TROPICAL ICE CORE AND GLACIER RETREAT EVIDENCE FOR ABRUPT HOLOCENE CLIMATE CHANGE

The 20th Century has seen the acceleration of unprecedented global and regional-scale climatic and environmental changes to which humans are vulnerable, and by which we will become increasingly more affected in the coming centuries. One-half of the Earth's surface area lies in the tropics between 30oN and 30oS, and this area supports about 70% of the global population. Thus, temporal and spatial variations in the occurrence and intensity of coupled ocean-atmosphere phenomena such as El Niño and the Monsoons, which are most strongly expressed in the tropics and subtropics, are of world-wide significance. Ice core records are available from low-latitude, high-altitude glaciers, and when they are combined with other high-resolution proxy histories provide an unprecedented view of the Earth's climatic history over many millennia. This paper provides an overview of these unique glacier archives of past climate and environmental changes on millennial to decadal time scales with special emphasis on abrupt climate change in the Holocene. Also included is a review of the recent, global-scale retreat of these alpine glaciers under present climate conditions, and a discussion of the significance of this retreat with respect to the longer-term perspective which can only be provided by the paleoclimate records. The abrupt climate change now underway in the Peruvian Andes is placed within a longer time perspective using tropical ice core records and carbon dating of plant material exposed by the retreating ice. A ~5,200 year old plant, growing above its current altitudinal range, indicates warmer Early Holocene conditions, consistent with stable isotopic records from tropical ice cores. Multiple lines of evidence from Africa, the Middle East, Europe to South America indicate an abrupt mid-Holocene climate event marking the transition from early ‘Hypsithermal' conditions to cooler, late Holocene ‘Neoglacial' conditions. This abrupt ‘cold' event was widespread and spatially coherent through much of the tropics and was coincident with structural changes within several civilizations.