for National Geographic News
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).
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