Do you want to freeze a cloud? Desert dust might help

A new study shows that natural dust particles, swirling in from faraway deserts can cause clouds in Earth¡¯s Northern Hemisphere to form ice crystals (or to glaciate). This subtle mechanism has major implications for climate projections.

Drawing on 35 years of satellite observations, an international research team led by ETH Zurich found that mineral dust¡ªtiny particles swept up by the wind and carried into the upper atmosphere¡ªcan trigger the freezing of cloud droplets. This process is particularly important in northern regions, where clouds often form in a temperature range just below freezing.

¡°Where there¡¯s more dust, clouds are much more likely to freeze at the top,¡± explains Diego Villanueva, a post-doctoral researcher for Atmospheric Physics at ETH Zurich and lead author of the study.

Dust turns clouds to ice

The researchers focused on clouds that contain both supercooled water and ice, forming between ?39?¡ãC and 0¡ãC. Such clouds are common in mid- and high-latitude regions, particularly over the North Atlantic, Siberia, and Canada. Researchers have shown that such clouds are extremely sensitive to environmental changes¡ªespecially to the presence of ice crystal nuclei which primarily form from desert dust aerosols.

Comparing the frequency of ice clouds with the level of dust content, the researchers observed a remarkably consistent pattern: The more dust and the cooler the clouds, the more frequent the ice clouds develop. What is more, according to the researchers, this pattern corresponded almost perfectly with laboratory experiments on how dust causes droplets to freeze.

¡°This is one of the first studies to show that satellite measurements of cloud composition agree with what we know from the laboratory,¡± says Ulrike Lohmann, senior co-author, and Professor of Atmospheric Physics at ETH Zurich.

A new benchmark for climate models

¡°The way clouds form ice crystals or glaciate directly influences how much sunlight is reflected back into space and how much water they release as precipitation,¡± says Villanueva. These factors are vital for climate models. However, until now, many of these models lacked a solid reference point for how cloud glaciation really works on a global scale.

The new findings establish a measurable link between dust in the air and the abundance of ice on the tops of clouds, thus providing a critical benchmark for improving climate projections. ¡°This helps identify one of the most uncertain pieces of the climate puzzle,¡± Villanueva explains.

A complex picture ¨C with a clear signal

For decades, climate and atmospheric scientists have studied droplet freezing at a microscale. This study shows, for the first time, that cloud ice formation (or glaciation) follows the same pattern as droplet freezing¡ªbut on a much larger scale.

The new findings demonstrate the enormous impact that tiny dust particles can have on the atmosphere: Nanometre-sized defects on the particles¡¯ surface in clouds trigger the freezing of water droplets that grow into ice crystals at a kilometre scale. This expands the field of atmospheric research in this area - from nanometre-scale to large-scale observations from space.

However, the dust¨Cice connection does not play out equally across the globe. In desert regions like the Sahara, cloud formation is sparse, and the strong movement of hotter air can suppress freezing. In the Southern Hemisphere, marine aerosols often take on the role of dust.

According to the researcher team, further studies are needed to clarify the influence of other factors such as updraft strength or humidity on cloud glaciation. For now, one thing is certain: Tiny dust particles from distant deserts contribute to shaping the clouds above our heads¡ªand with them, the future of our climate.