Alaska’s North Slope is warming rapidly, causing sea ice, snow, permafrost, and glaciers to melt at unprecedented rates. Such changes have significant societal implications, impacting energy and transportation infrastructure, shipping and commerce, and food security.

A new CU Boulder-led study analyzing 26 years of winter observations from this region reveals that increasing cloud opacity—cloud thickness or density—helps explain why the surface retains more heat as temperatures rise. 

The work provides direct observational evidence that cloud changes amplify Arctic warming during the coldest, darkest months. 

CIRES scientists and collaborators from Brookhaven National Laboratory led the research, published today in Nature Communications. The team analyzed ground-based measurements of surface radiation and cloud observations collected during winter at a long-term Arctic observatory in Utqiaġvik, Alaska. The observations helped researchers better understand cloud feedback, or how changing clouds redistribute energy, and whether or not changing Arctic clouds are increasing or dampening warming in northern Alaska. 

“There aren’t a lot of constraints on how wintertime Arctic clouds respond to warming—a lot of different things might be happening,” said Leah Bertrand, lead author and CU Boulder PhD candidate. “But in Utqiaġvik, it turns out that changing clouds were the decisive factor in less energy leaving the surface with warming.”

The research team used data from on-the-ground instruments operated by the Department of Energy’s (DOE) Atmospheric Radiation Measurement (ARM) User Facility. The site has been stationed on Alaska’s northern tip in Utqiaġvik since the 1990s. The North Slope facility is one of two long-term DOE ARM atmospheric observatories in the U.S., and consists of shelters and platforms that house over 50 state-of-the-art instruments built to collect atmospheric measurements at high latitudes.

The instruments survey the atmosphere around the clock to measure heat, water vapor, and clouds. Long-term data show temperatures in the region have increased. The data here is unique—and valuable. The instruments used in the study observe lower levels of the atmosphere, including clouds and radiation, and help scientists track and evaluate changes over time.  

“This Alaska DOE ARM facility provides valuable long-term observations for studies like this one,” said Jennifer Kay, CIRES Fellow, Atmospheric and Oceanic Sciences professor, and co-author of the study. “It also offers environmental insights for people living and working in and around the northernmost city of the United States.”

While many researchers have used data from this DOE site to study Arctic clouds, CIRES scientists are the first to pinpoint how changing opacity in Arctic clouds—specifically the increasing ice and water content of clouds—accelerates surface warming. 

To isolate clouds and their impact on surface warming, the team focused on data collected from December to March each year, when Alaska experiences near-total darkness, coupled with colder temperatures. The observations reveal that wintertime clouds are becoming more opaque, with both ice clouds and liquid-containing clouds trapping more heat. Researchers found that these cloud changes are the key reason why more heat remains at the surface as temperatures rise during winter, rather than escaping to space. Such heating can have significant implications on permafrost and sea ice, which are both critical to energy industry efforts in this environment.

For this paper, Bertrand made a strategic decision to study Arctic clouds in the winter to eliminate the sun’s impact on energy. Up next, she wants to study the different changes in Arctic clouds throughout different seasons. 

“It’s really exciting to put a new piece in the puzzle of Arctic clouds,” Bertrand said. “By adding sunlight into the mix, we’ll be able to take an even broader look at what’s going on and what role clouds play in it.”