Perspective Ice-core evidence of abrupt climate changes Richard B. Alley* Environment Institute and Department of Geosciences, The Pennsylvania State University, Deike Building, University Park, PA 16802 Ice-core records show that climate changes in the past have been large, rapid, and synchronous over broad areas extending into low latitudes, with less variability over historical times. These ice-core records come from high mountain glaciers and the polar regions, including small ice caps and the large ice sheets of Greenland and Antarctica. s the world slid into and out of the last carbon dating (2), and other techniques oxygen contains one or two ‘‘extra’’ neu- Aice age, the general cooling and are possible. trons). The vapor pressure of this heavy warming trends were punctuated by An especially powerful technique for water is less than for ‘‘normal’’ light water. abrupt changes. Climate shifts up to half correlation is to use the composition of As an air mass is cooled and precipitates, as large as the entire difference between atmospheric gases trapped in bubbles in it preferentially loses heavy water and ice age and modern conditions occurred the ice (6). Because most gases reside in must increasingly precipitate light water. over hemispheric or broader regions in the atmosphere long enough to be well At very low temperatures, heavy water has mere years to decades. Such abrupt mixed globally, ice cores around the world been greatly depleted and precipitation is changes have been absent during the few record the same atmospheric composition isotopically light. Empirically and theoret- key millennia when agriculture and indus- in bubbles trapped at the same time. Sev- ically, isotopic composition of precipita- try have arisen. The speed, size, and extent eral species show sufficiently variable his- tion and site temperature are strongly of these abrupt changes required a reap- tories to allow accurate correlations at correlated in time and space (10, 11); praisal of climate stability. Records of many times in the past. A complication is colder places and colder times have iso- topically lighter precipitation. these changes are especially clear in high- that air diffuses through spaces in snow Atmospheric and glaciologic factors resolution ice cores. Ice cores can preserve and firn (old snow) in the upper tens of other than temperature can affect the histories of local climate (snowfall, tem- meters, until the weight of additional snow isotopic paleothermometer, but several perature), regional (wind-blown dust, sea accumulation squeezes the firn to ice and traps bubbles. The trapped gas is thus a other paleothermometers allow calibra- salt, etc.), and broader (trace gases in the tion and validation. The physical temper- air) conditions, on a common time scale, little younger than the ice in which it occurs; uncertainty in this gas age͞ice age ature of the ice is important. Just as it demonstrating synchrony of climate takes a while to warm the center of cold changes over broad regions. difference complicates some interpreta- tions but still allows rather accurate dating food placed in a hot oven, deeper regions of the large ice sheets have not completed Ice-Core Interpretation in most cases (6, 7). In some circum- stances, the gas age͞ice age difference can warming from the low temperatures of the Dating and Accumulation. On some glaciers be determined precisely with gas-isotope previous global ice age, revealing how cold and ice sheets, sufficient snow falls each PERSPECTIVE anomalies that record rapid temperature the ice age was. Joint interpretation of the year to form recognizable annual layers, change, as detailed below (8). ice-isotopic and ice-temperature paleo- marked by seasonal variations in physical, The amount of ice between two time thermometers gives greater confidence in chemical, electrical, and isotopic proper- lines in a core, corrected for the layer the results (12, 13). ties. These can be counted to determine thinning from ice flow, is the snow accu- Additional paleothermometers are pro- ages (e.g., refs. 1 and 2). Accuracy can be mulation (9). The flow corrections range vided at times of rapid climate change. An assessed by comparison to the chemically from trivial and highly accurate to difficult abrupt air-temperature change causes a temperature difference between the snow identified fallout of historically dated vol- and uncertain, depending on the site and surface and the bubble-trapping depth, canoes and in other ways (3); errors can be its history. Buried snow drifts introduce SPECIAL FEATURE and this temperature difference then re- less than 1% of estimated ages. Ice flow noise in the records, and sublimation may laxes over a century or so as the deeper may disrupt layers quite close to the bed be important in especially low-accumula- layers adjust to the new surface tempera- (4, 5), and ice flow progressively thins tion zones, but accumulation typically pro- ture. Temperature gradients cause gas- layers with increasing burial so that