The trees of the South American Amazon rainforest and the flat grassland of the African savannas are literally a world apart but they both may have an impact on weather conditions around the globe. Large tracts of the Amazon rain forests are being cut down for lumber and to create farmland.
According to David Werth, a meteorology professor at Duke University in North Carolina, told a recent meeting of the American Meteorological Society that the amount of deforestation has not yet had a severe effect on the local or world climate, but he warns it could in the future.
"Computer models have predicted strong effects on local climate when the Amazon is deforested," he said. "That usually involves extreme deforestation, much more than it has actually happened. One thing we are modeling is trying to see is the global effects due to Amazon deforestation. If you get rid of the Amazon rain forest, will that affect areas other than the Amazon? So, what we've seen is that there are some areas that do seem to have a reduction in precipitation when the Amazon is deforested."
To make those calculations, scientists have 'told' the computer that the rain forest has been replaced by light-colored vegetation, like grass, that does not release moisture from the ground as well as trees do.
"If you replace the Amazon rain forest with grass, moisture cannot be removed from the deep layers of the soil as quickly," he said. "So you are sort of removing a source of moisture from the local weather, which has a kind of a drying effect."
A similar effect is seen in Africa, where the savanna or grassland is also shrinking, as the land is converted to crops and animal pasture.
According to Sharon Nicholson, Distinguished Research Professor of Meteorology at Florida State University, Removal of much of the vegetation already has led to changes in the local climate since more of the Sun's radiation is being reflected back into space, and the soil can't retain as much moisture as before.
"But one of the most important factors is the increased dust production," she said. "And this is related to things that happen at the surface, like vegetation removal, changes in the soil surface. The increased dust production can be both the result of the decreasing rainfall in the region, as well as human activities."
During the past 30 years, rainfall over many of the productive regions of West Africa has declined by 30 to 40 percent. And scientists believe one reason for this is the loss of the savanna just as they worry that deforestation in the Amazon will lead to decreased precipitation there. At the American Meteorological Society meeting, Professor Nicholson recommended a very careful look at how changes in the land surface affect weather processes on a small scale, especially whether individual storm systems that bring rainfall in periods of one to three days might be influenced by what is happening on the ground.
"We do know that degradation of the land surface can change the heat and moisture exchange with the atmosphere in ways that affect the storms, you can institute conservation measures at the ground that try to bring things back to a more natural state," she said. And Professor Nicholson points out that degradation of the land surfaces in West Africa and the Amazon has an effect beyond the local climate. The computer model David Werth used to analyze atmospheric moisture if the rainforest is cut down suggests that the air would get drier, winds above the Amazon would become a little faster and that rainfall would therefore be reduced. Through an atmospheric mechanism meteorologists call "wave dynamics," this decrease in precipitation could spread to faraway regions of the globe.
"When you alter the local climate of the Amazon, you are altering the heating at different altitudes," he said. "And when you have changes like that, you can stimulate waves in the atmosphere, which are roughly similar to waves in the water. They propagate away from the Amazon into other parts of the world and have an effect on those other parts of the world."
According to the computer modeling, this effect would be felt as far north as the United States and Canada, and across the Atlantic and southern Africa to the Indian Ocean. Similar effects might follow the loss of the savanna in Africa. The meteorologists say this gives the entire world a stake in what happens in the rainforests of South America and the savannas of Africa.