Recent Changes in tropical feeezing heights

A widespread retreat of alpine glacier and melting of tropical ice-cap margins has been observed in recent deades, over which time a general climate warming at lower altitudes has been documented. 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. 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 amodel atmosphere forced by the observed global pattern of sea surface temperature in a 19 year simulation. The comparison strongly indicates that the observed changes in the freezing-level height (the altitude of the 0 deg. 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 world- wide, 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.