This article lists estimates of world population over the course of history and prehistory, as well as projections of future developments. In summary, estimates for the progression of world population since the late medieval period are in the following ranges:
Contents
Estimates for pre-modern times are necessarily fraught with great uncertainties, and few of the published estimates have confidence intervals; in the absence of a straightforward means to assess the error of such estimates, a rough idea of expert consensus can be gained by comparing the values given in independent publications. Population estimates cannot be considered accurate to more than two decimal digits; for example, world population for the year 2012 was estimated at 7.02, 7.06 and 7.08 billion by the United States Census Bureau, the Population Reference Bureau and the United Nations Department of Economic and Social Affairs, respectively, corresponding to a spread of estimates of the order of 0.8%.
Deep prehistory
Most published estimates of historical world population begin at "year zero" of the Common Era, when world population was in the nine digits (estimates range between 150 and 330 million).
Some estimates extend their timeline into deep prehistory, to "10,000 BC", i.e. the last glacial maximum, when world population estimates range roughly between one and ten million.
Estimates for yet deeper prehistory, into the Upper Paleolithic, are of a different nature. At this time human populations consisted entirely of non-sedentary hunter-gatherer populations, which fall into a number of archaic species or sub-species, some but not all of which may be ancestral to the modern human population due to possible archaic human admixture with modern humans taking place during the Upper Paleolithic. Estimates of the size of these populations are a topic of paleoanthropology. A late human population bottleneck is postulated by some scholars at approximately 70,000 years ago, during the Toba catastrophe, when the Homo sapiens population may have dropped to as low as between 1,000 and 10,000 individuals.
For the time of speciation of Homo sapiens, ca. 130,000 years ago, Sjödin et al. (2012) estimate an effective population size of the order of 10,000 to 30,000 individuals, inferring an actual "census population" of early Homo sapiens of roughly 100,000 to 300,000 individuals.
Historical population
The following table uses astronomical year numbering for dates, negative numbers corresponding roughly to the corresponding year BC (i.e. -10000 = 10,001 BC, etc.). The table starts counting around the Late Glacial Maximum period, in which ice retreated and humans started to spread into the northern hemisphere.
1950 to present
For times after World War II, demographic data of some accuracy becomes available for a significant number of countries, and population estimates are often given as grand totals of numbers (typically given by country) of widely diverging accuracies. Some sources give these numbers rounded to the nearest million or the nearest thousand, while others give them without any rounding. Taking these numbers at face value would be false precision; in spite of being stated to four, seven or even ten digits, they should not be interpreted as accurate to more than three digits at best.
Projections
As of 2015, the population of the world is projected to reach 8 billion in 2025, and 9 billion by about 2040/42. Kapitza (1996) estimated an asymptotic limit of population growth of 14 billion, 90% of which (12.6 billion) expected to be reached by 2135.
Reasonable predictions of population development are possible for the next 30 years or so, representing the period of fertility of the children alive today. Projections of population reaching more than one generation into the future are highly speculative: Thus, the United Nations Department of Economic and Social Affairs report of 2004 projected the world population to peak at 9.22 billion in 2075 and then stabilise at a value close to 9 billion; By contrast, a 2014 projection by the United Nations Population Division predicts a population close to 11 billion by 2100 without any declining trend in the foreseeable future. On the other hand, a conservative scenario published in 2012 assumes that a maximum of 8 billion will be reached before 2040.
The following table shows projections of world population for the 21st century.
Other, historical projections include
By world region
Population estimates for world regions based on Maddison (2007), in millions. The row showing total world population includes the average growth rate per year over the period separating each column from the preceding one.