How many people will live on the planet 20 years from now? Where will they live? Where will population grow, and where will it decline?
The map on this page illustrates a projected possible answer to these questions, applying new methods of mapping population density and of projecting its future.
No one can predict the future of human population with any certainty, but United Nations demographers offer a series of projections that suggest a world population of between 7.5 billion and 8.3 billion in 2025. (A projection is a conditional forecast based on specific assumptions about the future.) That compares to today’s 6.5 billion people. Even the highest of these projections assumes continued declines in family size and increases in life expectancy worldwide, and there is no certainty of either. Still, such projections are taken as the best expert guess at the range of likely future population sizes.
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Although demographers have projected the populations of individual countries for decades, until recently more detailed projections of highly localized population change have not been attempted. But the advent of high-resolution mapping and new detailed datasets of population density begin to make such maps possible. This map, a product of a collaboration of researchers atColumbia University’s Center for Climate Systems Research(CCSR) and Population Action International (PAI), uses both population projections and population density maps produced in the 1990s to project population density in spatial detail for the year 2025. Although at first glance the map may appear to be illustrating population density, it does that only indirectly. The colors and shading of each “grid cell”—which are 2.5 minutes of arc or about three miles to a side at the equator—indicate broad categories of projected population gain or loss in that grid cell between 1995 and 2025. Projected increases in population density are shown as progressively darker shades of orange and decreases in density in blue.
Population density varies widely, from the one or two people who may inhabit a square mile of scrub to the hundreds of thousands on the same amount of land in Hong Kong. The maps handle this diversity of densities by using a logarithmic scale for the different gradations of color, meaning that each shade of color represents about 10 times more people, added or lost to the grid cell, than the lighter shade next to it on the scale.
The medium projection, most commonly cited among UN projections, is the one used to make this world map of projected population distribution in 2025. The upper and lower curves define the range of what the UN demographers consider possible paths for world population between the present and the end of the 23rd century.Source: United Nations Population Division, 2003
How the Maps Are Made
How did the researchers at CCSR and PAI arrive at the projected population gain or loss of each grid cell? First, they took two maps of past population density called the Gridded Population of the World, which represent density in 1990 and 1995. These maps, maintained by Columbia University’s Center for International Earth Sciences Information Network (a separate center at the university from CCSR), are based on detailed census and similar information for spatial distribution of human population worldwide. Taken together, the two maps show population density in nearly 9 million grid cells at two different times in the 1990s. They can also be compared to show changes in each grid cell between 1990 and 1995. These changes can then be extended to 2025 for each grid cell and adjusted so that all grid cells within a specific country added together are equal to the projected population under the United Nations Population Division’s medium projection of population for 2025.
Thus the maps shown here reconcile two very different sets of data: a detailed map of population distribution in the recent past by small grid cells, and a population projection made for a future date and for entire countries. The adjustment necessary for each cell is more complicated than it might seem, however. Given the diversity of population change between 1990 and 1995, there’s no easy or obvious way to bring all the grid cells smoothly to the point where a nation’s population equals the total that UN demographers project for 2025. The PAI and Columbia University researchers elected to use a technique known as share-of-growth for most of the grids, calculating each grid’s share of a nation’s population growth from 1990 to 1995. The researchers then extrapolated these shares of growth into the future as the national population grew to its projected 2025 total.
In a few cases, however, this technique actually produced the impossible result of negative population density. In such cases, the researchers used an alternative method known as shift-share, which extrapolated changes in each cell’s share of total national population rather than a share of that population’s growth. Where some cells still “went negative,” the researchers simply declared the grid cells uninhabited in 2025 and adjusted the rest of that nation’s grid cells accordingly to add up to the UN projection total.
There may be other methods or refinements for spatially projecting population data, especially as data on later population distribution become available. The researchers consider this a work in progress and hope to continue their work in this arena.
What They Show
Can we say anything new about world population as a result of looking at these maps and the spatial projections of future density that they depict? Several observations emerge, some more surprising than others:
- Globally, population is still growing and is projected to be much larger in 2025 (at 7.9 billion people) in the medium projection than today (6.5 billion). The vast majority of the world’s inhabited land surface is currently experiencing, and is expected to experience, population growth in the 30-year period from 1995 to 2025.
- Nonetheless, large swaths of the earth’s inhabited land surface are projected to experience population decline between 1995 and 2025. This projected decline is particularly significant in Eastern Europe, much of which has already begun experiencing decreases in population. More surprisingly, small pockets of such developing regions as sub-Saharan Africa and South America are projected to lose population by 2025. The presence of dispersed areas of population decline in these regions reminds us of the incredible diversity of population change around the world.
- Although it is only indirectly evident in these maps, most of the areas of greatest projected population growth—regions such as northern South Asia and eastern Asia—happen to be the most densely populated today. These areas, along with many others around the world, are likely to face significant challenges in adjusting to rapid population growth over the next few decades.
Maps to Where?
A final fact is not evident in any map of human population: Little about population change is inevitable. Decisions that societies and their governments make today can have a significant impact on the paths that population size, structure and distribution follow in the years to come.
Population Action International hopes that this map will help spark further efforts to map in detail the population-density implications of major population projections. In each region and country, such maps could benefit greatly from adjustment by local demographers, geographers and other experts who know the specific developments likely to happen in their areas that may affect future population density. We hope also that the maps will spur an increase in public and policymaking discussion in all countries about the implications of the world’s population growth as it heads to 7 billion people and beyond.