Clumps of dust in the desert shatter like glass on a kitchen floor. This similarity may mean the atmosphere carries more large dust particles than climate models assume.
Dust and other airborne particles’ effect in the atmosphere is “one of the most important problems we need to solve in order to provide better predictions of climate,” said climate scientist Jasper Kok of the National Center for Atmospheric Research in Boulder, Colorado. Other researchers suspect current models also neglect a large fraction of the climate-warming dust that clogs the skies after dust storms.
Most climate models use dust data from satellites that measure how many particles of different sizes are suspended in the atmosphere. These measurements reveal an abundance of tiny clay particles roughly 2 micrometers across (about one-third the width of a red blood cell), which can reflect sunlight back into space and cool the planet.
But satellites may be missing larger particles, called silts, which don’t hang around in the air as long. Silts up to 20 micrometers in diameter can act as a warm blanket to trap heat inside the Earth’s atmosphere.
To figure out how much clay and silt is actually kicked up from the Earth’s deserts, Kok turned to a well-studied problem in physics: how glass breaks.
Cracks spread through breaking glass in specific patterns, creating predictable numbers and sizes of glass shards. The final distribution of small, medium and large glass fragments follows a mathematical law called scale invariance.
“It shows up all across nature, from asteroids to atomic nuclei,” Kok said. “It’s really just beautiful.”
In a paper published Dec. 28 in the Proceedings of the National Academy of Sciences, Kok showed that the physics of how dust clumps break apart is similar to glass breaking.
Soil scientists have long known that dust clumps act like brittle materials, and physicists have well-tested mathematical descriptions of how brittle materials break. “But no one had put one and two together,” Kok said.
When wind picks up in the desert, Kok says, the particles that move first are large sand particles, up to 500 micrometers across. Silt-sized and smaller dust grains tend to stick together until a bouncing sand particle smacks into them.
“It’s physically analogous to hitting your windshield with a hammer, or dropping a drinking glass on the kitchen floor,” Kok said.
Cracks spread through the clump of soil as they would through a pane of glass, sending the same fraction of small, medium and large particles bouncing into the atmosphere. Kok compared his theory to ground measurements made in the middle of dust storms in six locations around the world and found they matched perfectly.
“Even though we don’t have an abundance of measurements, I think we have sufficient measurements to say this theory is a step in the right direction,” Kok said.
Kok’s theory suggests that dust storms produce two to eight times more silt-sized particles than climatologists previously thought. Neglecting the boost in particles suggests that climate models, and even short-term weather models for dusty regions, are somewhat off. Until climate scientists better understand how dust changes over time, however, Kok said it’s tough to gauge the effects.
“I thought it was a breakthrough, a real original idea,” said atmospheric physicist Charles Zender of the University of California at Irvine, who was not involved in the new work. Similarities to fractured glass may show up in other earth science systems, like earthquakes or glacier calving, he added. “Whether it’s submicron and invisible to the human eye, or as large as Greenland, it doesn’t matter. It’s the same property.”
Dust expert Tom Gill of the University of Texas at El Paso thinks Kok’s theory is elegant, though it will have to be backed up by lab and field experiments. If it holds up, however, “it has the potential to make some real improvements in modeling how dust and dust-like things move around and disperse and fall out of the air. That has implications for everything from global climate to volcanoes to hurricanes,” he said. “I’m very encouraged by it.”
By Lisa Grossman