In fact, the world is physically capable of feeding, sheltering and enriching many more people in the short term. Between 1820, at the dawn of the industrial age, and 2008, when the world economy entered recession, economic output per person increased elevenfold.

Life expectancy tripled in the last few thousand years, to a global average of nearly 70 years. The average number of children per woman fell worldwide to about 2.5 now from 5 in 1950. The world’s population is growing at 1.1 percent per year, half the peak rate in the 1960s. The slowing growth rate enables families and societies to focus on the well-being of their children rather than the quantity.

Nearly two-thirds of women under 50 who are married or in a union use some form of contraception, which saves the lives of mothers who would otherwise die in childbirth and avoids millions of abortions each year — an achievement that people who oppose and people who support the availability of legal abortions can both celebrate.

But there is plenty of bad news, too. Nearly half the world lives on $2 a day, or less. In China, the figure is 36 percent; in India, 76 percent. More than 800 million people live in slums. A similar number, mostly women, are illiterate.

Some 850 million to 925 million people experience food insecurity or chronic undernourishment. In much of Africa and South Asia, more than half the children are stunted (of low height for their age) as a result of chronic hunger. While the world produced 2.3 billion metric tons of cereal grains in 2009-10 — enough calories to sustain 9 to 11 billion people — only 46 percent of the grain went into human mouths. Domestic animals got 34 percent of the crop, and 19 percent went to industrial uses like biofuels, starches and plastics.

Of the 208 million pregnancies in 2008, about 86 million were unintended, and they resulted in 33 million unplanned births. And unintended births are not the whole problem. Contraceptives have been free since 2002 in Niger, where the total fertility rate — more than seven children per woman in mid-2010 — was the world’s highest. Women in Niger marry at a median age of 15.5, and married women and men reported in 2006 that they wanted an average of 8.8 and 12.6 children, respectively.

Human demands on the earth have grown enormously, though the atmosphere, the oceans and the continents are no bigger now than they were when humans evolved. Already, more than a billion people live without an adequate, renewable supply of fresh water.