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Giant particles of dust are being spread across the globe, and the physical forces responsible are still up in the air, hidden somewhere in the wind.
Whatever it is, this mysterious influence is so strong, it can transport particles 50 times bigger than we ever thought possible, carrying these huge pieces all the way from the arid Sahara Desert to the tropical Caribbean.
It’s an achievement that should defy the laws of physics, which is rarely a good sign. It probably means we’ve gotten something wrong along the way, and in our ignorance, we may have overlooked an important driver of climate change.
For nearly 30 years, scientists have known that small particles of dust, kicked up in the Sahara, often ride on global winds to the Caribbean, 3,500 kilometres (2,000 kilometres) away from home.
Originally, this foreign cloud of desert dust was thought to contain particles no bigger than 0.01 to 0.02 millimetres in diameter.
But recently, when scientists began collecting samples of dust from floating buoys and underwater traps in the Atlantic Ocean, the sheer size of the particles defied their expectations.
Between 2013 and 2016, the Royal Netherlands Institute for Sea Research (NIOZ) found some dust particles measuring 0.45 millimetres in diameter, nearly 50 times bigger than what global winds were once thought capable of carrying.
“These dust particles are whipped up from the Sahara Desert and carried between continents, and most people know them best when they end up settling on our cars or cause the kind of eerie orange skies we saw a year ago,” explains co-author Giles Harrison, a researcher in atmospheric electricity at the University of Reading.
“However, existing ideas do not allow for such massive particles travelling in the atmosphere for such vast distances, suggesting that there is some as-yet-unknown atmospheric process or combination of processes keeping them airborne.”
The bad news is, by underestimating what the winds can carry, we may have seriously thrown off our climate models.
After all, if large particles, like quartz, can be transported such vast distances, these materials could influence both cloud formation and the global climate system.
“This evidence of dust and ash being carried so far is significant because these particles influence radiation transfer around the Earth and carbon cycles in the oceans,” says Harrison.
Dust may seem like an inconsequential bother, but if enough of it gathers in our atmosphere, it can tip a delicate balance, changing the course of incoming sunlight and heat emitted from the Earth.
As they scatter and absorb incoming solar radiation, these large particles have the power to actually change the clouds above, influencing both our planet’s weather and climate.
By hanging out in the sky, all of this dust can even have an indirect impact on tropical cyclone development.
The amount of time that all of this heavy dust can hang out in the sky is staggering. And in the end, the research suggests that rain, and not just gravity, is responsible for many of the particles that end up falling from the sky.
Weighed down by such large burdens, acidic droplets of water are thus carried to the deepest parts of the ocean, further impacting food chains and the ocean carbon cycle.
But despite their far-reaching effects, most climate models to date have ignored these large dust particles. The physical laws on which these calculations are based just don’t allow for particles bigger than 10 μm to travel so far, even at high wind velocities.
There has to be something else keeping these dust particles aloft – we just don’t know what that something is. And while researchers have explored a few explanations, including vertical mixing, electric forces and turbulence, the answer continues to remain elusive.
“The fact that larger particles of dust keep floating in the atmosphere for a long time is considered to be in conflict with the physical laws of gravity,” says lead author Michele van der Does, a researcher at NIOZ.
“We show that through a combination of forces and movements in the atmosphere the large dust pellets can indeed stay in the atmosphere for a longer time and have their influence there.”
Now, the authors are calling for future climate models to incorporate the effects of much larger dust particles than ever before.
The research has been published in Science Advances.