diffu- vides a useful history of atmospheric de- isotope fractionation by the process of sion or sampling limitations eventually livery of snowfall to a site (9). obscure annual layers. thermal diffusion, with heavier isotopes migrating toward colder regions. Diffu- Where annual layers are not observed Paleothermometry. Ice cores are local pa- sion of gases through pore spaces in firn is because of depositional or postdeposi- leothermometers, telling past tempera- tional effects, by dating is conducted by faster than diffusion of heat, so the isotope ture where they are (or where the snow signal reaches the bubble-trapping depth correlation to other well-dated records, fell, if glacier flow has caused ice in a core before the heat does, and the isotope radiometric techniques in favorable cir- to have come from a significant distance). anomaly is recorded as the air is trapped cumstances, and by ice-flow modeling if The classic paleothermometer is the sta- needed. Most ice lacks sufficient appro- ble-isotopic composition of water in the priate materials to allow precise radiomet- ice core (10). Natural waters typically con- Abbreviation: GISP2, Greenland Ice Sheet Project 2. ric dating, but mountain glaciers some- tain a fraction of a percent of isotopically *To whom reprint requests should be addressed. E-mail: times contain enough material for radio- heavy molecules (in which the hydrogen or [email protected]. PNAS ͉ February 15, 2000 ͉ vol. 97 ͉ no. 4 ͉ 1331–1334 Downloaded by guest on September 24, 2021 in the bubbles (8). The degree of enrich- enough that when methane sources are deep cores (Fig. 1) in central Greenland ment reveals how big the temperature predominantly in the Northern Hemi- often are used as reference standards for difference was, and thus the magnitude of sphere, Greenland ice shows significantly abrupt climate changes. These records any abrupt climate change. In addition, higher methane concentrations than sim- provide annual resolution for some indi- the number of annual layers between the ilar-age samples from the Antarctic; cators through 110,000 years (older ice has record in the ice and in the bubbles of an hence, changes in the concentration dif- been disturbed by ice flow; refs. 4 and 5) abrupt climate change is a known function ference between Greenland and Antarc- and provide an exceptionally clear picture of temperature and snow accumulation; tica record changes in the latitudinal dis- of events in Greenland (temperature and using snow-accumulation data, one can tribution of methane sources (5, 22–24). accumulation), regionally (wind-blown learn the absolute temperature just before sea salt and continental dust), and more the abrupt climate change (8). Ice-Core Results broadly (trapped-gas records, especially of These paleothermometers agree closely Changes in Greenland. The ice-core records methane). on the size, speed, and timing of surface- from the Greenland Ice Core Project and The Greenland records show that climate temperature changes in central Green- Greenland Ice Sheet Project 2 (GISP2) changes have been very large, rapid, and land. Results from other regions rest on fewer paleothermometers and are some- what less secure, especially in meteorolog- ically complex areas (10, 14). Aerosols. Anything in the atmosphere eventually can end up in an ice core. Some materials are reversibly deposited (15), but most remain in the ice unchanged. The details of the air-snow transfer process are very complex but are being elucidated (16). Careful statistical and physical anal- yses are needed to make sense of small, short-lived changes, but large changes in concentrations of most materials in ice reflect changes in their atmospheric load- ing, with high confidence (16, 17). Isotopic composition of dust allows ‘‘fingerprinting’’ of source regions (18). Major ions provide information on sea salt, continental dust, and biogenic con- tributions; pollen tracks productivity on land nearby; methane sulfonate responds to oceanic productivity; and other insights are possible. Cosmogenic and extraterres- trial indicators also are of interest for some studies. Gases. Trapped gases in ice-core bubbles are highly reliable records of atmospheric composition, as shown by intercompari- sons among cores from different ice sheets and intercomparison with instrumental records and the air in firn above the bubble-trapping depth (19, 20). The slight differences between bubble and air com- position caused by gravitational and ther- mal effects are well understood and rec- ognizable (8). Chemical reactions in im- pure ice can produce anomalous compositions for some gases (21). How- ever, the ice chemistry warns of trouble, Fig. 1. High-resolution data from the GISP2 ice core, Greenland, and the Byrd ice core, Antarctica, and the close association of the gas and covering the Younger Dryas interval (YD) and adjacent times, modified slightly from ref. 37. (a) Byrd ice-chemistry anomalies, rather than be- ice-isotopic data (6, 38, 39). (b–e) GISP2 data mostly from ref.