Ice core evidence for climate change in the Tropics: implications for our future

Reliable meteorological observations for climate reconstruction are limited or absent prior to A.D. 1850 for much of the Earth and particularly in bot h tropical South America and the Tibetan Plateau region of central Asia. Ov er 50% of the Earth's surface lies between 30 degrees N and 30 degrees S an d 75% of the world's inhabitants live and conduct their activities in these tropical regions. Thus, much of the climatic activity of significance to h umanity, such as variations in the occurrence and intensity of the El Nino- Southern Oscillation and monsoons, are largely confined to lower latitudes. Moreover, the variability of these tropical systems and particularly that of the tropical hydrological system in response to regional and global clim ate forcing are not well understood. Fortunately, ice core records are also available from selected high altitude, low and mid-latitude ice caps. The ice core studies described here were undertaken as part of a long-term prog ram to acquire the global-scale, high-resolution climatic and environmental history essential for understanding more fully the linkages between the lo w and the high latitudes. Two ice core records, one covering the last full glacial cycle from the Guliya Ice Cap, China (35 degrees N; 6200 m asl) and one from Huascaran, Peru, which reveal significant cooling during the Last Glacial Cycle Maximum (LGM similar to 20,000 yr BP) are compared with prel iminary data coming from the new Sajama, Bolivia (18 degrees S, 6550 m asl) and the Dasuopu, Himalaya (China, 28 degrees N, 7200 m asl) cores. Lower d elta(18)O values (equivalent to cooling of similar to 8 degrees C) contribu te to the growing body of evidence that the tropical climate was cooler and more variable during the last glacial cycle and has renewed current intere st in the tropical water vapor cycle. The new tropical ice core records rai se additional questions about our understanding of the role of the tropics in global climate. Unfortunately, as a result of recent warming, all known tropical glaciers and ice caps are retreating and soon will no longer conti nue to preserve viable paleoclimatic records. The characteristics of the cu rrent warming will be examined and compared to earlier periods of climatic warming such as the transition form the last glacial into the current inter glacial as well as other periods within the Holocene. It is important to di stinguish natural variation in the climate system from the anthropogenic in fluences superimposed during the: last century. These tropical ice cores of fer long-term perspectives of accumulation, temperature, atmospheric dust a nd "greenhouse" gas concentrations against which recent variations may be a ssessed, with particular relevance for lower latitude regions where most pe ople live. (C) 1999 Elsevier Science Ltd. All rights reserved.