Tuesday, August 11, 2009

World’s dust bowl - Diatom dust - Africa to South America


World’s dust bowl

Changing ecosystems: Recent studies highlight the potential for changes in the quantities of desert dust, and the consequent alterations in ecosystems functioning in places far flung from the sources of desert dust. So, half of all the Amazonian dust supply is from the Sahara desert, writes Meera Iyer

We already know that climate change might drastically alter landscapes around the world. Interestingly, one of the prime agents of changing ecology might be dust from deserts, and often where you least expect it.

Several studies have shown that the Sahara desert is the world’s largest source of desert dust. A mind boggling 240 ± 80 million tons of dust is transported from the Sahara desert to the Atlantic Ocean and beyond every year.

Dustiest place in the world
Within the Sahara, the Bodélé Depression, on the northeastern end of Lake Chad, is notable as the single largest source of dust, responsible for about half of all the dust in the Sahara! This seems extraordinary, considering that the Bodélé Depression is only 150 km2, or about 0.2 per cent of the area of the Sahara desert.

The Bodélé owes its status as the world’s premier dust source to its past history, its unique topography and the resultant weather patterns.

Today’s Lake Chad is but a poor shadow of the vast lake that existed here about 7,000 years ago, at which point Mega-Chad was the world’s biggest lake. Diatoms – a type of algae found in water bodies – thrived here.
The remains of their silicaceous shells were deposited in thick layers on the lakebed, forming a soft rock called diatomite, which was exposed once the lake began drying. This very fine-grained mineral is easily dislodged and transported by the strong near-surface winds here (called the low-level jet) which are accelerated and funnelled by mountains on the north and southeast of the Bodélé.

About 50 million tons of all the dust transported out of Africa reached the Amazon rainforest every year. And about half of all the Amazonian dust supply is from the Bodélé, making it the largest supplier of dust to the Amazon.

For most of us, dust is merely something to be periodically cleared off surfaces, an irritant that we would gladly be rid of. But Saharan dust is in fact a lifeline for the rainforest. The soils of the Amazon basin are typically nutrient-poor, so that the rainforest trees are able to maintain their nutrient balance only through the inputs of nutrient-rich desert dust from the across the Atlantic Ocean.

Impacts of climate change
Two recent studies highlight the potential for changes in the quantities of desert dust, and the consequent alterations in ecosystem functioning in places far flung from the sources of desert dust.

In a paper published in late July in the journal Proceedings of the National Academy of Sciences (PNAS), Richard Washington from the University of Oxford, along with colleagues from Universite Blaise Pascal in France and the Leibniz Institute for Tropospheric Research in Germany, outline how the Bodélé Depression could be considered a tipping element for climate. Taking off from Malcolm Gladwell’s enormously popular book on tipping points – about how “little things can make a big difference”, a tipping element in the earth’s climate describes components of the Earth system that may pass a tipping point.

To determine how climate change may impact the production of dust from the Bodélé, Washington et al.’s paper focused on the controls on the amounts of dust produced, viz., controls on the strong near-surface winds, and on the amounts of diatomite available for erosion.

The authors used leading models of the Program for Climate Model Diagnosis and Intercomparison for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) to estimate the effects of climate change on the Bodélé.

Based on these climate models, and with the caveat that there is a great deal of uncertainty in the models, the authors expect an increase in rainfall over the region. Lake Chad has in fact emptied and refilled more than three times in the last 3-4000 years, with dust output dropping to zero during wetter periods and increasing once again during dry periods.

However, the authors aver that the rainfall increases predicted by the climate change models would be insufficient to cause a drop in dust production.
Instead, the researchers believe that climate change could in fact increase the amounts of Bodélé dust produced over the coming century.

This is based on climate models that also predict an increase in near surface wind speeds in this region in the later decades of the coming century.

Impact on alpine ecosystem
A second paper, also in PNAS, examines the impacts of increasing desert dust on the alpine ecosystem of the San Juan mountains, Colorado, USA. Dust here comes largely from the southwestern United States, with minor inputs from Asia. In the last two hundred years, the introduction and expansion of livestock rearing and railways in the southwest has led to an astonishing 500% increase in dust deposition in the San Juan.
To understand the possible implications of such an increase, Heidi Steltzer from Colorado State University and colleagues, set up experimental plots in the mountains, adding desert dust to some plots, removing naturally arrived dust from a second set and leaving a third set of test plots unchanged.

They found that increased dust deposition caused snows to melt 7 to 13 days earlier. Interestingly, climate warming in the region also advances snowmelt, but because early snowmelt caused by dust is not accompanied with higher temperatures, it has different biological consequences. The researches found that it led to synchronised growth and flowering across species, a result which could impact ecosystem functioning, including nutrient cycling, and inter-species interactions and hence species compositions.

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