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The Anthropocene is a proposed name for the interval of Earth's history in which human activity has become a dominant influence on the planet's climate, biogeochemical cycles, ecosystems, and sedimentary record. The term was popularized by the atmospheric chemist Paul Crutzen and the biologist Eugene Stoermer in 2000, and it is widely used in Earth system science and in the public discussion of global change. It has not been adopted as a formal unit of the geologic time scale. In March 2024, the International Union of Geological Sciences, on the recommendation of the International Commission on Stratigraphy and its Subcommission on Quaternary Stratigraphy, declined to ratify a proposal to define it as an epoch following the Holocene. The Anthropocene therefore remains an informal term, which is used with several meanings by geologists, climate scientists, ecologists, and scholars of the humanities. Human influence is not in doubt. The disagreement concerns whether, when, and how to formalize it as a stratigraphic unit.

The Holocene began about 11,700 years ago, at the end of the last glacial period. It has had a relatively stable climate, within which agriculture and complex societies developed. The Anthropocene, in the sense used by Earth system scientists, describes a departure from this state, with changes in atmospheric composition, global temperature, ocean chemistry, land cover, nutrient cycles, and biodiversity that are rapid compared with most changes in the geological record.

Ideas that humans change the planet on a geological scale are older than the term. In the nineteenth century, George Perkins Marsh documented human effects on landscapes, and the Italian geologist Antonio Stoppani wrote in 1873 of an "anthropozoic era". The Russian geochemist Vladimir Vernadsky described humans as a geological force in his work on the biosphere and the "noosphere" in the 1920s. Crutzen and Stoermer proposed the name "Anthropocene" in 2000 and suggested the late eighteenth century, with the rise of the steam engine, as a start. In 2009, the Subcommission on Quaternary Stratigraphy set up the Anthropocene Working Group (AWG) to evaluate whether the interval could be defined formally. In 2019, the AWG voted by a large majority to recommend that the Anthropocene be treated as a formal unit, with a start in the mid-twentieth century. In 2023, it proposed a candidate boundary section, and in 2024, the higher committees voted against the proposal.

Requirements for a formal epoch

Formal units of the geologic time scale are defined by a Global Boundary Stratotype Section and Point (GSSP), a specific location in a sedimentary sequence that records the base of the unit, along with a primary marker that can be correlated worldwide. A formal Anthropocene epoch therefore needs a global, synchronous signal in sediments or ice that will be preserved in the rock record.

Proposed markers

Candidates for a stratigraphic signal include:

• Radionuclides. Atmospheric tests of nuclear weapons, from 1945 and especially after 1952, produced a global fallout of artificial radionuclides, such as plutonium-239 and plutonium-240, cesium-137, and an increase in atmospheric radiocarbon (carbon-14) that peaked in the mid-1960s, after which it fell following the 1963 Partial Test Ban Treaty. The plutonium signal was proposed as the primary marker for the AWG's candidate boundary, because it is global, rises sharply in the early 1950s, and can be measured at very low concentrations. These isotopes are radioactive, so the signal decays. Plutonium-239 has a half-life of about 24,100 years, so the amount remaining after a time t is:

• Technofossils and novel materials. Human-made materials that are new to the geological record include plastics and microplastics, concrete, fly ash from the burning of coal, aluminum metal, and persistent chemicals. Plastic production has increased from a very small amount in 1950 to hundreds of millions of tonnes per year, and spherical carbonaceous particles from fossil fuel combustion appear in sediments worldwide from the mid-twentieth century. Persistent organic pollutants and PFAS (per- and polyfluoroalkyl substances) are also found worldwide.
• Carbon isotopes and the Suess effect. Fossil carbon is depleted in carbon-13 (with a δ¹³C of about −28 per mil, compared with about −6.4 per mil for the preindustrial atmosphere). Its release has lowered the δ¹³C of atmospheric CO₂ by about 2 per mil since the Industrial Revolution, which is called the Suess effect, and it is recorded in tree rings, corals, and ice cores. The signal began in the nineteenth century and so it is not a sharp marker of the mid-century.
• Other signals. These include changes in sedimentation and in the geochemistry of lake sediments (for example, nitrogen isotopes from fertilizer use and the enrichment of lead and other metals), shifts in the species of microfossils and pollen, and the rise of atmospheric methane and nitrous oxide concentrations.

