Tropical Glacier Retreat

In a previous post entitled Worldwide Glacier Retreat, we highlighted the results of a study by J. Oerlemans, who compiled glacier data from around the world and used them to estimate temperature change over the last ~400 years. A question that arose in subsequent online discussion was to what extent Oerlemans had relied on glaciers from tropical regions (answer: he didn’t), and what the reasons are behind retreat of glaciers in these regions. Raymond Pierrehumbert, a climate dynamicist at the University of Chicago, kindly offered to write a guest editorial to further clarify what we do and don’t know about tropical glacier retreat. Pierrehumbert’s editorial follows below. –eric

1. INTRODUCTION

The Tropics, loosely defined as the region from 30N to 30S latitude, make up nearly half the surface area of the Earth; they are home to 70% of its people, and the vast majority of its biological diversity. Moreover, the tropical region is the “accumulation zone” for the Earth’s energy balance, with a great deal of excess solar energy being exported to help warm the rest of the planet. Detection and characterization of climate change in the Tropics is therefore a matter of great concern. Assessing the ability of climate models to reproduce this change is an important part of determining the fidelity with which the models can be expected to forecast the way climate will change in response to future increases in greenhouse gas content.

Figure 1: The Qori Kallis Glacier in the Peruvian Andes

Throughout the Tropics, glaciers are in retreat. Well-documented examples include Quelccaya [Thompson, et al. 1993], Huascaran [Byers, 2000; Kaser and Osmaston,2002], Zongo and Chacaltaya [Francou,et al 2003; Wagnon et al. 1999] in S. America; and the Lewis, Rwenzori and Kilimanjaro (more properly, Kibo) glaciers in East Africa [Hastenrath, 1984; Kaser and Osmaston, 2002]. There have been indications of widespread retreat of Himalayan glaciers, including Dasuopu in the subtropics, but a quantitative understanding of this region must await peer-reviewed analysis of the recently completed 46000-glacier Chinese Glacier Inventory. The case of Quelccaya, in the Andes, is especially interesting, because it provides direct evidence of an unusual recent warming trend. When the summit core was first drilled in 1976, the chemical composition of the ice showed well-preserved annual layering throughout its depth, accounting for a time span of 1500 years. When attempts were made to update the record by redrilling in 1991, it was found that the annual cycle had been wiped out over the top 20 meters of the core by percolation of meltwater from extensive melting of the ice surface since 1976. Melting of this sort had not occurred at the summit at any time during the previous 1500 years, and indicates an increase of 150 m, between 1976 and 1991, of the altitude at which significant melting occurs. A vivid animation showing the retreat of the Qori Kalis glacier flowing out of Quelccaya can be seen here.

The widespread retreat is all the more notable because tropical mountain glaciers are old. They have survived thousands of years of natural climate fluctuations, only to dwindle at a time when other climate indicators — notably surface temperature — are showing the imprint of human influence on climate. Quelccaya is at least 1500 years old, Dasuopo is 9000 years old, and Huascaran has seen 19000 years. A date for the ultimate demise of these glaciers has not been fixed, but the Northern Ice Field on Kilimanjaro may be gone in as little as twenty years, after having survived the past 11,000 years.

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