"This is the first study to show that human activities have led to an increase in upper tropospheric water vapour," Brian Soden of the University of Miami Rosenstiel School of Marine and Atmospheric Science told environmentalresearchweb. "This is important because upper tropospheric water vapour provides an important positive feedback, which greatly amplifies the sensitivity of the climate system to external forcings like the increase in carbon dioxide from burning fossil fuels."

To come up with the results, Soden and colleagues compared satellite measurements of water vapour from 1979 to 2005 with levels projected by climate models from the Coupled Model Intercomparison Project (CMIP5). After 2005 the type of satellite measurement changed so the team was unable to look beyond this date.

"The methodology itself is pretty standard within the climate-change community," said Soden. "We compared observations of upper tropospheric water vapour to two sets of model simulations. One set only considered natural causes of climate change, e.g. volcanoes, solar cycles, natural climate cycles like El Niño. The other set contained both natural causes and anthropogenic causes, e.g. emissions of carbon dioxide from the burning of fossil fuels. The models with only natural causes were unable to reproduce the observed increase in upper tropospheric water vapour, but the models with both natural and anthropogenic causes were."

According to Soden, the study verifies the largest positive feedback in climate models and so adds further credibility to model projections of future anthropogenic climate change.

"Although the absolute increase in water vapour is small at these [upper troposphere] levels, the absorptivity scales with the fractional changes in water vapour, which are typically 2–3 times larger in the upper troposphere compared with the surface," writes the team in PNAS.

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