THE ROGER REVELLE COMMEMORATIVE LECTURE SERIES Abrupt Climate Changes: Oceans, Ice, and Us BY RICHARD B. ALLEY Presenting the Sixth Annual Roger Revelle Commemorative Lecture given November 10, 2004 at the Baird Auditorium at the Smithsonian Museum of Natural History in Washington, D.C. This lecture series was created in 1999 by the National Research Council’s Ocean Studies Board in honor of the late Dr. Roger Revelle, a leader in the field of Oceanography for almost a half a century, in order to highlight the important links between ocean sciences and public policy. SYNOPSIS change. Although a major, potentially community recognized that variabil- Modern climate science indicates that rapid change in climate is a daunting ity includes very large and widespread rising carbon dioxide levels in the at- prospect, I am convinced that today’s abrupt climate changes. The relatively re- mosphere and other effects of human students, if given suffi cient training and cent revelations gleaned from the climate activities will cause large climate changes fi nancial support, will be able to address record tell us that change will occur in requiring signifi cant adaptation by hu- the climate challenges of the future. fi ts and starts, with potentially large and man societies around the globe. Climate rapid jumps and detours along the way. records and human history provide in- INTRODUCTION: A CHALLENGE After presenting some of the evidence sight into the nature of climate change How should we handle our climate fu- for abrupt climate changes and describ- and the types of challenges a shifting ture? In trying to decide what if anything ing the likely causes, I will discuss wheth- environment will pose for humankind. to do about climate change, policy-mak- er the past has anything to tell us about The geologic record shows that climate ers may consider a wide range of future the future (the answer is yes). sometimes changes abruptly, but this as- scenarios, with larger or smaller, scarier Then I will offer a few opinions about pect of climate history has received rela- or more acceptable changes. But almost the future based on the science of cli- tively little attention in efforts to under- all those scenarios postulate change that mate, my impressions of recent environ- stand the consequences of future climate occurs smoothly and gradually, leaving mental history, and the potential of this the government and the governed plenty generation of students. Richard B. Alley ([email protected]) of time to respond. is Evan Pugh Professor, Department of This view of climate is surprising be- MIRED IN MUD Geosciences and Earth and Environmental cause Earth’s climate history is anything Earth’s history is recorded in sediments. Systems Institute, Th e Pennsylvania State but smooth. Variability is the rule, not Because plants and animals that grow in University, University Park, Pennsylvania, the exception. However, only in the past warm environments can be readily dis- United States of America. decade or so has the broader scientifi c tinguished from those that grow in cold Th is article has been published in Oceanography, Volume 17, Number 4, a quarterly journal of Th e Oceanography Society. Copyright 2003 by Th e Oceanography Society. All rights reserved. Reproduction of any portion of this article by photo- 194 Oceanography Vol.17, No.4, Dec. 2004 copy machine, reposting, or other means without prior authorization of Th e Oceanography Society is strictly prohibited. Send all correspondence to: [email protected] or 5912 LeMay Road, Rockville, MD 20851-2326, USA. environments, variation in the plant and local temperatures changing as much as animal skeletons found in sediment lay- 10°C (18°F) or more over a decade or ers indicates that there were shifts in the so. Additionally, the climate shift moved climate at that location. For instance, quickly, spreading around much of the skeletons of planktonic microorganisms globe within about a decade, possibly as that accumulate on the seafl oor beneath quickly as a single year. the warm waters of the Gulf Stream are readily distinguished from those of INTO THE ICE nearby cooler waters. Similarly, pollen Although sediments of ocean, lake, and that accumulates on a tundra lake bed bog fl oors have provided important has little in common with pollen from evidence of past climate changes, it is a temperate or tropical forest. Sediment diffi cult to resolve how rapidly events cores have revealed alternating accumu- occurred because sediments often accu- lations of layers of cold- and warm-water mulated very slowly. Burrowing animals organisms. Sometimes these layers occur literally can stir up the past, thus, dis- close together, suggesting sudden shifts rupting parts of the record. Ice provides