Sahara dust reflects sunlight and has a cooling effect on the climate. However, there is also an indirect and significant connection between the fine particles coming from the Sahara, microscopic marine life, and the climate. This is all related to the chemical composition of Sahara dust and the greenhouse gas carbon dioxide (CO2). How does this work exactly?
Air and Sea Currents Spread Sahara Dust
Powerful winds and dust storms in the Sahara lift sand and dust high into the atmosphere, up to several kilometers. Eastern trade winds carry the dust westward across the Atlantic Ocean. The dust can travel vast distances and reach as far as the Gulf of Mexico (see animation in image 1). With the typical wind pattern in the Northern Hemisphere, the dust eventually spreads over a large part of the North Atlantic Ocean. Then, mainly due to precipitation, the Sahara dust ends up in the sea. The ocean currents in the North Atlantic, Caribbean, Gulf of Mexico, and Mediterranean Sea further distribute the dust across the ocean, where it serves as food for phytoplankton.
Sahara Dust Feeds Phytoplankton
Phytoplankton is a type of microorganism that lives in the upper few meters of the ocean and drifts with the ocean currents. There are several types, from something resembling algae to microscopic animals and bacteria. What they have in common is that they perform photosynthesis, the same process that plants and trees do. They absorb CO2 and use the energy from sunlight to break down the CO2 molecules. The oxygen (O2) is exhaled and with the carbon atoms (C) they build their skeleton.
A nutrient that is limiting for the growth of phytoplankton is iron. It just so happens that Sahara dust contains an iron-rich substance (hematite). There is only about 1 percent hematite in the dust. But because so much dust from the Sahara ends up in the ocean, this is still enough to significantly promote the growth of phytoplankton. In this way, Sahara dust increases the breakdown of CO2 in the ocean. When the plankton dies, it drifts to the ocean floor, where the carbon is stored in the sediment. Thus, Sahara dust helps to limit the amount of CO2 in the atmosphere.
