Abrupt Tropical Climate Change: Past and Present

Abrupt tropical climate change: Past and present Lonnie G. Thompson*†‡, Ellen Mosley-Thompson*§, Henry Brecher*, Mary Davis*, Blanca Leo´ n¶, Don Lesʈ, Ping-Nan Lin*, Tracy Mashiotta*, and Keith Mountain** *Byrd Polar Research Center and Departments of †Geological Sciences and §Geography, Ohio State University, Columbus, OH 43210; ¶Herbarium, Plant Resources Center, University of Texas, Austin, TX 78712; ʈDepartment of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269; and **Department of Geography and Geosciences, University of Louisville, Louisville, KY 40292 This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on May 3, 2005. Contributed by Lonnie G. Thompson, May 12, 2006 Three lines of evidence for abrupt tropical climate change, both fields, which have persisted for at least 11,000 yr and have lost 82% past and present, are presented. First, annually and decadally of their areal extent since 1912 (8). Aerial photographs taken in averaged ␦18O and net mass-balance histories for the last 400 and January 2006 confirm the continued reduction of ice cover. 2,000 yr, respectively, demonstrate that the current warming at The chemical and physical properties preserved within glaciers high elevations in the mid- to low latitudes is unprecedented for at and ice caps also record climatic and environmental changes that least the last 2 millennia. Second, the continuing retreat of most provide a longer-term context for 20th-century changes. When mid- to low-latitude glaciers, many having persisted for thousands the first two Quelccaya cores were drilled in 1983, it was of years, signals a recent and abrupt change in the Earth’s climate impossible to return frozen samples from the field; thus, the system. Finally, rooted, soft-bodied wetland plants, now exposed cores were cut into Ϸ6,000 samples that were melted, bottled, along the margins as the Quelccaya ice cap (Peru) retreats, have and returned to the Byrd Polar Research Center at Ohio State been radiocarbon dated and, when coupled with other widespread University, where their dust content and liquid conductivity were proxy evidence, provide strong evidence for an abrupt mid-Holo- measured. The ␦18O measurements were made at the University cene climate event that marked the transition from early Holocene of Copenhagen and the University of Washington (3). Stable (pre-5,000-yr-B.P.) conditions to cooler, late Holocene (post-5,000- isotopic ratios (␦18O, ␦2H) preserved in mid- to low-latitude yr-B.P.) conditions. This abrupt event, Ϸ5,200 yr ago, was wide- glaciers provide a valuable tool for paleoclimate reconstruction spread and spatially coherent through much of the tropics and was and, when they are combined with other ice-core properties such coincident with structural changes in several civilizations. These as dust concentrations, aerosol chemistry, and accumulation three lines of evidence argue that the present warming and rates, a richer picture of past regional climate conditions associated glacier retreat are unprecedented in some areas for at emerges. In 2003, a new ice core was drilled to bedrock on the least 5,200 yr. The ongoing global-scale, rapid retreat of mountain summit of Quelccaya, but this time it was returned frozen to Byrd glaciers is not only contributing to global sea-level rise but also Polar Research Center, where concentrations of dust and various threatening freshwater supplies in many of the world’s most chemical species and the stable isotopic ratios were measured. populous regions. A.D. 1600 to the Present icecore ͉ isotopes ͉ paleoclimate ͉ precipitation ͉ Holocene Fig. 3 illustrates the seasonally varying constituents used to con- struct a highly accurate time scale for the 2003 Quelccaya core. oday most glaciers outside the polar regions are retreating at Comparison of the annually averaged ␦18O records (A.D. 1540– Taccelerating rates (1, 2). This retreat is particularly evident in 1983) from the 1983 core (blue) and 2003 core (red) confirms that the high Andes of Peru, which contain 70% of the Earth’s tropical the time scales are virtually identical (Fig. 4). This agreement is glaciers. The first long tropical ice cores were retrieved in 1983 from significant for two reasons. First, it finally confirms the time scale the Quelccaya ice cap (Fig. 1), the Earth’s largest tropical ice cap, for the 1983 Quelccaya core. Second, it demonstrates that the ␦18O which is 5,670 m above sea level in the southeastern Andes of Peru record has been excellently preserved within the ice cap even (3). Because of its dome shape, Quelccaya is very sensitive to the though its surface has been subjected to warming conditions for the current increase in the elevation of the 0°C isotherm (4), and it loses intervening 20 yr. Thus, the Quelccaya ␦18O record, if correctly more mass per unit rise in