From 10,000 to 5000 years ago, the Sahara Desert was wet, green, and covered with lakes! Over those thousands of years, minerals precipitated out of the water and were deposited on the lake beds. Now that it is a dry desert, high temperatures cause rapidly rising hot air currents to lift those minerals high into the atmosphere, where prevailing western winds carry the dust across the Atlantic to the Americas.
While the dust movement varies in intensity over time, it is almost always present to some degree. In the winter, dust is typically sent to South America. In the summer, it sends the clouds toward North America.
The effects of the dust on human respiratory allergies are regrettable, but plants love the minerals contained in the dust, primarily the phosphorus and iron. Saharan dust is a primary nutrient source of the Amazon rain forest, the “lungs of the world”, enabling the production of oxygen and the sequestering of carbon dioxide. For plants, and us, as we depend on them, this brown cloud does have a silver lining.