Continental divide: why Africa's are change burden is greater

Environmental Health Perspectives, August, 2005 by Scott Fields

People who depend on livestock will be just as hard-hit as pastures go brown, Mendelsohn says. But in this case smaller spreads fare better than large ones. Big operations are usually committed to herds of cattle, which demand plentiful water and easy-to-reach areas in which to graze. When water is scarce, large pastoralists are forced to move their herds southward to relatively wetter areas that are usually occupied by sedentary farmers, thus precipitating intergroup conflicts. However, people who have just a few animals can switch to goats and sheep, animals that tend to be more inventive when it comes to finding food and water.

Ecosystem Changes

With changes in land use and climate, some areas in East Africa have become drier, Olson says, and water sources are becoming intermittent or disappearing. Streams that used to run year-round are now seasonal. The expansion of agriculture into savannas also blocks migration routes for large animals such as zebras, wildebeest, and elephants, she says.

As a result of climate-related ecosystem changes, some wild sources of food are also becoming harder to find, says Catherine O'Reilly, an assistant professor of environmental science at Vassar College. The fish stock in the deep Rift Valley lakes of East Africa, for example, are decreasing as average air temperatures rise. These lakes--a chain of fresh and brackish bodies including lakes Malawi, Tanganyika, and Victoria--contain greater biodiversity than any other of the world's freshwater systems, she says. That diversity depends on algae that are supplied when surface waters mix with nutrient-rich deep waters.

"With climate change," O'Reilly explains, "there is less of this mixing, because the [temperature-mediated] density difference between the surface waters and the deep waters has gotten greater, and so it takes more energy to mix deep water up to the surface." Less algae means less food for the entire food web, and the result, she says, is big decreases in fish catches in all of these lakes.

O'Reilly and colleagues reported in the 214 August 2003 issue of Nature that climate change had contributed to a 30% decline in Lake Tanganyika fish stocks over the past 80 years. Such declines can be disastrous for the villages in the region, where the average income is less than US$250 per year, and where the people depend on the fish from these lakes for all of their protein.

When this important food source fades, every aspect of the regional environment is affected. As fish yields go down, increased demands are put upon the land as some fishermen switch to arable farming, O'Reilly says. This in turn leads to more intensified farming, and thus more deforestation, increased erosion, and degradation of the shoreline. Degradation of the shoreline destroys in-shore habitat and spawning grounds for many fish species, further impacting the fish population. "You get a positive feedback loop started," she says, "whereby a small decrease in productivity in the lakes can cycle through all these factors and impact [yet] another aspect of the fish life-history cycle."