Dust Vegetation Climate

So far, we have considered albedo, roughness and evaporation of water in the feedbacks between vegetation cover and climate. Another potentially important vegetation-climate feedback comes from dust. The dust in the atmosphere mostly consists of particles of soil, fragments of the sorts of minerals that make up rocks and clays. These tiny particles tend to scatter sunlight. Dust is really a product of vegetation cover, or rather a lack of vegetation cover; areas with lots of bare soil between clumps of grass and scattered bushes tend to be the biggest contributors of dust to the atmosphere, when the wind blows across the bare surface and carries particles of dry soil aloft. In contrast, when the vegetation cover forms a continuous mat, the roots bind the soil together. With such a root mat in place, even if the soil dries out sometimes it does not crumble at the surface and get blown away. The leaves and stems of the vegetation also interrupt and slow the wind, preventing it from picking up dust from the soil surface.

As one might expect, heavily forested areas (with a deep dense root mat to bind the soil, a dense canopy to interrupt the wind and plenty of rain to keep the soil moist) contribute almost no dust to the atmosphere. Conversely, one might think that the entirely bare desert surfaces such as the central Sahara would contribute the most dust to the atmosphere—because there are no plants there to stop dust being whipped up by the wind. In fact, far more of the dust that floats around in the world's atmosphere comes from the relatively thin strips of desert margin, such as the Sahel, than from the extreme desert interiors. Why would this be? Because it is only in climates with some moisture and some vegetation activity that the rock underneath can be broken down to supply dust (a process known as weathering). In a totally dry environment, whatever dust there once was has blown away, ending up either in the sea or fixed into the soil of zones of wetter climate. There is no supply of new material and all that is left are bare stony surfaces and sand fields—the sort of landscape that makes up the Sahara. In a very moist, forested environment, breakdown of rocks can be rapid but the soil is never exposed to dry out in the open, so the clays and other minerals within it do not get picked up by the wind. A semi-arid environment such as the Sahel offers the ideal combination from the point of view of getting dust into the atmosphere: enough biological activity to chemically break down rocks into fine particles, and enough bare dry soil to allow those particles to be picked up by the wind.

By scattering some light but absorbing some too, dust has an ambivalent effect on climate. Much of the light that dust particles scatter goes back into space, so the earth is cooler because less of the warming sunlight can reach it. Also, dust that floats high in the atmosphere "wastes" some of the heat from the light that it does absorb, because it is floating above the main blanket of greenhouse gases. Without much greenhouse gas above it to trap the heat, the dust loses its heat easily back into space instead of heating the atmosphere around it and the surface below, and the earth is left cooler than it would otherwise be. On the other hand, dust itself can act like a greenhouse gas. It absorbs some infra-red radiation on its way out from the earth's surface, and then sends some of that back down to earth. So infra-red that could have been lost to space is bounced around between each dust particle and the earth's surface. This has the effect of warming the earth slightly. Also, although dust is "bright" and scatters a lot of sunlight, it is not as bright as snow and ice. If there is dust in the atmosphere above a snow-covered ice cap, the dust will actually trap more of the sun's heat than the brilliant white surface below would, and so it will help to warm the air.

The overall balance between these opposing factors determines whether dust warms or cools the earth. There is some uncertainty amongst climate scientists as to whether the cooling or warming influence is more important, and by how much.




Figure 5.10. The opposing effects of dust in the atmosphere on temperature at ground level.


Figure 5.10. The opposing effects of dust in the atmosphere on temperature at ground level.

However, opinion favors a cooling influence for dust, in most places and at most times in the earth's history.

Dust can also have two opposing effects on rainfall (Figure 5.10). It can potentially increase rainfall, by providing condensation nuclei for water droplets. This helps the clouds to form more quickly and more abundantly, giving more rain. On the other hand, a layer of dust in the atmosphere that heats up under the sun can act as a "lid", preventing upwelling of air. This makes rain less likely. Generally, it is thought that the "rain-suppressing" influence of dust dominates over the "rain-making'' influence.