The candidate section and the decision

The AWG selected Crawford Lake, a small meromictic lake in Ontario, Canada, as the proposed stratotype. Its bottom waters do not mix with its surface waters, and the sediments accumulate in annual layers (varves), which allow the dating of the plutonium signal to the early 1950s with a precision of a year. Eleven other sites around the world were studied as candidates, including lakes, peat bogs, a coral, an estuary, and a marine basin. The AWG proposed a start date of about 1952. The proposal was not accepted by the Subcommission on Quaternary Stratigraphy in 2024, with several of its voting members objecting that the effects of humans are diachronous, that a boundary defined by one decade of nuclear fallout is too narrow, and that the concept is as much about processes and societies as about rocks. The decision of the International Union of Geological Sciences confirmed that the Anthropocene would not be added to the formal time scale, and also stated that the term would continue to be used.

Alternative start dates

Several start dates have been proposed:

• Early Anthropocene. William Ruddiman proposed that early agriculture and forest clearance began to raise CO₂ and methane thousands of years ago. The idea is debated, because the size of the effect and the interpretation of the ice core records are uncertain.
• 1610, the "Orbis spike". The decline in atmospheric CO₂ recorded in Antarctic ice about 1610 was attributed to forest regrowth on abandoned farmland after the population collapse in the Americas that followed the arrival of Europeans. It also coincides with the start of the global exchange of species (Columbian exchange).
• The Industrial Revolution. Coal and the steam engine, from the late eighteenth century, followed by the rise of atmospheric CO₂ above the Holocene range.
• The mid-twentieth century. The Great Acceleration and the global signals from nuclear testing, which the AWG preferred.

Some geologists propose that the Anthropocene should be treated as an informal or an ongoing "event", like the Great Oxidation Event, and not as an epoch with a precise lower boundary.

The term "Great Acceleration" was used by Will Steffen and colleagues for the rapid increase, since about 1950, in indicators of human activity and of changes in the Earth system. In their 2015 analysis, they compiled 24 indicators, 12 for socio-economic trends and 12 for Earth system trends. Socio-economic indicators include population (about 2.5 billion in 1950 and about 8 billion in 2022), real gross domestic product, primary energy use, fertilizer consumption, water use, urbanization, international tourism, and the number of large dams. Earth system indicators include atmospheric concentrations of CO₂, methane, and nitrous oxide, surface temperature, ocean acidification, the loss of tropical forest, and the degradation of the biosphere. Many of these indicators show growth that is close to exponential, at least in some periods, but the rate differs among them, and not all of them show an acceleration in recent decades. For example, the stratospheric ozone layer was depleted by chlorofluorocarbons, but, because of the Montreal Protocol, ozone-depleting substances are declining and the layer is slowly recovering. The statement that the increase in all indicators is "explosive and synchronized" is a simplification, and trends differ among regions and among groups of people. Large differences in the use of resources and in emissions exist between wealthy and poor countries and among individuals.

Burning of fossil fuels, cement production, and land-use change have moved carbon from geological reservoirs and the biosphere to the atmosphere and ocean. Fossil fuel and cement emissions are about 10 gigatons of carbon per year (about 37 to 38 gigatons of CO₂) in the early 2020s. The natural flux of CO₂ from volcanoes is of the order of 0.1 to 0.3 gigatons of carbon per year, so that human emissions are roughly 30 to 100 times larger than the geological source. Atmospheric CO₂ has risen from about 280 ppm in the preindustrial period to more than 420 ppm, which is higher than at any time in at least several hundred thousand years, and by some estimates in millions of years.

Only part of the emitted CO₂ stays in the atmosphere, because the ocean and the land biosphere take up a large share. The airborne fraction is about 45 percent. Because 1 ppm of atmospheric CO₂ corresponds to about 2.1 gigatons of carbon, annual emissions of 10 gigatons of carbon and an airborne fraction of 0.45 give an expected rise of 10 × 0.45 / 2.1 ≈ 2.1 ppm per year, which is close to the observed rise of about 2 to 2.5 ppm per year. The rate of change of the concentration is much faster than that in the glacial-interglacial cycles, in which CO₂ changed by about 80 to 100 ppm over about ten thousand years. The ocean has taken up about a quarter of the emissions, and the absorbed CO₂ lowers the pH of surface seawater (by about 0.1 units since the preindustrial period) in a process called ocean acidification. Global mean surface temperature has risen by about 1.1 to 1.3 °C relative to 1850 to 1900, and the ocean has taken up more than 90 percent of the excess heat in the climate system. The Earth system responds on a range of timescales. A large part of the emitted CO₂ will remain in the atmosphere for centuries to millennia, and the removal by silicate weathering takes hundreds of thousands of years.

