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

Abstract

A WIDESPREAD retreat of alpine glaciers¹ 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 documented⁸. 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 years⁷. 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 simulation⁹. The comparison strongly indicates that the observed changes in freezing-level height (the altitude of the 0°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.