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Coarse dust lingers in the atmosphere

27 April 2020

Climate models substantially underestimate both the amount of large mineral particles in the air and their warming effect.

A caravan of camels in the Sahara
Earth’s radiation budget is affected by dust from places such as the Sahara desert, pictured here. Credit: ©Sergey Pesterev/Wikimedia Commons/CC BY-SA 4.0

When shortwave solar radiation reaches Earth’s atmosphere, clouds and certain aerosols reflect some of it back to space. Other aerosols absorb some of the energy, and the ground absorbs the remainder. Although scientists confidently know the total amount of radiation entering and exiting the atmosphere, the effect on the radiation budget from particles larger than 5 μm in diameter is less clearly understood. Now Adeyemi Adebiyi and Jasper Kok of UCLA have found that coarse dust warms the climate far more than previous simulations suggest.

The researchers estimated the global dust-size distribution by calculating the minimum difference between measured distributions and those simulated by several global climate models. For most diameters, the results (solid red line in the graph) agree well with an earlier observational constraint on the dust-size distribution in the atmosphere (solid black line). The analysis also reveals that other models (additional colored lines) underestimate the fraction of coarse dust in the atmosphere by about 75%, and the discrepancy is more pronounced at larger dust diameters.

Chart of atmospheric mass distribution measurements
Credit: A. A. Adebiyi, J. F. Kok, Sci. Adv. 6, eaaz9507 (2020)

The mismatch between models and observations increases farther from the dust source, which suggests that models underestimate the length of time coarse dust remains in the atmosphere. One reason for the discrepancy could be that models assume spherical particles: Compared with their observed aspherical counterparts, they have lower surface-to-volume ratios and consequently higher velocities. Convective turbulent mixing, the potential for charged dust interactions that counteract gravity forces, and uncertainties in the numerical simulations could also explain the disparity between observations and models. To estimate radiative forcing more accurately, Adebiyi and Kok included coarse dust in Earth’s radiation budget and found that the top of the atmosphere warms five times as much as in previous model simulations. (A. A. Adebiyi, J. F. Kok, Sci. Adv. 6, eaaz9507, 2020.)

Editor’s note, 4 May: The article was updated to correct the size of the dust grains that were the focus of the study. The particles are larger than 5 μm in diameter, not 5 mm.

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