How many people could live on Earth? Many scientists have tried to calculate that number, with widely divergent results. Seventeenth-century biologist Antonie van Leeuwenhoek put the upper limit at 13.4 billion; in 1967, biochemist C.T. De Wit said one trillion. Population modelers now say there is no single answer. The population that Earth can ultimately carry, and the quality of life that those people will have, depends on political and environmental choices. sustainability-research organization in Boston, has made some of the most sophisticated predictions yet of how those decisions will play out. PoleStar, the computer simulation that Tellus developed, starts with projections of population and economic growth from consumption, land use and pollution. The simulation's outcomes show that how we shape policy now will determine whether the next century's population lives in a pleasant world or one of degradation and scarcity. It's not too late to choose one of the brighter paths, says Richard Rosen, the executive vice president of Tellus, "but you'd have to get going immediately. There's no leisurely way.

Forces

Business as usual—the economy grows, technology advances. Poorer regions build up industries, and environmental problems become more serious.

Reform

Governments take rapid action to meet U.N. climate targets and other sustainability goals, but economic growth remains the strongest factor in developing new policies.

World

Environmental, economic and social problems overwhelm current systems, and governments become authoritarian. The wealthy retreat to protected enclaves, leaving poor masses in a degraded wasteland.

Transition

Society's values change radically to prioritize environmental preservation, social equality and cooperation.

Policies that make family planning available to all social strata will help control population growth. Without such measures, the global population could top 10 billion by the end of this century.

The maximum number of people that could ultimately be packed onto the planet, with all other land used solely to grow food, according to geophysicists at the Potsdam Institute for Climate Impact Research in Germany.

In 2011, the global population reached
7 billion.

In 2005, 14 percent of the population went hungry. Depending on how resources are allocated, this number could shrink to zero by 2100.

|15| |---| |12| |---| |9| |---| |6| |---| |3| |---| |0| |---| 2025 2050 2100

Exajoules

|1200| |---| |900| |---| |600| |---| |300| |---| 2025 2050 2100

Gigatons of Carbon

Megatons

In 2005, the poorest 20 percent of society made 12 cents for every dollar the richest 20 percent made. How will policy decisions affect wages?

Market Forces

Policy Reform

Fortress World

Great Transition

2005
27%

In 2005:

Cropland
12%

Grazing
26%

Forest
30%

Built-up
2%

In four possible futures:

Transition

Reform

Forces

World

2034

2047

2049

2071

2088

2105

2123