the equilibrium line than steeper alpine interpreted, provides an Ϸ1,500-yr context for the recent period of glaciers (5). This sensitivity makes Quelccaya an obvious choice for accelerating glacier retreat. ␦18 monitoring changes in both ice mass and ice-cover extent. The O–air temperature (Ta) relationship is fairly well estab- The retreat of Quelccaya’s largest outlet glacier, Qori Kalis, has lished for Antarctica and Greenland (9) but remains more contro- been evaluated by terrestrial photogrammetry since 1978 (6). These versial for high mountain regions in mid- to low latitudes. Empirical ␦18 observations, along with a map produced from aerial photographs investigations are limited. It was found in a study of the O–Ta taken in 1963, allow changes in ice area and volume over the last 42 relationship for discrete precipitation events at three sites on the yr to be assessed. A sequence of maps documents the rapid and Tibetan Plateau (TP) that although dynamical processes dominate accelerating retreat of the glacier front (Fig. 2A). In the last 14 yr ␦18O on short time scales, air temperature is the primary control on (1991-2005), Qori Kalis has been retreating Ϸ10 times faster (Ϸ60 longer-term (more than annual) ␦18O variations at individual sites m⅐yrϪ1) than during the initial measurement period of 15 yr from (10). In the Andes, the controls on ␦18O in recent snowfall have 1963 to 1978 (Ϸ6m⅐yrϪ1). A small proglacial lake that first been investigated by using observations (11) and models (12, 13). appeared in 1991 and covered Ϸ6 hectares has grown contempo- Strong linkages have been demonstrated (14) between sea-surface raneously with the retreat of the ice front and now covers Ϸ34 temperatures across the equatorial Pacific Ocean and ␦18Oinice hectares (Fig. 2C). On the basis of sonar observations in 2004, the lake is up to Ϸ60 m deep. The accelerating rate of retreat of the Qori Kalis terminus is consistent with that observed for six other Conﬂict of interest statement: No conﬂicts declared. glaciers in the Cordillera Blanca (Fig. 2B) that have been monitored Freely available online through the PNAS open access option. by the power company ElectroPeru (7). This recent and rapid loss Abbreviations: TP, Tibetan Plateau; LIA, Little Ice Age; ka, thousand years (ago). of ice on Quelccaya is characteristic of most low- and mid-latitude ‡To whom correspondence should be addressed. E-mail: [email protected]. ice fields and is best typified by Kilimanjaro’s three remaining ice © 2006 by The National Academy of Sciences of the USA 10536–10543 ͉ PNAS ͉ July 11, 2006 ͉ vol. 103 ͉ no. 28 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0603900103 Downloaded by guest on October 1, 2021 INAUGURAL ARTICLE Fig. 1. The locations of ice cores and evidence for abrupt climate change Ϸ5,000 yr ago discussed in the text are shown along with areas of large-scale ice retreat. GEOLOGY cores from the tropical Andes and the Himalaya (Dasuopu glacier). in air temperature more strongly than variations in precipitation (6, More recently, it has been suggested (15) that ␦18O ultimately 10, 11, 16, 17). records changes in the atmospheric circulation that integrate all Over the past two decades, the Byrd Polar Research Center aspects of transport from source to sink. Although not fully ice-core group has recovered cores from ice fields across Tibet resolved, the weight of available empirical evidence, including that and in the Andes (see Fig. 1 for locations). Various aspects of presented in Fig. 5, strongly suggests that on longer time scales, ␦18O these records have been published, but they have not been variations in low-latitude mountain precipitation reflect variations integrated and interpreted collectively. Here the 5-yr averages of Fig. 2. Glacier retreat as documented in the Peruvian Andes. (A) Retreat of Qori Kalis from 1963 to 2005. (B) Retreat records for Qori Kalis and six other Andean glaciers. (C) The photos document the expansion of the proglacial lake from 1991 to 2005 as Qori Kalis retreated. Thompson et al. PNAS ͉ July 11, 2006 ͉ vol. 103 ͉ no. 28 ͉ 10537 Downloaded by guest on October 1, 2021 Quelccaya provides the only reliable ice-core-derived An history for the past 400 yr in the Andes. A recently reconstructed 400-yr high-resolution pollen history from Sajama (24) indicates wetter conditions from A.D. 1500 to 1700 and drier conditions from A.D. 1700 to 1880, generally consistent with the An record from Quelccaya to the north (Fig. 5B). Meteorological data show that annual precipitation decreases from east to west and from south to north across the TP, whereas in the South American Andes precipitation decreases from north to south (25). The ice fields located in more arid regions of the TP (Dunde, Guliya, Puruogangri), have rather similar An histories (Fig.

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