Rising temperatures are believed to diminish the amount of ice crystals in clouds, which results in clouds that are more dependent on liquid. Recent research, however, indicates that warming in the Arctic is actually increasing the release of natural aerosols from regions that are barren and have vegetation, which are free from snow and ice. These aerosols may promote the formation of ice crystals in mixed-phase clouds, potentially influencing both cloud characteristics and the climate in the Arctic.
The Arctic often encounters temperatures that facilitate the creation of mixed-phase clouds, which have both supercooled liquid droplets and ice crystals. The structure of these clouds is vital to the region’s energy regulation and climate dynamics. Clouds with a higher liquid content tend to persist longer and reflect more sunlight compared to those with a greater concentration of ice crystals.
As temperatures in the Arctic continue to rise, meteorologists are keen to understand how this warming impacts cloud structures and its wider implications for the area. Climate models typically suggest that warmer conditions will lead to clouds in the Arctic containing more liquid water and fewer ice crystals since warmer temperatures usually hinder ice crystal development. Nonetheless, the formation of clouds is also affected by aerosols, which serve as nuclei for both liquid droplet condensation and ice crystal creation.
A research study published in Communications Earth & Environment on September 18, 2024, led by Associate Professor Yutaka Tobo from the National Institute of Polar Research in Japan, explored the connection between rising surface air temperatures and the aerosol particles known as ice-nucleating particles (INPs), which facilitate ice crystal growth in clouds. Their research indicated that increased surface warming in the Arctic expands snow and ice-free zones, which in turn release more active INPs. These INPs can prompt the formation of ice in clouds, consequently decreasing the liquid water content in mixed-phase clouds and potentially exacerbating warming trends.
“We discovered that the INPs increased significantly as surface air temperatures rose beyond 0°C, especially in Svalbard, a region undergoing warming at a rate five to seven times that of the global average,” remarked Associate Professor Tobo.
The findings stem from continuous measurements of INPs acquired during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) project from September 2019 to early October 2020 at the Zeppelin Observatory in Svalbard. The research team collected aerosol samples and employed a well-established droplet-freezing technique, exposing these samples to low temperatures to assess their ability to form ice.
The results showed an uptick in the number of INPs during warmer months (mid-April to September), particularly when surface air temperatures exceeded 0°C. Using advanced scanning electron microscopy with energy-dispersive X-ray analysis, the researchers identified that the INPs prevalent during warmer months primarily consisted of mineral dust and carbon-based particles resembling microorganisms or plant detritus.
So, where do these aerosols originate? Analysis of the Normalized Difference Vegetation Index (NDVI), which monitors vegetation density, revealed that during the summer months, around 35% of Svalbard exhibited NDVI values between 0 and 0.5, indicating the presence of snow-free barren areas and vegetation consisting of grasses, mosses, and lichens. The data suggest that the INPs are made up of dust and biological organic aerosols released from these regions.
These findings raise alarms, particularly considering that winter warming is more pronounced than summer warming, with temperatures in Svalbard increasing by over 2°C per decade. As the prevalence of snow and ice-free areas rises in the Arctic winter in the coming decades, it is likely that INP emissions will also increase, altering the makeup of mixed-phase clouds. “Our findings imply the potential for higher contributions of active INPs from terrestrial sources at high latitudes as surface warming progresses, necessitating their inclusion in climate models to enhance our understanding of Arctic mixed-phase clouds’ behavior,” concluded Associate Professor Tobo.
Funding information
This research was partially funded by JSPS KAKENHI (JP19H01972, JP20H00638, JP22H01294, JP22H03722, JP23H00523, JP23H03531, JP23KK0067, JP23K18519, JP23K24976, and JP24H00761), the Arctic Challenge for Sustainability (ArCS) Project (JPMXD1300000000), ArCS II Project (JPMXD1420318865), the Environment Research and Technology Development Fund (JPMEERF20172003, JPMEERF20202003, and JPMEERF20232001) from the Environmental Restoration and Conservation Agency of Japan, and grants for the Global Environmental Research Coordination System from the Ministry of the Environment of Japan (MLIT1753 and MLIT2253).
