[4] The global population is still increasing, but there is significant uncertainty about its long-term trajectory due to changing fertility and mortality rates.

[14][15] Even earlier, genetic evidence suggests humans may have gone through a population bottleneck of between 1,000 and 10,000 people about 70,000 BC, according to the now largely discredited Toba catastrophe theory.

By contrast, it is estimated that around 50–60 million people lived in the combined eastern and western Roman Empire in the 4th century AD.

[23] Starting in AD 2, the Han dynasty of ancient China kept consistent family registers to properly assess the poll taxes and labor service duties of each household.

[26] New crops that were brought to Asia and Europe from the Americas by Portuguese and Spanish colonists in the 16th century are believed to have contributed to population growth.

[31] The pre-Columbian population of the Americas is uncertain; historian David Henige called it "the most unanswerable question in the world.

[33] Encounters between European explorers and populations in the rest of the world often introduced local epidemics of extraordinary virulence.

[42] Population growth in the Western world became more rapid after the introduction of vaccination and other improvements in medicine and sanitation.

[46] The first half of the 20th century in Imperial Russia and the Soviet Union was marked by a succession of major wars, famines and other disasters which caused large-scale population losses (approximately 60 million excess deaths).

The points at which it reached three and four billion were not officially noted, but the International Database of the United States Census Bureau placed them in July 1959 and April 1974 respectively.

[75] In June 2012, British researchers calculated the total weight of Earth's human population as approximately 287 million tonnes (630 billion pounds), with the average person weighing around 62 kilograms (137 lb).

[76] The IMF estimated nominal 2021 gross world product at US$94.94 trillion, giving an annual global per capita figure of around US$12,290.

[81] The Han Chinese are the world's largest single ethnic group, constituting over 19% of the global population in 2011.

[86] Approximately 4.6 billion people live in these ten countries, representing around 57% of the world's population as of July 2023.

[105] In fact, during the 2010s, Japan and some countries in Europe began to reduce in population, due to sub-replacement fertility rates.

[100] In 2019, the United Nations reported that the rate of population growth continues to decline due to the ongoing global demographic transition.

[106] An alternative scenario is given by the statistician Jorgen Randers, who argues that traditional projections insufficiently take into account the downward impact of global urbanization on fertility.

[107] Adrian Raftery, a University of Washington professor of statistics and of sociology, states that "there's a 70 percent probability the world population will not stabilize this century.

The United Nations and the US Census Bureau both give different estimates – according to the UN, the world population reached seven billion in late 2011,[110] while the USCB asserted that this occurred in March 2012.

[129] Some analysts have questioned the sustainability of further world population growth, highlighting the growing pressures on the environment,[130][131] global food supplies, and energy resources.

[141] The PRB puts the figure at 117 billion as of 2020, estimating that the current world population is 6.7% of all the humans who have lived since 190,000 BCE.

[143][144][145] Haub characterized this figure as an estimate that required "selecting population sizes for different points from antiquity to the present and applying assumed birth rates to each period".

In many early attempts, such as in Ancient Egypt and the Persian Empire, the focus was on counting merely a subset of the population for purposes of taxation or military service.

The latter equally depended on period, location and social standing, but calculations identify averages from roughly 30 years upward.

Notable proponents of this notion include: agronomist and insect ecologist David Pimentel,[149] behavioral scientist Russell Hopfenberg (the former two publishing a study on the topic in 2001),[150] anthropologist and activist Virginia Abernethy,[151] ecologist Garrett Hardin,[152] science writer and anthropologist Peter Farb, journalist Richard Manning,[153] environmental biologist Alan D. Thornhill,[154] cultural critic and writer Daniel Quinn,[155] and anarcho-primitivist John Zerzan.

[157][158][159][160][161][162] Furthermore, certain scientific studies do lend evidence to food availability in particular being the dominant factor within a more recent timeframe.

The world human population began consistently and sharply to rise, and continues to do so, after sedentary agricultural lifestyles became common due to the Neolithic Revolution and its increased food supply.

Criticism of this theory can come from multiple angles, for example by demonstrating that human population is not solely an effect of food availability, but that the situation is more complex.

[168] Another criticism is that, in the modern era, birth rates are lowest in the developed nations, which also have the highest access to food.

The United Nations projects that the population of 51 countries or areas, including Germany, Italy, Japan, and most of the states of the former Soviet Union, is expected to be lower in 2050 than in 2005.