Desert Dust, Dust Storms and Climate
Dust storms in three shapes. The whirl. The column. The sheet. In the first the horizon is lost. In the second you are surrounded by 'waltzing Ginns.' The third, the sheet, is 'copper-tinted. Nature seems to be on fire.'
—Michael Ondaatje, The English Patient
Dust storms occur most frequently over deserts and regions of dry soil, where particles of dirt are loosely bound to the surface. Grains of sand lofted into the air by the wind fall back to the ground within a few hours, but smaller particles remain suspended in the air for a week or more and can be swept thousands of kilometers downwind. Dust from the Sahara desert regularly crosses the Atlantic, causing bright red sunrises and sunsets in Miami, traveling as far as the Caribbean and the Amazon basin. Here we describe our understanding of how desert dust alters the Earth's surface temperature.
Airborne dust particles, or dust aerosols, alter the climate by intercepting sunlight intended for the surface. By shading the Earth from the sun's radiation, dust aerosols have the same effect as a rain cloud. The reduction by dust of net radiation at the surface during the months of June through August is shown in Fig. 1. (Net radiation equals the absorbed sunlight minus thermal radiation emitted by the surface back to the atmosphere. At the surface, changes in the net radiation caused by dust are mostly in the solar component.) Because atmospheric dust concentrations are measured at only a few locations, Fig. 1 was created using a computer model that takes into account the global distribution of dry soil, along with the winds that carry the dust away from its source.
Between June and August, the deserts surrounding the Arabian Sea are the dominant source of dust aerosols, which are carried to the northeast towards India and Asia by the Indian monsoon winds, as well as to the west over North Africa by the strong westward flow near the surface. This westward plume is augmented by dust from the Sahara and Sahel regions and can be traced across the Atlantic. In the Southern Hemisphere, the Australian outback is the largest source of dust. The region of reduced surface radiation in Fig. 1 is an indicator of the low-level circulation during the Northern Hemisphere summer.
While solar radiation is reduced beneath the dust cloud, the absorption of sunlight by dust particles heats the cloud itself. The dust cloud therefore displaces heating from the Earth's surface into the atmosphere. In this way, dust aerosols differ from the sulfate aerosols lofted into the stratosphere following the eruption of Mount Pinatubo. The latter reflect sunlight back into space, thus reducing the amount of radiative heating both within the atmosphere and at the surface.
The displacement of solar heating away from the surface by dust aerosols alters the Earth's climate. The average change in surface temperature during June through August resulting from dust aerosols is shown in Fig. 2. This change is calculated by comparing two versions of the NASA Goddard Institute for Space Studies computer climate model, one including the effect of dust aerosols and the other omitting that effect. Fig. 2 shows that beneath the dust cloud, temperatures at the surface are typically reduced by 1°C. This is similar to the effect of a rain cloud passing overhead, which can cause a drop in temperature in response to the diminished sunlight. However, in some regions, the atmosphere adjusts to this cooling by bringing heat from surrounding warmer regions, returning the temperature to its original value. This occurs over the Arabian Sea, where the temperature remains unchanged under the dust cloud, despite the large reduction in surface radiation (Fig. 1).
Note also that there is cooling far downwind of the Arabian dust cloud, extending from northern Asia to the Pacific and North America. Such cooling is possible because of the atmospheric circulation that connects regions beneath the dust cloud to regions downwind. It is not currently well-understood why cooling occurs in one region downwind of the dust cloud and not another.
|Figure 2. Change in surface temperature by desert dust between June and August. The units are degrees Celsius.|
So far, we have assumed that any differences between the two computer simulations, such as the difference in surface temperature shown in Fig. 2, are the result of adding the radiative effect of dust to only one model. However, another potential source of difference arises from the atmospheric fluctuations that are simulated by each model. Consider, for example, the possibility that several consecutive summers may be unusually warm. An average temperature constructed during this period would overestimate the true climatological temperature that would be recognized over a longer period of time. Even if dust were absent from both models, an extended heat wave could occur in one model and not the other, resulting in a difference in average temperature. As the models continue to run, the average temperature of each simulation will approach the true average, reducing the difference to zero. Nonetheless, we must still distinguish cooling caused by the dust cloud from differences in temperature that occur simply because the simulations are insufficiently long for the true average to emerge.
A number of statistical tests designed to isolate changes forced by dust aerosols suggest that the changes in surface temperature over Australia and most of the Northern Hemisphere are likely to be the result of the dust cloud. In contrast, temperature changes along the Antarctic coastline are more likely the result of fluctuations within the model that are unrelated to dust.
In summary, dust aerosols, lofted into the air by the wind-erosion of dry, loosely-packed soil, can lead to cooling of the surface below. However, the winds that extend this cooling to regions far beyond the dust cloud can also bring in warm air beneath the cloud itself, thus offsetting the effect of diminished sunlight, as occurs over the Arabian Sea.
Approximately half of the dust in today's atmosphere may be the result of changes to the environment caused by human activity, including agriculture, overgrazing, and the cutting down of forests. Thus, a significant part of the temperature change in Fig. 2 has occurred within this century. The cooling due to dust may partially obscure the warming that is attributed to increasing greenhouse gases, such as CO2. In a future study, we hope to isolate the temperature change resulting from anthropogenic dust aerosols, so that the greenhouse temperature signal can be detected with greater confidence.
Miller, R., and I. Tegen 1998. Climate response to mineral dust aerosols. J. Climate 11, 3247-3267.
Miller, R., and I. Tegen. 1997, submitted. Interaction of mineral dust aerosols with a tropical direct circulation. J. Climate.
Tegen, I., A.A. Lacis and I. Fung 1996. Modeling of particle size distribution and its influence in the radiative properties of mineral dust aerosol. Nature 380, 419-422.
Please address all inquiries about this research to Dr. Ron Miller.