Following the steady economic growth that many African countries have experienced in recent years and continued population growth, urbanization has accelerated rapidly on the continent as people turn to cities to take advantage of new economic opportunities. But growing cities have led to another problem. According to a new Food and Agriculture Organization (FAO) report, Growing Greener Cities in Africa, urban populations are exceeding the capacity of African cities to provide food for them, putting nearly 300 million people at risk of hunger and malnutrition, and greener strategies – urban agriculture and better water use – could help considerably.

The poorest households gain the least when countries reap the demographic dividend, according to a preliminary draft of a new study by four Harvard School of Public Health researchers. In “Microeconomic Foundations of the Demographic Dividend,” authors David Bloom, David Canning, Günther Fink, and Jocelyn Finlay write that 18 years of Demographic Health Surveys (DHSs) from 60 developing countries show that while declining fertility rates and dependency ratios can lead to rising incomes on a nationwide level, sub-nationally those trends occur unevenly and mostly benefit wealthier households. Poorer households, meanwhile, are likelier to see slower fertility declines, delaying the economic gains that can result from demographic transitions, and increasing inequality. Importantly, the authors emphasize that the study reflects “the early stages of the demographic transition”; long-term economic effects of fertility decline, they write, “remain ambiguous.”

It’s not often that a change in accounting rules could reduce the probability of war. But that’s exactly what happened at the U.S. Securities and Exchange Commission (SEC) last month.

In 1979, Egyptian President Anwar Sadat famously said that “the only matter that could take Egypt to war again is water.” Sadat’s message was clear: the Nile is a matter of national security for Egypt.

Indeed, Egypt relies on the Nile for 95 percent of its water. But it is not the only state with an interest in the world’s longest river. There are 11 states in the Nile River basin, which stretches from Africa’s Great Lakes region – Tanzania, Uganda, Kenya, Rwanda, Burundi, and the Democratic Republic of the Congo – to the Ethiopian and Eritrean highlands through South Sudan, Sudan, and Egypt to the Mediterranean Sea.

The challenges for cities in the coming century will be many, but accounting for swelling numbers of new residents – due to more open avenues of communication and flows of goods, economic opportunity, population growth, and potential climate change-induced displacement – is perhaps the biggest.

Over the next two decades, as many as three billion people will join the middle class, even as billions more live without electricity, modern cooking fuel, and safe and reliable access to food and water. Resources are becoming more scarce and more difficult to extract, and combined with environmental factors ranging from climate change to soil erosion, those changes will make meeting middle class demand all the more difficult while leaving the world’s poorest more vulnerable to price shocks and resource shortages. In a recent report, the McKinsey Global Institute concludes that nothing less than a “step change” in how resources are managed will be required if individuals, businesses, and governments are to overcome these trends and pave the way for a more sustainable and equitable future.

The original version of this article appeared on The Economist.

Economies benefit when people start having smaller families. As fertility falls, the share of working-age adults in the population creeps up, laying the foundation for the so-called “demographic dividend.” With fewer children, parents invest more in each child’s education, increasing human capital. People tend to save more for their retirement, so more money is available for investment. And women take paid jobs, boosting the size of the workforce. All this is good for economic growth and household income. A recent National Bureau of Economic Research study estimated that a decrease of Nigeria’s fertility rate by one child per woman would boost GDP per head by 13 percent over 20 years. But not every consequence of lower fertility is peachy. A new study by researchers at the Harvard School of Public Health identifies another and surprising effect: higher inequality in the short term.

June’s Rio + 20 Conference on Sustainable Development left environmentalists little to cheer about. Kumi Naidoo, executive director of Greenpeace International, called the meeting “a failure of epic proportions.” And Washington Post reporter Juliet Eilperin said the conference “may produce one lasting legacy: convincing people it’s not worth holding global summits.”