Recent changes in tropical freezing heights and the role of sea surface temperature

A WIDESPREAD retreat of alpine glaciers' and melting of tropical ice-cap margins(2-7) has been observed in recent decades, over which time a general climate warming at lower altitudes has been documented(8). Moreover, some ice-core records provide evidence suggesting that mid-tropospheric temperatures in the tropics have been greater in recent decades than at any time during the past 2,000-3,000 years(7). Here we examine the processes controlling mountain glacier retreat by comparing high-altitude air-temperature measurements for the past few decades, to the temperatures predicted by a model atmosphere forced by the observed global pattern of sea surface temperature in a 19-year simulation(9). The comparison strongly indicates that the observed changes in freezing-level height (the altitude of the 0 degrees C isotherm) are related to a long-term (over decades) increase in sea surface temperature in the tropics, and the consequent enhancement of the tropical hydrological cycle. Although changes in this cycle are likely to affect high-elevation hydrological and ecological balances worldwide(10,11), tropical environments may be particularly sensitive because the changes in tropical sea surface temperature and humidity may be largest and most systematic at low latitudes.