about 160,000 years ago. (This represents about 11,350 feet of ice accumulation.) The graph clearly shows how a rise in gases will mean a rise in global temperature (though whether rising gases trigger rising temperatures, or vice versa, remains unknown). Also note that (though the graph, which has data up to two decades old, does not show this), at about 360 parts per million, the amount of CO₂ in the atmosphere today far exceeds levels at any time in the past 160,000 years--indeed, in the past few million years. For those worried about global warming, this is a sobering statistic.

Graph data taken from:
Barnola, J. M., D. Raynaud, Y. S. Korotkevich and C. Lorius, 1987, Vostok ice core provides 160,000-year record of atmospheric CO2, Nature, 329, 408-414.

Chappellaz, J., J.-M. Barnola, D. Raynaud, Y. S. Korotkevich and C. Lorius, 1990, Atmospheric CH4 record over the last climatic cycle revealed by the Vostok ice core, Nature, 345, 127-131.

Jouzel, J., C. Lorius, J. R. Petit, C. Genthon, N. I. Barkov, V. M. Kotlyakov and V. M. Petrov, 1987, Vostok ice core: a continuous isotope temperature record over the last climatic cycle (160,000 years), Nature, 329, 403-407.

Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N. E. Barkov and Y. S. Korotkevich, A., 1985, 150,000-year climatic record from Antarctic ice, Nature, 316, 591-595.