Lichens that help hold together soil crusts in arid lands around the world are dying off as the climate warms, new research shows. That would lead deserts to expand and also would affect areas far from the drylands, as crumbling crusts fill winds with dust that can speed snowmelt and increase the incidence of respiratory diseases.
Biologically rich soil crusts, sometimes called cryptobiotic soils or biocrusts, are spread out across dry and semi-dry regions of every continent, including Antarctica. In total, the crusts cover more than 6 million square miles—an area about the size of Russia.
They are assemblages of hundreds of organisms, mostly algae, fungi, lichens, mosses and even cyanobacteria. Woven together by eons of evolution, the organisms become keepers of soil, building intricate organic structures with bacterial filaments and sticky polysaccharides to hold grains of earth and sand in place. Soil crusts build up land, slow erosion and suck a lot of carbon dioxide out of the atmosphere, storing it in the soil. Some crusts even fix nitrogen that fertilizes plants.
Some strains of lichen, which are communities of fungi and algae in symbiotic relationships, survived the planet’s last three mass extinction events, but a study published last week in the Proceedings of the National Academies of Sciences shows global warming is likely causing a decline of lichens that are key components of biological soil crusts. If the organisms die and crusts crumble, deserts will expand and soils in arid regions will dry up and potentially blow away as dust in the wind.
The airborne dust can also increase aridity far from the soil crusts when it settles on distant snow-covered mountains, where studies show that it can change the snowmelt cycle, often reducing river flows. More intense and frequent dust storms are also projected to increase cases of respiratory diseases, such as valley fever, in the Southwest.
The study published last week in the Proceedings of the National Academies of Sciences analyzed soil crust and climate data from 12 football field-size research plots in Canyonlands National Park. It found that nitrogen-fixing lichens that are part of the soil crust in that region have declined sharply since 1996, coinciding with regional temperature spikes far above the global average and a multidecadal drying trend that shows no signs of ending.
U.S. Geological Survey soil researcher Rebecca Finger-Higgens, lead author of the new study, said the biocrusts may have reached a tipping point beyond which there is a permanent shift in their compositions of organisms. That could lead communities of soil crusts and plants to shrink.
When biocrusts disappear, soils dry out and are more likely to blow away, Finger-Higgens said. A biocrust that hangs on with fewer lichens will produce less nitrogen fertilizer, and so fewer plants may be able to survive, leaving ever more bare ground.
All lichens are declining, but hardest hit are those that help take nitrogen from the air and put it in the soil where vegetation can use it. And because some animals rely on the plants nourished by nutrients in the biocrust, the loss of soil crust can have a cascading effect on the entire ecosystem, she said.
In 1996, nitrogen-fixing lichen made up 19 percent of the biocrust in the Canyonlands test plots, but that dropped to just 5 percent in 2019 and shows no sign of recovering. The protections of the national park prevent direct impacts from activities like grazing or fossil fuel extraction, which are known to destroy biocrusts in other areas, although the footsteps of park visitors can still crush crusts that took decades to develop.
“A lot of these crusts are really old, centuries, maybe thousands of years, and the changes we documented are happening in what is literally a hot spot of climate change,” Finger-Higgens said.
The Four Corners region, where Colorado, Utah, Arizona and New Mexico meet, is one of the fastest warming regions of the world, with temperatures increasing between 3.6 and 5.4 degrees Fahrenheit over the last 100 years. Other research has linked warming to increasing drought severity, a loss of grass cover and an increase in dust storms. Global warming, Finger-Higgens said, has also affected the Southwest monsoon season.
“The late summer precipitation events that are important for recharge are just not happening as much anymore,” she said.
The research shows there was a steep decline in the nitrogen-fixing lichens in the early part of the study period, coinciding with a period of extended drought from which they never recovered. Global warming is pushing the biocrust communities close to “a vital tipping point,” the authors wrote. “Such tipping points or nonlinear changes make it extremely difficult or even impossible for an ecosystem to return to its original state.”
Even though it might be hard to extrapolate the findings because untouched areas like the study plots are rare, the findings add to the ever-growing worries about the impacts of warming on Western water supplies, said Brad Udall, a climate and water researcher at Colorado State University, who was not involved in the research.
“We know that increasing dust on snow, forest fires and many other land cover changes have the potential to negatively impact water supplies,” he said. “Unfortunately, I’ve yet to see a land cover study that allays any of my concerns.”
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Mónica Ladrón de Guevara, a drylands researcher in Spain who was not involved in the new study, said the results are similar to research in other countries, all linking the presence of biocrusts with the protection and stabilization of soil, making it “an essential biological cover for preventing dust storms.”
In the American West, livestock grazing has caused the most damage to the soil crusts, followed by mining and drilling, urbanization and recreation and climate change exacerbates the other human impacts, intensifying dryland degradation and desertification, Ladrón de Guevara said. “The last report of IPCC forecasts an increase in atmospheric dust with high confidence,” she added.
That increase is already affecting millions of people far from where winds pick up the once-crusted soils by reducing water supplies and driving spikes in dust-related respiratory diseases.
At the end of the snow season, multiple dust layers in snowpack often merge and darken, making the snow absorb more heat from the sun. That can speed evaporation, cutting the amount of snowmelt trickling into rivers. One study estimated that dust reduces flows in the Colorado River by 5 percent, which is about 250 billion gallons per year—twice the amount of water that Las Vegas uses annually.
Biocrusts are key to maintaining soil stability in global drylands, said Carrie Havrilla, a dryland researcher at Colorado State University who was not involved in the new study. Some research projects global biocrust cover could decline up to 40 percent in the next half century, she added.
“Biocrust organisms “glue” soils together and prevent soil losses via wind and water erosion,” she said. “As such, biocrust cover losses can result in soil destabilization, and increased erosion rates and dust storm activity. Mounting evidence suggests biocrust cover and functioning will be negatively impacted by climate change and that these responses may be even more dramatic than those of vascular plants in drylands.”
Ladrón de Guevara said there are similar studies showing that more frequent and intense dust storms from the Sahara blowing toward Europe are speeding the melt of glaciers in the Alps. Researchers also suspect that same dust, when it’s carried westward, may contribute to the spread of Sargassum seaweed that is choking some coastal ecosystems in the western Atlantic by over-fertilizing the ocean.
In the long term, the breakdown of biocrusts could also have a big impact on human health. Some dust includes small amounts of harmful inorganic materials that can damage health as they accumulate. The finest particles can embed themselves in lungs. And airborne dust can also carry pathogenic microbes. The dust-driven disease valley fever has already increased in California and Arizona, where it is endemic, and is spreading to new regions as soils dry out.
A 2019 study showed that if global warming continues at its current pace, fine dust levels in the Southwest could increase by 57 percent, leading to a 230 percent increase in dust-driven deaths. Emergency room visits could surge by more than 200 percent for cardiovascular issues and 88 percent for asthma treatments.
The research found that climate-driven changes in dust concentrations account for 34 to 47 percent of these health impacts, with economic damages that could run as high as $47 billion per year.
Although there have been a few biocrust restoration efforts, the vast scale of damage is challenging and cryptobiotic soils can take decades or centuries to develop naturally. The only meaningful response is large-scale climate mitigation, Finger-Higgens said.
“Otherwise,” she added, “there’s not much we can do.”