Climate models have long predicted that the upper troposphere — a region of the Earth’s atmosphere that lies beneath the stratosphere at an altitude of 10–12 km — should be warming at least as fast as the surface. However, since the 1970s temperature measurements carried out by weather balloons have found the lower-troposphere temperature to be fairly constant. This conclusion was backed up in 1990, when researchers used data taken from satellites to measure temperature changes in the troposphere.

For a while climate scientists have known that weather-balloon instruments are affected by the warming effect of the Sun’s light. They have also struggled to interpret the extent to which the satellite data of the troposphere could be influenced by the stratosphere. But the awareness of these uncertainties has not made it any clearer as to what temperature changes, if any, are taking place in the upper troposphere.

Now, Robert Allen and Steven Sherwood of Yale University have used wind data taken from weather balloons as a proxy for direct temperature measurements to give the first conclusive evidence that the upper troposphere has been warming after all. Although they are an indirect measure of temperature, these wind records can be backed up by satellite and ground instruments, making them more reliable than existing direct temperature measurements (Nature Geoscience doi: 10.1038/ngeo208).

‘Put the controversy to rest’

Allen and Sherwood took wind data from 341 weather-balloon stations — 303 in the northern hemisphere and 38 in the southern hemisphere — covering a period from 1970 to 2005. To covert the data to temperature measurements, they employed a relationship known as the thermal-wind equation, which describes how vertical gradients in wind speed change with horizontally varying temperature. They found that the maximum warming has occurred in the upper troposphere above the tropics at 0.65 ± 0.47 °C per decade, a rate consistent with climate models.

“This research really does show the tropical troposphere has been warming over the past three decades,” says Benjamin Santer of Lawrence Livermore National Laboratory. “And it will, I hope, put this controversy of weather balloon and satellite data to rest.” Santer, who was one of the lead authors of the 1995 report by the Intergovernmental Panel on Climate Change, thinks the next step is to confirm Allen and Sherwood’s findings with direct temperature records. These, he explains, must be taken with advanced weather-balloon instruments that can be calibrated against older models to remove biases.

“The approach by Allen and Sherwood is a promising start,” says John Lanzante of Princeton University. “But more confidence can be established as other investigations further scrutinize the wind data and method used to translate winds into temperature-equivalent measures.”