In April 2001, the U.S. Census Bureau announced that the total population of the United States on April 1, 2000, was 281,421,906. The apparent precision was entirely spurious. The Director of the Census Bureau later remarked that the true population of the United States in 2000 is known only to within 5 million persons or so
(so plus or minus 2%). The amount of error and uncertainty remains or increases for historical estimates, current estimates and future estimates.
According to a preliminary announcement of the United Nations Population Division (2001), the difference between the medium projected population of the world in 2050 according to its 1998 projection (8.9 billion) and its 2000 projection (9.3 billion) is 413 million people, roughly 5 percent of the 1998 projected population size for 2050. The population project at the International Institute for Applied Systems Analysis projected a median (not medium) world population in 2050 of 8.8 billion. These three projections for 2050 differ in methods and assumptions. The differences in output among them of half a billion people express demographers’ ignorance of the future, not differences in genuine knowledge.
The demographic present, past, and future are surrounded by uncertainty. Population projections are uncertain for several reasons. The initial data may be erroneous. The rates of birth, death, and migration in cohort-component models may be projected erroneously. (Projections for countries or small areas normally are more uncertain than those for the world because of the greater importance of migration for local areas.) External factors may change unexpectedly (such as AIDS, natural hazards, or climate). External factors may change as expected but the relationship between those factors and demographic rates may change. Policies and programs may develop to influence the rates of birth, death, and migration.
Estimates or guesses about historic, current and future economic activity (GDP etc…) build upon the uncertainty around population.
Most demographers would probably agree to four statements about the future of global population during the 25-50 years.
1. the population will be bigger than it is now. The world will be bigger by 2 billion to 4 billion people by the middle of the century, and nearly all of that growth will be in poor countries, not rich.
2. the population will increase less rapidly, absolutely and relatively, than it has recently. Whether population growth ends depends on choices that we make right now about reproductive health, education for women, and many other investments.
3. the population will be more urban than it is now. Practically all of the additional people will be living in cities in poor countries, and that will be an unprecedented epidemiological challenge to infectious disease control.
4. the population will be older than it is now.
Canada and US Population estimates
Under the scenarios considered, Canada’s population would be between 36.0 and 42.0 millions in 2031 and between 36.0 and 50.0 millions in 2056. In the medium-growth scenario, the population would be 39.0 millions in 2031 and 42.5 millions in 2056.
Economic estimates for the size of China’s economy can vary over 1200% for 2040. Nobel Prize winning economist Robert Fogel projects that China’s economy will be $123 trillion in 2040
Researchers at the Institute of Quantitative & Technical Economics of the Chinese Academy of Social Sciences forecasted in 2007 that China’s economy would grow to about $5.9 trillion in 2030 and $6.7 trillion in 2035.
An economist who could predict annual economic growth to within 1% would be doing pretty well. But if there was a consistent under-estimation or overestimation of economic growth for China, USA, Europe or the world of 2% over the course of decades then the result could easily be off by 100% by 2040. The annual economic figures themselves are inaccurate to start with. Then you start getting into surprises like a pandemic or financial crisis or surprising successes like new technology or business processes or policies which increase growth. The policies which helped India breakout of the Hindu growth rate. China shifting to opening its economy in the late 1970s and 1980s. Containerized shipping for more growth. Walmart providing economic productivity gains. The internet.
Even getting a handle on how much those things helped or hindered in the past is uncertain, so it is tough to determine how much things are helped going forward.