Research from the University of Leeds and the National Centre for Atmospheric Science (NCAS) in the UK suggests that large-scale dust storms in southern Alaska, which can last for many days, have a bigger than realized impact on the global climate system, by sending tons of fine vegetation-rich silt into the atmosphere.
Dust particles in the atmosphere, composed of fine sediment and silt, are important agents in ice formation. Without dust, water in clouds can remain in liquid form even though temperatures may be well below freezing.
Whether ice formation in clouds will contribute to global warming or help cool the planet depends on how much ice they contain, how many ice-nucleating particles are present and the nature of those particles.
“Only a small fraction of the dust particles in the atmosphere has the capacity to nucleate ice and we are only just starting to understand their sources and global distribution,” said Professor Ben Murray, an atmospheric scientist at the University of Leeds who is part of the research team.
“Whether a cloud becomes more or less reflective of sunlight depends on how much ice is in it, so we need to be able to understand and quantify the various sources of ice-nucleating particles around the globe. At present, climate models tend not to represent these high-latitude sources of dust, but our work indicates that we need to.”
Previous research has focused on dust particles whipped up into the atmosphere from across Africa and Asia, all of which are at mid to low latitudes and involve dust generated from desert environments. The researchers at Leeds and NCAS took a different approach and decided to focus on sources of high-latitude dust. They analyzed dust coming from the Copper River, which extends for more than 275 miles before draining into the Gulf of Alaska. The river is estimated to transport 70 million metric tons of glacial sediment every year.
During periods of low water – in the summer and autumn – the silt from the Copper River is picked up by winds and carried over hundreds of miles across North America, reaching altitudes where it can cause ice formation in clouds.
Sarah Barr, a doctoral researcher at NCAS and Leeds University and author of the paper, collected samples during dust storms in the Copper River area. The material was analyzed and compared with the types of dust particles that originate from desert environments. Unlike the dust from the Sahara, dust particles from the Copper River valley were found to contain a greater volume of biological material, deposited by the rich vegetation and wildlife that lives in the region.
These particles from Alaska are more effective at forming ice than dust from the Sahara, due to the greater presence of microscopic fragments of biogenic substances.
Barr said, “We knew that deserts like the Sahara are very important at supplying ice-nucleating particles to the atmosphere, but this paper shows that river deltas like the Copper River valley are also very important. Huge quantities of dust are emitted from places like the Copper River, and we need to understand these emissions to improve our climate models.”