In December, an interdisciplinary team of climate scientists, historians, and archaeologists at the University of Maine’s Climate Change Institute and the Initiative for the Science of the Human Past at Harvard published a study in the Journal of Geophysical Research. It detailed how they used a new, high-resolution laser technology to analyze ice from the Colle Gnifetti Glacier on the Swiss-Italian border, and how this technology allowed the team to trace the history of Saharan dust events as well as the atmospheric conditions that promote them.
Saharan dust storms are an influential weather phenomenon for both human and natural systems. Though they fertilize flourishing ecosystems, they can also harm human respiratory health, alter the frequency of North Atlantic hurricanes, and speed the melting of glaciers. Because future occurrences of Saharan dust storms are uncertain given the changing climate, many studies have looked to the past to understand the connection between these dust events and climatic patterns.
Lead author Heather Clifford is a graduate student at the University of Maine Climate Change Institute. She explained that the Saharan dust record held inside the Colle Gnifetti ice core revealed that increased dust transport historically occurs when the atmosphere creates high pressure systems over the Mediterranean and drier conditions over North Africa. Climate change models indicate that these conditions will become more vigorous, indicating a dustier future.
Clifford’s coauthor, Dr. Alex More, is a research professor at the Climate Change Institute at the University of Maine, as well as a researcher at Harvard University and an associate professor in the School of Health Sciences at Long Island University in New York. More explained that in 2012, three of his colleagues (Paul Mayewski (CCI), Michael McCormick (Harvard), and Dietmar Wagenbach (Heidelberg)) wanted to extract an ice core in Europe, instead of the typical polar locations, as the site is closer to impacts from human civilization. Greenland traps signals from a mix of North American regions and Antarctica traps signals from a mix of continents as well –– but the Alps provide a unique look into the history of the Mediterranean region.
Enter the Colle Gnifetti Glacier. “The glacier has been studied for many years because it’s a low accumulation glacier which gives a very clear, particularly high-quality ice for this type of study,” said More. The 72-meter ice core –– the deepest core ever to be dug out of the European Alps –– was extracted in 2013. “This was the first time that researchers from history, climate science, archaeology, volcanology, public health and multiple other disciplines came together for a project like this: from grant-writing to publication,” More said.
Clifford took charge of the lab work and data analysis. Normally, ice cores are melted for analysis by a mass spectrometer, the instrument used to determine the elemental signature of a sample. “Imagine spending hundreds of thousands of dollars, sometimes millions, to get tens of meters of ice from the remotest places of the world… and then melting the entire thing.” More added: “We are losing a record of climate change because of climate change, so it’s crazy that we would destroy that ice.” Therefore, the team opted to find a better way to collect their data without melting the ice core.
Clifford’s team is the first to use a non-destructive laser method to sublimate microscopic circles of ice from a core. More explained that the laser moves slowly over the ice inside a vacuum to create a 10-micrometer groove in the core. An argon gas carrier then transfers the sublimated gas from the core to the mass spectrometer where it is analyzed in real time. The core remains intact and the glacier’s record can be preserved forever in a specialized refrigerated depository, even when the actual glacier itself ceases to exist.
While this project is the first to use laser technology for ice core analysis, it has previously been used for lake sediment cores and for archaeological purposes, More explained. Artifacts like ancient coins are precious, and a laser helps determine composition without damaging the structure.
The new laser provides an ultra-high-resolution analysis of glacial ice. More calls it the “gold standard” because it has already produced eight million data points, which is unmatched in their field. “The max data collected for one year is 1,100 data points,” Clifford told GlacierHub. This means they are able reconstruct past atmospheric conditions in much more detail, on subannual to storm-scale event time frames. By contrast, the quality of data obtained by the traditional method of melting ice is coarse and does not provide continuous detail. Using the laser, the researchers created the longest and most accurate record of Saharan dust transport to the European Alps. It spans the past 2,000 years.
Ice core extraction on the Colle Gnifetti Glacier and the laser ablation method: courtesy of Alex More, Climate Change Institute, University of Maine.
“It took four of us to put the record together and we all lasered over two years,” Clifford said. Lasering was an entirely new methodology that had to be calibrated very carefully, and the laser proceeds slowly, in 100-micrometer increments. “We can only measure the concentration of a few chemical elements at a time, so we often laser the same ice multiple times in order to measure the concentration of different elements. Each element is a piece of the puzzle in understanding climate change, pollution and the human impact on the planet,” More said.
“A calcium spike alongside an iron spike indicates dust,” said More, referring to the mass spectrometer readings. Then, depending which elements occurred alongside those dust spikes, the researchers could identify how different behaviors in atmospheric circulation deposited the Saharan dust in the Alps.
Some elemental signatures indicated a marine origin. “If they are not present, the air probably took a more direct route across Europe into the Alps,” explained More. Clifford pulled all the elemental data together and found that dust was more likely to be picked up by winds when conditions were dry over the African Sahel, or more arid over the Sahara, or when there was high sea level pressure over the Mediterranean. Periods of drought are expected to become more severe with climate change, so the study predicts an intensification of Saharan dust storms.
Saharan dust is rich in iron, and when iron mixes with oxygen, it rapidly oxidizes and gives off a distinctive red hue. Dust storms have long been observed by NASA satellites, and was a rare enough phenomenon that they were written about in European historical records, referred to as blood-rain. More, McCormick and their team combed through thousands of historical records to match what they read in the ice with what people wrote about their experiences of these events. This level of detail is only possible thanks to the quality of the laser and historical data combined.
Saharan dust events in Europe happen 43 times a year on average, on a scale of two to eight days. But More says “these dust storms sometimes occur and people in Europe don’t even notice… They’ll just say ‘oh it’s a hazy day today.’” The study indicates that dust storms are becoming more intense as climate change results in dryer conditions over north Africa. With more drought, stronger winds will have more dust to lift into the atmosphere, carrying more particles to human populations, an intensification already being seen in North Africa.
A NASA article describes how trade winds carry Saharan dust across Western Africa toward the Gulf of Guinea, forming the Harmattan Haze (named after the dusty easterly trade winds) which in Twi means “tears your breath apart.” Susanne Bauer of NASA’s Goddard Institute for Space Studies found that “air pollution in Africa likely caused the premature deaths of about 780,000 people in 2016, more than the number killed by HIV/AIDS,” and 70 percent of these deaths were attributable to dust. “Air pollution is the greatest silent killer,” affirmed More.
In the record, Clifford found that there has already been a significant increase in dust transport over the past century. Climate data show that the conditions she found necessary to fuel dust events will become more severe in the future with climate change. The team predicts that the increase in Saharan dust transfer will worsen air quality and pose a threat to human health, while increased deposits on glaciers will speed up melting and exacerbate the effects of climate change on nature and society.
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