On May 26th, 2008, Germany turned red. The winds of change, though, were meteorological, not political. Unusual weather brought iron-rich dust from Africa to Europe, not only altering the colour of roofs and cars on the continent but also, according to recent calculations by Max Bangert, a graduate student at the Karlsruhe Institute of Technology, making the place about a quarter of a degree colder for as long as the dust stayed in the air.
Unusual for Germany; commonplace for the planet as a whole. The Sahara and other bone-dry places continually send dust up into the atmosphere, where it may travel thousands of kilometres and influence regional weather, the global climate and even the growth of forests halfway around the planet.
Earlier in 2008, for instance, Ilan Koren and his colleagues at the Weizmann Institute of Science, in Israel, detected a particularly voluminous burst of dust from the Bodélé Depression. This low-lying bed of silt in Chad, across which powerful jets of wind are wont to blow, constitutes less than 1% of the Sahara’s area but is reckoned the world’s dustiest place. It is thought to be responsible for a quarter or more of the Sahara’s output of airborne dust.
Dr. Koren observed the dust rise with a camera on a satellite called Aqua; watched it obscure the sun using an automated photometer in Ilorin, Nigeria; followed it across the Atlantic with another satellite, CALIPSO; and finally saw a spike in levels of silicon, aluminium and iron as it landed on detectors in Manaus, Brazil. His results, presented at a meeting of the American Geophysical Union held in San Francisco in December, provide a remarkable account of the intercontinental transfer of dust.
Blowin’ in the wind
The importance of this long-distance logistical chain has become apparent only in the past few years, and researchers are still working out its many repercussions—for the more you look at dust, the more effects it seems to have. African dust is thought, for example, to stimulate plant growth in the Amazon by bringing in phosphorus (which is in short supply there). This may put a check on global warming by removing what would otherwise be a long-term constraint on the forest’s ability to suck up carbon dioxide as it grows.
Dust which does not reach land may do something similar to the sea. Some parts of the ocean are short of iron, which red desert dust has in abundance. Dust from the Gobi desert seems to stimulate plankton blooms in the nutrient-poor waters of the North Pacific, though it is not clear whether this results in a net reduction of atmospheric carbon dioxide, since that would require some of the plankton to sink to the seabed, never to return.
Dust aloft cools the land below, as Europe’s meteorologists found out in May 2008. It does this directly, by reflecting sunlight back into space, and indirectly, by helping clouds to form. The effect is significant. The carbon dioxide which has been added to the atmosphere since the industrial revolution began has a greenhouse effect equivalent to the arrival of about 1.6 watts of extra solar power per square metre of the Earth’s surface. The direct effects of dust are estimated to provide a countervailing cooling of about 0.14 watts per square metre. Add the indirect effect on clouds and this could increase markedly, though there are great uncertainties.
This dust-driven cooling, though, is patchy—and in some places it may not even be helpful. Dust that cools a desert can change local airflow patterns and lessen the amount of rain that falls in surrounding areas. This causes plants to die, and provides more opportunities for wildfires, increasing the atmospheric carbon-dioxide level.
To get a better sense of the net effects brought about by the ups and downs of dust, it would help to have a detailed historical record of the dustiness of the planet. And this is what Natalie Mahowald of Cornell University and 19 colleagues have achieved. They analysed cores from glaciers, lake bottoms and coral reefs and measured how the levels of some telltale chemicals changed with depth, and thus with time. They then used models of global wind circulation to deduce which dust sources have become stronger and which weaker. Their conclusion, published recently in Atmospheric Chemistry and Physics, is that in fits and starts over the past century the air became twice as dusty.
Part of the increase stems from human activities—directly, in the case of construction, or indirectly, when it results from clearing vegetation from marginal land in order to farm it. Another part of the explanation may be global warming itself, shifting the boundaries of deserts and intensifying dust production in some areas.
How many times must a man look up?
The amount of dust actually injected into the atmosphere, though, may have been significantly underestimated. In a recent paper in the Proceedings of the National Academy of Sciences, Jasper Kok of the National Centre for Atmospheric Research, in Boulder, Colorado, writes that the amount of coarse dust driven into the atmosphere by wind is at least double and may be eight times as much as previously thought.
He arrived at this conclusion not by measuring dust directly from planes or satellites, which see only a bit of the atmosphere at a time and are not necessarily good at picking up all signs of dust, but by reasoning his way to a model of how loose soil, some fine and some coarse, is affected by the wind and lifted into the air.
Fine particles of dust do not simply lie around until they are blown away by the wind. Rather, they stick together in clumps. Only when these clumps are broken up is the dust liberated. That happens when heavier particles are lifted by the wind and then fall back to the ground, hammering and shattering the dust-clumps as they do so. Dr. Kok shows that this shattering, like the shattering of all sorts of other things, produces a distinctive mix of particle sizes. This mix does not match those currently used in climate models. It has more bigger particles and fewer smaller ones. The discrepancy seems not to have been noticed before because existing ways of measuring dust are biased towards the finest material—that which most influences air temperature and cloud formation.
The consequences of this reassessment are unclear, since the effects of coarse dust are not well understood. Also, the larger particles fall out of the atmosphere more quickly. What is clear is that it is yet another example of how fiendishly complicated the atmosphere is, and what a broad set of approaches is required to understand it.
