A New Wrinkle in the Role of Clouds in Climate Change

Clouds continue to be one of the greatest sources of uncertainty in climate change models. At night, clouds keep Earth warm by preventing heat from escaping. During the day, most clouds cool Earth by keeping some sunlight from reaching the surface. Understanding the balance of these opposite effects on a global scale, as clouds constantly form, move, and break up, is a major challenge to climate forecasting.

Jasper Lewis and Simone Lolli, scientists at UMBC’s Joint Center for Earth Systems Technology, contributed to new research that reveals clouds’ effects on climate may be even more complex than previously understood. The study focuses on cirrus clouds, the wispy ribbons that float up to 20 km above Earth’s surface.

Cirrus clouds are known to contribute minimally, if at all, to cooling during the day because they are so thin. Lewis and Lolli’s study was the first to show that cirrus clouds can sometimes cool Earth during the day, depending on factors like the cloud’s thickness, height, and the temperature. “A cooling effect during daytime was something before known only in theory,” says Lolli, “but for the very first time we verified it with measurements.”

The study, led by the Naval Research Laboratory in Monterey, California and in collaboration with UCLA, analyzed 2012 data collected by the NASA Micro Pulse Lidar Network (MPLNET) at Goddard Space Flight Center. The team’s previous work used data from NASA’s CALIPSO, an orbiting satellite.

“With the satellite you get many locations, but only one measurement per day,” said Lolli. “With ground-based instruments you can take measurements 24 hours per day.”

These measurement capabilities offer a huge advantage if you’re looking for variations between day and night. The copious data also allow the team to hypothesize how cirrus clouds affect climate differently across seasons and latitudes, which is critical to understanding global energy balance.

The research improved on previous work because Lewis designed new protocols to analyze the data, taking into account the huge size of cirrus clouds (up to hundreds of kilometers across) and their high elevation, which creates noise in the data.

Lolli’s task was to input the data into an existing model to determine whether the clouds were warming or cooling the atmosphere, a process known as “climate forcing.” Data input posed a major challenge, as the type of data MPLNET collected was not in a form the model could use. Lolli had to convert the data into something the model could accept, which required significant time, attention to detail, and, as Lolli puts it, a lot of math.

Recognizing the team’s innovative methodology and groundbreaking study results, the Naval Research Laboratory just honored Lewis and Lolli’s paper with the 2016 Alan Berman Research Publication Award. According to the award ceremony program, these competitive awards “not only honor individuals for superior scientific accomplishments in the field of naval research, but also seek to promote continued excellence in research and its documentation.”

For Lewis, the best part about receiving the award “is recognition of the significance of the work that you’ve done.” Lolli adds, “It’s very important, because it’s motivation for others to get going in this area of research.”

“Uncertainty about climate forcing is one of the big things that people don’t fully understand yet” in climate science, Lewis shares, “so this is a small step in the direction of moving that understanding a bit further.”

Read “Daytime cirrus cloud top-of-the-atmosphere radiative forcing properties at a midlatitude site and their global consequences“ in the Journal of the American Meteorological Society.