Saharan dust fertilizes new world rainforests

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As winds sweep eastward into the Atlantic off the northwest African coast, a remarkable thing happens: plumes of aeolian dust particles are swept off the surface of the Sahara. They will meander along the varied paths of the easterly trade winds, only to settle again in places as remote from each other as they are from the source: North America, the Caribbean, the Amazon Basin, the southern Mediterranean, eastern Europe, and occasionally even the chilly southern shores of Scandinavia.

A major Saharan dust plume event, November 1988

Where does the dust come from? Total Ozone Mapping Spectrometer (TOMS) suggest two major source areas: the Bodélé depression at the southern edge of the Sahara and an area covering eastern Mauritania, Western Mali and southern Algeria.

A little bit of dust blowing around shouldn’t be anything for meteorologists to bother about.  This dust, however, is anything but inconsequential- in the Caribbean alone, an estimated 20 million tones are deposited annually. It is the primary source of several essential trace elements, such as calcium and magnesium, to island rain forests whose soils have been leached through tens of thousands of years of erosion. Saharan dust enters the Amazon basin in bursts accompanying major wet season rains, feeding the soil with nutrients that the forest depends on. In fact, scientists now believe the Amazon to be so dependent on aeolian dust inputs that efforts are underway to model long-term expansions and contractions of the world’s largest rain forest in relation to the size of the Sahara over geologic time.

Dust not only nourishes the forests, it moderates their climates. African mineral dust is now considered the dominant light scattering aerosol throughout the tropical and subtropical Atlantic. The ability of airborne dust particles to scatter light decreases the amount of direct solar radiation hitting earth’s surface around the equator.

The dependence of major ecosystems across the world on Saharan dust underscores the deep connectivity of the biosphere, atmosphere, lithosphere and hydrosphere.

The world’s largest rainforest is nourished by mineral dust blown from across the Atlantic

1.Goudie, A. & Middleton, N. Saharan dust storms: nature and consequences. EARTH-SCIENCE REVIEWS 56, 179-204 (2001).

2.    SWAP, R., GARSTANG, M., GRECO, S., TALBOT, R. & KALLBERG, P. SAHARAN DUST IN THE AMAZON BASIN. TELLUS SERIES B-CHEMICAL AND PHYSICAL METEOROLOGY 44, 133-149 (1992).

Ice crystal formation in clouds stimulated by marine diatoms

The formation of ice crystals in the atmosphere is often facilitated by the presence of small, airborne particles that serve as a “nucleation” site for the growing crystal. Ice nucleation, with or without airborne particles, plays a large role in cirrus cloud formation. However, airborne particles allow ice crystals form in warmer, mixed-phase clouds that would otherwise have been ice-free.

A recent study published in Nature Geoscience reports that a common planktonic diatom, Thalassiosira pseudonana, can actually serve as a nucleation site for ice crystals. Diatoms are single-celled, marine photosynthetic organisms that are most famous for their often beautiful, glassy, silica-rich shells. They are found worldwide and are particularly abundant in cold, nutrient-rich ocean waters, such as the northern Pacific and Antarctic. Samples of T. pseudonana were exposed to water vapor and a supercooled salt solution under “typical tropospheric conditions” (ie, conditions that diatoms would be exposed to in the region of the atmosphere where cirrus-cloud formation takes place). The researchers found that the presence of diatoms in water allowed ice to form at substantially higher temperatures, and that the rate of ice nucleation in the presence of diatoms was generally rapid.

Thalassiosira pseudonana, a planktonic diatom

Small organisms that they are, the ability of diatoms and possibly other phytoplankton to initiate ice nucleation in clouds may have profound effects on climate. Increased ice crystal production due to diatoms could mean more incoming solar radiation reflected away from the earth by clouds (remember albedo effects?). Thus, diatom fragments in clouds may in fact increase the cooling potential of clouds (clouds are also important climate warmers, the water vapor contained within them is a powerful greenhouse gas).

A warming climate has been linked to changes in diatom populations. Warming is expected to lead to selection for smaller species of diatoms, which could be more easily aerosolized. Furthermore, warming may increase diatom populations due to enhanced ocean nutrient availability and decreased Arctic sea ice cover. These processes would both result in an increased concentration of diatomaceous aerosol material in clouds, leading to increased ice-crystal formation. Tiny glass cells, swept up unwittingly and unwillingly from their oceanic homes, may prove an important climate driver as they build icy shelters in the clouds.

Knopf et al. 2010. Stimulation of ice nucleation by marine diatoms. Nature Geoscience 2: 1037.