The Haber-Bosch process, developed in the early twentieth century, converts atmospheric nitrogen to ammonia for fertilizers and industrial uses. Together with the cultivation of nitrogen-fixing crops and the combustion of fossil fuels, human activity creates a flux of reactive nitrogen that is larger than the flux from natural biological fixation on land. Estimates of the human flux are about 200 teragrams of nitrogen per year, compared with about 50 to 60 teragrams from natural terrestrial sources, although estimates of the natural flux are uncertain. A large part of the fertilizer applied to land is not taken up by crops. The excess flows to rivers, groundwater, and the atmosphere, and it causes eutrophication and oxygen-depleted "dead zones" in coastal waters, such as the Gulf of Mexico and the Baltic Sea, and emission of nitrous oxide, a greenhouse gas. Mining of phosphate rock has greatly increased the flux of phosphorus to land and water, and that flux is several times larger than the natural weathering input, although the exact ratio depends on the estimate. Other changes include the large increase in the use of freshwater and the mobilization of sulfur, mercury, and other metals.

Human activity has altered most of the land surface. Agriculture covers about a third of the ice-free land, and the majority of the biomass of mammals is now livestock and humans, with wild mammals accounting for a few percent. The main direct drivers of the decline in biodiversity are changes in land and sea use, direct exploitation of organisms, climate change, pollution, and invasive species. Global transport and trade have moved many species across barriers, and have led to the spread of invasive organisms and to some homogenization of regional biotas. The current rate of extinction is estimated to be tens to hundreds of times higher than the average rate over the past ten million years, and an assessment in 2019 estimated that about one million species are threatened with extinction. Many biologists describe the present loss as the beginning of a sixth mass extinction, although the magnitude is less than that of the five great mass extinctions, in which about three quarters or more of species were lost, and some argue that the term depends on how fast the losses continue. Most species are not described, so numbers are uncertain.

The planetary boundaries framework, proposed by Johan Rockström, Will Steffen, and colleagues in 2009 and updated in 2015 and 2023, identifies nine Earth system processes that regulate the stability of the planet: climate change, biosphere integrity (the loss of genetic diversity and the loss of functional diversity), land-system change, freshwater change, biogeochemical flows (nitrogen and phosphorus), ocean acidification, atmospheric aerosol loading, stratospheric ozone depletion, and the introduction of novel entities (such as synthetic chemicals and plastics). For each, a boundary is proposed, which marks the edge of a "safe operating space" for humanity in a Holocene-like state. The 2023 update concluded that six of the nine boundaries had been transgressed, and a later assessment concluded that ocean acidification is also close to or beyond its boundary. The framework is a heuristic. It has been criticized because the locations of the thresholds are uncertain, because many processes, such as the use of freshwater, are regional and not global, and because it does not directly address equity. Closely related is the study of tipping points, in which a part of the Earth system, such as an ice sheet, a rainforest, or an ocean circulation, may change abruptly and in a way that is hard to reverse once a threshold is passed. The thresholds and timing for most proposed tipping elements are uncertain, and statements that the Earth system is "pushed into an unpredictable regime" should be understood as a description of risk and not as a settled outcome.

The concept has been criticized from several directions. Some scholars in the social sciences and the humanities argue that the name "Anthropos" implies that all of humanity is equally responsible, whereas emissions and the use of resources have been very unequal, and they have proposed terms such as the Capitalocene, Plantationocene, or Technocene to emphasize particular systems of production. Others argue that the concept can promote the idea that humans are in control of the planet. Stratigraphers have questioned the need for a formal epoch, and some regard the Anthropocene as a useful concept even without a formal definition. Geologists note that the sedimentary record of the last century is thin compared with the time spans of other epochs, and the stratigraphic usefulness of a unit depends on the ability to correlate it globally.

• Holocene
• Great Acceleration
• Planetary boundaries
• Holocene extinction
• Earth system science
• Climate change
• Geologic time scale