Two years ago, 24 million tons of dust lifted high above Africa to create a plume that swirled across the Atlantic Ocean and covered Puerto Rico in a pink patina. An unusually meandering jet stream helped launch the monster dust cloud. But this week, ecologists fingered another factor in such storms: the disappearance of biocrust, a microbial mat that coats dryland soil and helps keep dust in place. “It glues the sand together,” says Bettina Weber, an ecologist at the University of Graz and co-author of the new study.
Trampled by livestock and scorched by climate change, that glue is likely to become weaker in the future, researchers say, leaving the soil prey to wind. The dust study “demonstrates that biocrust loss in one area of the world can have wide-ranging and far-flung impacts,” says Rebecca Finger-Higgens, an ecologist at the US Geological Survey.
A biocrust is a hard surface coating or “skin,” typically a few millimeters thick, containing a thriving community of fungi, lichen, moss, cyanobacteria, and other microbes. Historically ecologists have paid little mind to biocrusts, which cover soils in arid, semiarid, and extremely cold places all over the world. But researchers have realized these coatings produce and process nutrients that nearby organisms need to thrive, especially in arid environments. They also help a dryland soil retain its little moisture.
In 2018, Weber, postdoc Emilio Rodriguez-Caballero, and their colleagues mapped all of Earth’s biocrusts, concluding they cover 12% of the land surface. They then teamed up with climate modelers and dust experts to figure out how much dust formation the biocrust glue currently averts. First, the researchers compiled data about how much wind it takes to destroy the biocrust and blow the soil away. They calculated dust emissions at 31 different sites, then fed the results into a model that predicted worldwide dust emission — and how much it would rise without the biocrusts. “[They] used a very elegant combination of field data and modeling, ”says Fernando Maestre, a dryland ecologist at the University of Alicante.
Biocrusts reduce dust in the air by 700 million tons per year, Weber and her team report this week in Nature Geoscience. That amount would bury all of New York City under 35 centimeters of dust. The study “places biocrusts as key players in preventing dust emissions globally,” Maestre says.
Over the next 65 years, between 25% and 40% of these crusts will vanish, says Rodriguez-Caballero, now at the University of Almeria. Climate change that threatens soil organisms will account for about half the destruction; other damage will come from trampling by humans, livestock, and farm machinery.
Finger-Higgens and colleagues have documented the impact of climate. In a long-term biannual survey of plots in Utah’s Canyonlands National Park, they found that biocrust lichens in particular suffer when temperatures rise. As Canyonlands temperatures rose by 0.27 ° C per decade, lichens, especially those that help convert nitrogen in the air to a form that other organisms can use, have almost disappeared, the team reported on 11 April in the Proceedings of the National Academy of Sciences. With less nitrogen, fewer plants can survive, leaving ever more bare ground and more dust emission, Finger-Higgens says.
Some implications of a dustier climate are unclear, researchers say. The impact of airborne dust on temperatures depends in part on the size of the particles. Dust particles provide nuclei for cloud formation and can cause snow to melt faster. Although dust helps transport important nutrients for plant life, it can make respiratory problems and other health issues worse for people.
Up to now, dusty places like the Sahel were expected to get greener and less dusty as higher carbon dioxide levels have a fertilizing effect, but biocrust loss will likely counteract this process to some degree, Weber and her colleagues note. “Biocrust, dust, [and] climate all exert feedback on each other, ”says Diana Francis, an atmospheric scientist at Khalifa University who is not part of the work.
Climate modelers have often overlooked how dust affects temperature and rainfall, says Michael Mann, an atmospheric scientist at Pennsylvania State University, University Park. The effects of diminishing biocrusts should not be dramatic enough to make much difference in global climate models, he says. But Joseph Prospero, an atmospheric chemist at the University of Miami, cautions that “there are large areas of the Earth for which we have essentially no information” about biocrusts. That may change. Weber, Maestre, Finger-Higgens, and other biocrust experts are seeking funding for standardized measurements of biocrusts around the world.
But researchers say the need to protect these fragile communities is already clear. Reducing emissions and changing farming and other land use practices can help reverse their decline, Maestre says. “The findings provide strong arguments for preserving biocrust communities worldwide.”