in conditions. higher resolution and better preserva- Figure 1. Ice cores, such as this one from Of particular interest, European bogs tion of past events, inspiring scientists Greenland Ice Sheet Project 2 (GISP2) in cen- showed that warming since the ice age to seek ice cores from Antarctica, high tral Greenland, produce wonderful records of staggered back to cold conditions several mountains, and, for my story, the ice climate change. Th e ice cores pictured here are being studied by Mark Meier (left) and the 2 times, with one especially prominent re- sheet of Greenland (Figure 1). author (right) at the National Science Founda- versal. This most recent major cold inter- Snow that falls on an ice sheet is grad- tion-U.S. Geological Survey’s U.S. National Ice Core Laboratory, located in Denver, Colorado. val (about 12,800 years ago) was called ually squeezed into ice, which piles up the Younger Dryas after a tundra fl ower over time. The ice in Greenland is now (Dryas octopetala) that appears in the hundreds of kilometers across and three mud record from that time, but disap- kilometers (about two miles) thick in pears when European conditions become the middle. The ice spreads slowly under temperate. Conditions characteristic of its own weight like pancake batter on a a tree. As snow falls, it traps bits of dust, cold periods, such as the regrowth of gla- griddle, dripping off the edges of Green- sea salt, smoke from forest fi res, volcanic ciers, appear to have started and ended land to form icebergs or melting in low- ash, and other materials from the atmo- abruptly during the Younger Dryas1. elevation coastal regions. The rate of ice sphere. Bubbles get trapped when the But how abruptly? How much cooling loss has been close to the rate of snowfall, snow is squeezed into ice, thus capturing occurred? How much of the world was keeping the volume of the ice sheet rela- a time capsule of air that can be analyzed affected? The discoverers of the Younger tively constant over the last millennia. for gases that fl uctuate with climate, such Dryas lacked the technology to fi nd Summer sunshine changes the physi- as methane and nitrous oxide. Measure- out, but now we are learning that the cal properties of snow, providing a sea- ments of subtle isotopic indicators and changes were rapid and dramatic, with sonal marker in the ice like the rings of trapped gases have been used to de- 1Weart, S, 2003, Th e discovery of rapid climate change, Physics Today 56(8), 30-36. 2Much has been written about Greenland ice cores, spanning at least hundreds and perhaps thousands of scientifi c papers now. I am biased in favor of the popular account in Alley, R.B., 2000, Th e Two-Mile Time Machine, Princeton University Press, New Jersey. For a more detailed starting point to the literature, see Hammer, C., P.A. Mayewski, D. Peel, and M. Stuiver, 1997, Preface to Greenland Summit Ice Cores, Journal of Geophysical Research 102(C12), 26,315-26,316, and all other papers in the same issue (approximately 47 papers) from pages 26,315 to 26,886. Th e paper by Severinghaus, J.P., T. Sowers, E.J. Brook, R.B. Alley, and M.L. Bender, 1998, Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice, Nature 391(6663), 141-146 is especially relevant in showing how widespread and abrupt the climate changes were. Abrupt climate change in general is treated in National Research Council, 2002, Abrupt Climate Change: Inevitable Surprises, National Academies Press, Washington, D.C. Oceanography Vol.17, No.4, Dec. 2004 195 termine the temperature in Greenland years ago. The ice ages were paced by few years (Figures 4 and 5). These jumps when the snow fell. The actual tempera- slow changes in Earth’s orbit. Remark- were accompanied by order-of-magni- ture of the ice today also provides evi- ably, the orbital changes had little effect tude changes in dustiness and almost dence of past cooling—the ice is coldest on the total energy received from the two-fold shifts in atmospheric methane. 1 to 2 km down (about a mile) because sun, serving instead to move sunshine When Greenland was cold and dry, more of the lingering chill from the ice age. around on the planet. Cooling occurred dust and less methane reached Green- By reading the records in a Greenland worldwide when sunshine dropped in land, indicating generally cold, dry, and ice core—temperature and snowfall in the far north, because northern sunshine windy conditions across much of the Greenland, wind-blown dust from Asia affected atmospheric carbon-dioxide globe. Greenland’s snowfall seems to (fi ngerprinted by its unique chemical concentrations and thus global tempera- have doubled in a single year at the end composition and minerals), methane tures—Antarctica experienced especially of the Younger Dryas.