Global Warming May Unleash "Sand Seas" in Africa, Model Shows
for National Geographic News
|June 29, 2005|
Global warming threatens to stir up southern Africa's enormous dune
fields, according to a new study.
Scientists warn that the Kalahari dune fields, which are presently stable and covered by vegetation, will undergo widespread reactivation this century as a result of declining rainfall, increasing droughts, and rising wind strengths.
"This could have major consequences for several states and for the people who farm the land in these areas," said David Thomas, a physical geographer at Oxford University in England. Thomas led the study, which is published tomorrow in the academic journal Nature.
Wind and Erosion
Many studies of the impacts of global warming on environments have tended to focus on ice caps, glaciers, and coastlines.
Although desert dunes cover 5 percent of the global land surfaceand up to 25 percent of Africa, albeit in largely stable formthere has been relatively little interest in how climate change will affect the dynamics of the landscape.
"Yet some of these [landscapes] are potentially very vulnerable to climate change," Thomas said. "We know how sensitive they have been to major climate changes over past millennia."
Desert dunes shift as winds pick up sand grains and dump them elsewhere, potentially turning vegetated land into desert. But dune field dynamics depend not only on winds, but also on the erodibility of the dune surface. Such erosion is determined by the amount of vegetation cover, and how dry the ground is.
The Kalahari region, which stretches for a million square miles (2.5 million square kilometers) from northern South Africa to Zambia and beyond, developed in multiple arid phases since the last interglacial period.
While stabilized and vegetated, the dune fields are often degraded, since in some cases it has been many thousands of years since they were last active.
To study the sand dunes and their sensitivity to global warming, Thomas and colleagues used a "dune mobility index." The index integrated the two key environmental elements that drive dune activity: erodibility (the ease with which dunes erode) and erosivity (the wind energy applied to dune erosion).
The researchers used different climate models and different emission scenarios to predict what effect global warming will have on the desert dunes.
"We found that as the century progressed, the dune fields of the mega-Kalahari reactivate, partly due to erodibility increasing as precipitation declines, and partly as wind energy increases, especially toward the end of the dry season when surfaces are least vegetated," Thomas said.
The study showed that the southern dune fields of Botswana and Namibia, which are the driest, will become activated by 2040. The more northerly and easterly dunes in Angola, Zimbabwe, and Zambia will begin to shift by 2070.
By the end of the 21st century, all the dunes in the Kalahari region are likely to be on the move.
"These are areas where dunes are currently wooded in many places," Thomas said. "We'll potentially see major environmental changes [with] currently vegetated but sandy landscapes reverting to active, blowing sand seas where life will potentially be very difficult."
Such widespread, desert-like conditions with active sand dunes throughout the Kalahari area have not occurred in more than 14,000 years.
The sand would not necessarily blow all year round, but mainly in the windy months. This is the case in the hyperarid dune fields that are found today in parts of Saudi Arabia and the Namib Desert in southern Africa.
"Often it is the high points of the dunes, the crests, that blow first as the wind strengths tend to be higher there," Thomas said. "But as lower dune slopes lose vegetation, they become susceptible to transport, too."
The dune field areas in the Kalahari often support livestock farming, which will become much more difficult if these presently quiet dune fields revert to activity, experts say.
"This is a very important [study] that shows how currently semi-arid area may respond to global warming," said Nicholas Lancaster, a research professor at the Division of Earth and Ecosystem Sciences at the Desert Research Institute, based in Reno, Nevada.
"The implications for southern Africa are hugeespecially for cattle herders, wildlife, and tourism," Lancaster added.
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