Cloud Scientists Confirm Dust's Role in Cloud Freezing, Possibly Revising Climate Simulations

In a groundbreaking analysis published in Science, researchers have shed light on the impact of dust on cloud formation in Earth's atmosphere. The study, led by Diego Villanueva from ETH Zürich in Switzerland, reveals that dust particles from deserts play a significant role in causing cloud water droplets to freeze at warmer temperatures than they would otherwise.

The researchers focused on cloud tops, which are visible in satellite imagery, and analysed 35 years of satellite data on cloud tops across the Northern Hemisphere's extratropics. They found that regions with more dust had a higher proportion of ice-topped clouds, with the effect being particularly strong in summer when desert winds lift the most dust.

This discovery has important implications for climate modeling, as models must account for dust and the ways it affects cloud freezing and helps shape precipitation. Daniel Knopf, an atmospheric scientist at Stony Brook University, likened the process of observing ice crystal nucleation to Schrodinger's cat, emphasizing the challenges of watching an ice crystal nucleate in nature.

Instruments on aircraft or balloons are often required to catch a micrometer-sized droplet in a cloud at the right moment. Knopf noted that there is more work to be done to understand exactly what the new observations mean for scientists' understanding of climate, particularly in terms of the number of liquid droplets or ice crystals.

The study builds on earlier research, such as the 2012 experiment conducted by German researchers who directly tested the effect of desert dust on cloud freezing in a cloud chamber. They re-created cloud conditions and observed the temperatures at which droplets froze with different types of desert dust. Dust remained the key factor for ice nucleation in most instances, though there are exceptions, such as above the Sahara where few clouds form despite the presence of dust.

The researchers found that a tenfold increase in dust roughly doubled the likelihood of cloud tops freezing, and 100 times more dust was needed to see freezing become 4 times as frequent. This finding has significant implications for the climate, as in the 20th century, scientists found that pure water can remain liquid even when cooled to -34.5°C, but once tiny amounts of material like dust are introduced, it freezes at much warmer temperatures.

The study's findings also have historical roots, with French scientist Joseph Louis Gay-Lussac observing in 1804 that clouds with more dust particles tended to have more frozen droplets. As the Earth may have drier surfaces due to climate change in the next 10-20 years, likely producing more dust aerosols in the atmosphere, Villanueva is motivated to keep studying clouds and aerosols to better understand their impact on our climate.

Cloud Scientists Confirm Dust's Role in Cloud Freezing, Possibly Revising Climate Simulations