The intricate ways that water vapor, ice crystals, and other aerosols combine in Earth's atmosphere can dramatically alter the overall brightness of the planet, creating a unique signature that can be detected from space. Known as the planetary emission fingerprint, this signal may not only help climate scientists better observe the effects of global warming but could also one day be used to assess the habitability of distant exoplanets. Understanding its characteristics is challenging, however, because planetary emissions are determined not just by the total amount of water vapor, aerosols, and ice in the scene—collectively known as relative humidity—but also by the vertical structure of the atmosphere, which until now has been difficult to measure.

Now, a team led by researchers from the U.K.'s University of Leicester and involving participants from the U.S. Department of Energy's (DOE's) Brookhaven National Laboratory has identified a way to infer details of the vertical makeup of Earth's atmosphere from passive observations. Their method involves making measurements from multiple wavelengths and combines ground-based observations with data from NASA satellites, such as the Atmospheric Infrared Sounder (AIRS). The team, including Brookhaven atmospheric scientist Hang Sun, reports their findings in the journal Geophysical Research Letters.

"It's well-known that clouds and aerosols have an impact on the brightness temperature of the atmosphere. But to understand climate—either present-day or on exoplanets—we also need to understand the vertical distribution of aerosols and water vapor," said Stephen English of the University of Leicester's School of Physics and Astronomy, lead author of the study. "Until now, we've only had a handful of snapshots from active sensors onboard satellites that can provide that information. Passive measurements cover the entire globe in much more detail, but they miss the vertical structure."

This team found a clever workaround that draws on previous theoretical work conducted by coauthor Paul O. Wennberg, also of the University of Leicester. When atmospheric conditions are just right—warm and moist, but with a cool surface—infrared radiation emitted by the planet's surface is almost completely absorbed by the low layers of the atmosphere. As the radiation rises, much of it is released, and some makes it all the way out into space. The leftover radiation is the part that instruments sitting on satellites passively detect, and its spectral signature can be subtly altered by the vertical structure of the atmosphere.

"These very warm and moist conditions happen over tropical regions where there is a lot of deep convection," Sun said. "These convection plumes are highly efficient in moistening and cooling the upper troposphere, and this changes the vertical structure of the atmosphere."

The team used two wavelengths—one highly sensitive to water vapor, the other to the combined impact of water vapor and aerosols—to identify conditions where they could best observe the vertical structure of Earth's atmosphere from space. They found that regions dominated by deep convection can be used as "windows" to probe the higher reaches of the atmosphere. They can also provide information about the lower part of the atmosphere at the same time, helping to distinguish between water vapor, clouds, and aerosols.

"With this method, we can retrieve vertical profiles in both the upper troposphere/lower stratosphere and lower troposphere for specific conditions," Sun said. "This can be valuable information for climate and Earth system models."

The team hopes their method can be applied to observations made by current and upcoming polar-orbiting satellites to infer the vertical structure of Earth's atmosphere under warm, moist conditions across the entire globe. As technology continues to improve, this method could be applied to detect potential signs of habitability on distant exoplanets.