The United Nations says that more than 850 million people in the world do not have enough to eat. Poverty, disease and conflict have always threatened food security. But other causes of hunger have emerged: rapid urbanization, the biofuels boom, a sudden spike in food prices, and crop damage from global warming. VOA's Leta Hong Fincher takes a closer look.

This is Kibera, the largest slum in Kenya's capital, Nairobi. It has no clean water or sewage system, but it is home to more than 600,000 people. John Otieno is one of them. "The settlement lacks everything. It's kind of a forgotten community."

Such "forgotten communities" also are home to more and more hungry people. Agriculture experts say that urban residents are suffering from food shortages, as tens of millions of people migrate from the countryside to the world's cities each year. Unlike traditional farmers, city dwellers live far from where their food is grown, and have little control over their resources.

Danielle Nierenberg is a researcher at the Worldwatch Institute, an environmental group in Washington. "For livestock keepers, for instance, chickens and pigs are like walking credit cards. They allow people to sell off their pigs or chickens or cows in times of need or when their children need school uniforms or when a person in their family gets sick."

Without livestock of their own, Nierenberg says, poor, urban residents spend between 50 and 80 percent of their income on food. This makes them especially vulnerable when food prices increase, as they have in recent months.

Historically, food prices have fluctuated. But some trade experts say the cost of many foods could stay high permanently, because of rising demand from population growth, slower growth in agricultural productivity and shrinking natural resources.

Charlotte Hebebrand is head of the International Food and Agricultural Trade Policy Council in Washington. She says governments must ask themselves a key question. "How do we get the most out of our agricultural production in a sustainable fashion, but also in a fashion that really allows us to draw on technological resources to increase yields, so that we can feed more people, using basically the same amount of land that we have available and possibly decreasing water supplies."

Hebebrand says that world food demand could double by 2050. But new research shows that world food production will face a serious decline in coming decades because of global warming.

William Cline is a senior fellow at the Center for Global Development and the Peterson Institute for International Economics in Washington. He says developing countries will be hit hardest by crop damage because of their closeness to the equator. "Global agricultural potential could fall by about five to 15 or 20 percent as a result of global warming if nothing is done by the 2080s. But secondly, that would mask a much deeper loss, something like 30 to 40 percent in India, for example, and something like 20 percent or more in Africa and Latin America."

Concern about global warming has led governments to invest in biofuels derived from plants as an alternative to oil. But some environmentalists warn that the boom in biofuels such as ethanol is driving up the price of grain, making food even more unaffordable for the poor.

Lester Brown is head of the Earth Policy Institute in Washington. "What we're now seeing is an entirely new historical phenomenon, something that we've never experienced before," he says. "And that is direct competition between the 860 million people in the world who own automobiles and the two billion poorest people in the world, who are simply trying to get enough food to survive."

In the future, environmentalists say biofuels can be produced from non-food products such as leaves and wood. Another way to increase food security is to support urban farming so that the world's city dwellers can feed themselves more efficiently. Agriculture experts agree that much more investment will be needed to produce more food from fewer resources.