Saharan dust once changed climate

Every year, trade winds over the Sahara Desert sweep up huge plumes of mineral dust and transport this dust across North Africa and over the Atlantic Ocean.

Researchers have reconstructed the African dust plume over the last 23,000 years and observed a dramatic reduction in dust beginning around 11,000 years ago[1]. They say this weakened plume may have allowed more sunlight to reach the ocean, increasing its temperature by 0.15 degrees Celsius - a small but significant spike that likely helped whip up monsoons over North Africa, where climate at the time was far more temperate and hospitable than it is today.

Around 11,000 years ago, Earth had just emerged from the last ice age and was beginning a new, interglacial epoch known as the Holocene. Geologists and archaeologists have found evidence that during this period the Sahara was much greener, wetter, and more habitable than it is today. There was extensive human settlement throughout the Sahara, with lifestyles that would never be possible today. Archaeologists have found fish hooks and spears in the middle of the Sahara, in places that would be completely uninhabitable today.

This evidence of wet conditions shows that the region experienced regular monsoon rains during the early Holocene. This was primarily due to the slow wobbling of Earth's axis, which exposed the Northern Hemisphere to more sunlight during summer; this, in turn, warmed the land and ocean and drew more water vapor -- and precipitation -- over North Africa. Increased vegetation in the Sahara may have also played a role, absorbing sunlight and heating the surface, drawing more moisture over the land.

To estimate how the African dust plume has changed over thousands of years, the team looked for places where dust should accumulate rapidly. The Bahamas looked like a good place, because sediment accumulates quickly, making it easier for scientists to determine the ages of particular sediment layers.

[Dust storm seen from space]

Scientists obtained sediment core samples from the Bahamas that were collected in the 1980s by colleagues from the Woods Hole Oceanographic Institution. They determined how much dust was in each sediment layer and determined how fast it had been accumulating. The team analyzed sediment layers from the last 23,000 years and showed that around 16,000 years ago, toward the end of the last ice age, the dust plume was at its highest, lofting at least twice the amount of dust over the Atlantic, compared to today. However, between 5,000 and 11,000 years ago, this plume weakened significantly, with just half the amount of today's windblown dust.

Computer simulations showed that a drop in long-range windblown dust would raise sea surface temperatures by 0.15 degrees Celsius, drawing more water vapor over the Sahara, which would have helped to drive more intense monsoon rains in the region.

[1] Williams et al: Glacial to Holocene changes in trans-Atlantic Saharan dust transport and dust-climate feedbacks in Science Advances- 2016