Humans implicated in Africa’s deforestation

Climate change alone cannot explain abrupt loss of rainforest 3,000 years ago, study suggests.

Humans may have played a significant part in the sudden disappearance of rainforests from Central Africa, according to a study published online in Science. The work contradicts the prevailing view that the expansion of farming practices on the continent was made possible by the increased incidence of long, severe dry spells that destroyed vast tracts of rainforest.

Geochemist Germain Bayon and his colleagues at the French Research Institute for Exploration of the Sea in Plouzané examined the weathering of sediment samples drawn from the mouth of the Congo River. Because deforestation would intensify weathering, the clay samples provide, in effect, a continuous record of the climate for the past 40,000 years.

When the researchers examined the sediment cores, they found that samples that were between 20,000 and 3,500 years old showed weathering that was consistent with the patterns of rainfall in the region. However, around 3,000 years ago, “there was a complete decoupling” between rainfall and the rate of weathering, Bayon says. The findings, he says, indicate that “climate could not be the only factor in explaining deforestation”.

The team suggests that Bantu-speaking peoples from present-day Nigeria and Cameroon, known to have begun emigrating across Central Africa around 4,000 years ago, had “a significant impact on the rainforest” as they cleared land for farming and iron-smelting.

Man versus nature

Many researchers are sceptical that humans wielding crude implements could have destroyed such large areas of forest.

“Even today with very intensive slash-and-burn cultivation, agricultural practices do not create bare terrain that would be subject to weathering,” says Katharina Neumann, head of archaeobotany at Goethe University in Frankfurt. “How can we imagine that early Bantu farmers with their simple tools and small population were more effective on the destruction of the rainforest than modern farming in Central Africa?”

Alfred Ngomanda, director of the Research Institute in Tropical Ecology in Libreville, Gabon, also believes the prevailing view — that climate change was largely responsible for the loss of rainforest from Central Africa — is correct. But “Bantu famers have probably played a part in forest loss,” he says, “accelerating local forest degradation.”

Bayon says that the latest work does not necessarily contradict existing theories, but rather illustrates how the combination of culture and climate can affect the environment. “Humans can have a huge impact on natural processes,” he says.

The study also raises important questions about the extent to which deforestation and other human activities may be exacerbating the effects of climate change, says David Harris, deputy director of science at the Royal Botanic Garden in Edinburgh, UK. “We need to be especially vigilant about what the present day human effects will be with logging, modern transport, groups displaced by conflict, and modern markets for food and other forest products,” he says.

below, the graph of CO2 emissions over time:



Global warming may trigger winter cooling

It seems counterintuitive, even ironic, that global warming could cause some regions to experience colder conditions. But a new study explains the Rube Goldberg-machine of climatic processes that can link warmer-than-average summers to harsh winter weather in some parts of the Northern Hemisphere.

In general, global average temperatures have been rising since the late 1800s, but the most rapid warming has occurred in the past 40 years. And average temperatures in the Arctic have been rising at nearly twice the global rate, says Judah Cohen, a climate modeler at the consulting firm Atmospheric and Environmental Research in Lexington, Massachusetts. Despite that trend, winters in the Northern Hemisphere have grown colder and more extreme in southern Canada, the eastern United States, and much of northern Eurasia, with England’s record-setting cold spell in December 2010 as a case in point.

A close look at climate data from 1988 through 2010, including the extent of land and sea respectively covered by snow and ice, helps explain how global warming drives regional cooling, Cohen and his colleagues report online today in Environmental Research Letters. In their study, the researchers combined climate and weather data from a variety of sources to estimate Eurasian snow cover, and then they speculated about how that factor might have influenced winter weather elsewhere in the Northern Hemisphere.

First, the strong warming in the Arctic in recent decades, among other factors, has triggered widespread melting of sea ice. More open water in the Arctic Ocean has led to more evaporation, which moisturizes the overlying atmosphere, the researchers say. Previous studies have linked warmer-than-average summer months to increased cloudiness over the ocean during the following autumn. That, in turn, triggers increased snow coverage in Siberia as winter approaches. As it turns out, the researchers found, snow cover in October has the largest effect on climate in subsequent months.

That’s because widespread autumn snow cover in Siberia strengthens a semipermanent high-pressure system called, appropriately enough, the Siberian high, which reinforces a climate phenomenon called the Arctic Oscillation and steers frigid air southward to midlatitude regions throughout the winter.

“This is completely plausible,” says Anne Nolin, a climate scientist at Oregon State University in Corvallis. The correlations between warm summers and cold winters that originally led the researchers to develop their idea don’t prove cause and effect, but analyzing these trends with climate models in future studies could help researchers bolster what Nolin calls “an interesting set of connections.”

“Northern Eurasia is the largest snow-covered landmass in the world each winter,” she notes. It only makes sense, she argues, that it would have a big influence on the Northern Hemisphere’s climate. Indeed, she adds, previous studies have noted the link between Siberian snow cover and climate in the northern Pacific.

The team’s analyses suggest that climate cycles such as the El Niño-Southern Oscillation, the Pacific Decadal Oscillation, and the Atlantic multidecadal oscillation can’t explain the regional cooling trends seen in the Northern Hemisphere during the past couple of decades as well as trends in Siberian snow cover do. If better accounts of autumn snow-cover variability are incorporated into climate models, scientists could provide more accurate winter-weather forecasts, the researchers contend