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.