So, by acting as a source of dust, semi-arid areas tend to make climates cooler and they also tend to make them drier. How extensive their influence is depends how long the dust stays aloft before it rains back down onto land or into the sea. Some climate scientists have suggested that the dust that builds up in the atmosphere in the Sahel during a drought is instrumental in intensifying and prolonging the drought. A study by Masuru Yoshioko and colleagues used a special dust model (designed to simulate how dust gets blown aloft into the atmosphere) in combination with a climate model to simulate the feedbacks from dust in the Sahel. They suggested that about 30% of the fluctuation in rainfall that occurs over a series of decades is actually due to the effects of dust from the region being whipped up by the wind into the air above. The initial "trigger" could be a change in sea surface temperature over the Atlantic that started off the drought, but the dust in the air would help to hold the climate system in a dry state for several years, with the dryness caused by the dust ensuring that more dust kept on reaching the atmosphere. So, this is a clear positive feedback, acting in parallel with other effects of vegetation cover such as albedo and evaporation. Their model is at odds with the prevailing picture, in suggesting that the dust effect is actually much more important than other vegetation feedbacks in controlling the rainfall over the Sahel. The importance of dust in drought phases in the Sahel and in other arid regions is still very much a moot point, but now we have some tantalizing clues that it might be very important.

Dust may also be able to influence climate globally on the timescale of tens of thousands of years. A geological history of the amount of dust in the atmosphere comes from coring down into ice sheets and deep ocean sediments. Dust particles that rain down out of the atmosphere end up buried in layers of snow if they land on ice sheets. If they land on the sea surface they will sink down to the sea bed and be buried in the sediment. Using sensitive techniques to study dust content in these ice and ocean bed cores, one can estimate how much dust was around in the atmosphere at particular times in the past.

The picture emerging from the last couple of million years is that the cold "glacial" climate phases tended to be much dustier than the warmer "interglacial" periods. In some areas there were tens of times as many dust particles in the atmosphere, showing up now in cores from the ice or sediment in which they were buried. Overall, the world seems to have had something like three times as much dust in the atmosphere during glacials as it has today. A big increase in the dustiness of the atmosphere is much as one might expect from comparing vegetation maps of the two types of climate phase. Glacial phases have much less forest vegetation, and far larger areas of desert and semi-desert, compared with interglacials. In fact, starting from a plausible vegetation map of a glacial phase, and using a climate model to estimate how much dust would be whipped up and carried by the wind, one can fairly accurately predict the extra amount of dust in the atmosphere during a glacial. Additional dust would also be coming from the edges of ice sheets where ground-up rock debris was being dumped and drying out. All the extra dust in the atmosphere during glacials surely had some significant effect on climate. Most likely it reinforced the cold and aridity of the time, by reflecting the sun's light back into space and suppressing the rain-giving convection of the atmosphere. The actual effect of this dust on global climate during glacials still needs to be simulated using a GCM that is ambitious enough to incorporate dust fluxes, in addition to everything else.

However, there might have been times when dust actually brought about a warming in climate. Jonathan Overpeck and colleagues at the University of Colorado pointed out that compared with the very bright surface of an ice sheet on land or floating sea ice on the ocean, dust in the atmosphere is rather darker in color. This means that if it blows over the top of a region of ice, either staying in the atmosphere or settling on the top of the ice, it will tend to mean that more of the sun's heat is absorbed. This will bring about a warming of climate, perhaps helping to melt the ice back. During ice ages, if the winds blow in certain directions and carry dust over ice sheets they might actually help to bring about the end of the glacial climate. Over-peck's group used a climate model to show that once ice sheets reach a particular size and extent, they might set off the process of their own destruction by sucking in dust.

An additional and very different effect of dust on climate may work through ocean plankton. It is thought that the growth of plankton out in the open ocean is often limited by lack of iron, and mineral dust happens to be very rich in iron. Experiments show that adding iron salts to a tank of surface ocean water will often produce a "bloom", a population explosion of algae floating in the water. It seems that, fueled by the iron they need, the algae are able to use up the additional small amounts of other nutrients—such as nitrogen and phosphorus—and multiply. Other more ambitious experiments have actually involved dumping iron salts off the back of ships traveling across the ocean. Within a few days, all along the path of the boat there is often (though not always) a bloom of phytoplankton, detectable by satellite. This burst of phytoplankton growth lasts between a few days and a couple of weeks before it disperses or is eaten up by hungry zooplankton. Given what a modest addition of iron can do, some oceanographers have wondered what effect a big increase in dust flux might have during glacial phases. Is it possible, for instance, that the increased iron input from all the dust greatly increases phytoplankton growth. The increased growth of the plankton could drag down more carbon to the deep sea, helping to decrease the CO2 level of the atmosphere (Chapter 7). Hence, the greater dustiness of the atmosphere during glacials could be part of the cause of lower CO2 levels. Since lower CO2 is likely to be part of the cause of the climate and vegetation conditions that produce more dust, what we have here is a positive feedback loop that reinforces the glacial climate.

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