The Earth's tropical regions are very important for understanding current and past climate change as 50% of the Earth's surface lies between 30°N and 30°S, and it is these regions where most of the sun's energy that drives the climate system is absorbed. The tropics and subtropics are also the most populated regions on the planet. Paleoclimate records reveal that in the past, natural disruptions of the climate system driven by such processes as large explosive volcanic eruptions1 and variations in the El Nino-Southern Oscillation2 have affected the climate over much of the planet.3 Changes in the vertical temperature profile in the tropics also affect the climate on large-spatial scales. While land surface temperatures and sea surface temperatures show great spatial variability, tropical temperatures are quite uniform at mid-tropospheric elevations where most glaciers and ice caps exist. Observational data before, during and after a major El Nino event demonstrate that within four months of its onset, the energy from the sea surface is distributed throughout the tropical mid-troposphere.4 Thus, the observation that virtually all tropical regions are retreating5 under the current climate regime strongly indicates that a large-scale warming of the Earth system is currently underway.
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