Moons of Jupiter

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Moons_of_Jupiter 

A montage of Jupiter and its four largest moons (distance and sizes not to scale)

There are 80 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers (or other sexual partners) or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 76 known moons and the rings together composing just 0.003% of the total orbiting mass.

Of Jupiter's moons, eight are regular satellites with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are nearly spherical in shape due to their planetary mass, and so would be considered at least dwarf planets if they were in direct orbit around the Sun. The other four regular satellites are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter's rings. The remainder of Jupiter's moons are irregular satellites whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. These moons were probably captured by Jupiter from solar orbits. Twenty-three of the irregular satellites have not yet been officially named.

Characteristics

The Galilean moons. From left to right, in order of increasing distance from Jupiter: Io; Europa; Ganymede; Callisto.

The physical and orbital characteristics of the moons vary widely. The four Galileans are all over 3,100 kilometres (1,900 mi) in diameter; the largest Galilean, Ganymede, is the ninth largest object in the Solar System, after the Sun and seven of the planets, Ganymede being larger than Mercury. All other Jovian moons are less than 250 kilometres (160 mi) in diameter, with most barely exceeding 5 kilometres (3.1 mi). Their orbital shapes range from nearly perfectly circular to highly eccentric and inclined, and many revolve in the direction opposite to Jupiter's rotation (retrograde motion). Orbital periods range from seven hours (taking less time than Jupiter does to rotate around its axis), to some three thousand times more (almost three Earth years).

Origin and evolution

The relative masses of the Jovian moons. Those smaller than Europa are not visible at this scale, and combined would only be visible at 100× magnification.

Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk. They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history.

Simulations suggest that, while the disk had a relatively high mass at any given moment, over time a substantial fraction (several tens of a percent) of the mass of Jupiter captured from the solar nebula was passed through it. However, only 2% of the proto-disk mass of Jupiter is required to explain the existing satellites. Thus, several generations of Galilean-mass satellites may have been in Jupiter's early history. Each generation of moons might have spiraled into Jupiter, because of drag from the disk, with new moons then forming from the new debris captured from the solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned so that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance with each other, which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io.

The outer, irregular moons are thought to have originated from captured asteroids, whereas the protolunar disk was still massive enough to absorb much of their momentum and thus capture them into orbit. Many are believed to have broken up by mechanical stresses during capture, or afterward by collisions with other small bodies, producing the moons we see today.

Discovery

Jupiter and the Galilean moons through a 25 cm (10 in) Meade LX200 telescope.

 

The number of moons known for each of the four outer planets up to October 2019. Jupiter currently has 80 known satellites.

 

See also: Timeline of discovery of Solar System planets and their moons

Chinese historian Xi Zezong claimed that the earliest record of a Jovian moon (Ganymede or Callisto) was a note by Chinese astronomer Gan De of an observation around 364 BC regarding a "reddish star". However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609. By January 1610, he had sighted the four massive Galilean moons with his 20× magnification telescope, and he published his results in March 1610.

Simon Marius had independently discovered the moons one day after Galileo, although he did not publish his book on the subject until 1614. Even so, the names Marius assigned are used today: Ganymede, Callisto, Io, and Europa. No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892.

With the aid of telescopic photography, further discoveries followed quickly over the course of the 20th century. Himalia was discovered in 1904, Elara in 1905, Pasiphae in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974. By the time that the Voyager space probes reached Jupiter, around 1979, 13 moons had been discovered, not including Themisto, which had been observed in 1975, but was lost until 2000 due to insufficient initial observation data. The Voyager spacecraft discovered an additional three inner moons in 1979: Metis, Adrastea, and Thebe.

No additional moons were discovered for two decades, but between October 1999 and February 2003, researchers found another 34 moons using sensitive ground-based detectors. These are tiny moons, in long, eccentric, generally retrograde orbits, and averaging 3 km (1.9 mi) in diameter, with the largest being just 9 km (5.6 mi) across. All of these moons are thought to have been captured asteroidal or perhaps comet bodies, possibly fragmented into several pieces.

By 2015, a total of 15 additional moons were discovered. Two more were discovered in 2017 by the team led by Scott S. Sheppard at the Carnegie Institution for Science, bringing the total to 69. On 17 July 2018, the International Astronomical Union confirmed that Sheppard's team had discovered ten more moons around Jupiter, bringing the total number to 79. Among these is Valetudo, which has a prograde orbit, but crosses paths with several moons that have retrograde orbits, making an eventual collision—at some point on a billions-of-years timescale—likely.

In September 2020, researchers from the University of British Columbia identified 45 candidate moons from an analysis of archival images taken in 2010 by the Canada-France-Hawaii Telescope. These candidates were mainly small and faint, down to a magnitude of 25.7 or over 800 m (0.50 mi) in diameter. From the number of candidate moons detected within a sky area of one square degree, the team extrapolated that the population of retrograde Jovian moons brighter than magnitude 25.7 is around 600, within a factor of 2. Although the team considers their characterised candidates to be likely moons of Jupiter, they all remain unconfirmed due to their insufficient observation data for determining reliable orbits for each of them.

Naming

Main article: Naming of moons

 

Galilean moons around Jupiter   Jupiter ·   Io ·   Europa ·   Ganymede ·   Callisto

 

Orbits of Jupiter's inner moons within its rings

The Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century. The astronomical literature instead simply referred to "Jupiter I", "Jupiter II", etc., or "the first satellite of Jupiter", "Jupiter's second satellite", and so on. The names Io, Europa, Ganymede, and Callisto became popular in the mid-20th century, whereas the rest of the moons remained unnamed and were usually numbered in Roman numerals V (5) to XII (12). Jupiter V was discovered in 1892 and given the name Amalthea by a popular though unofficial convention, a name first used by French astronomer Camille Flammarion.

The other moons were simply labeled by their Roman numeral (e.g. Jupiter IX) in the majority of astronomical literature until the 1970s. Several different suggestions were made for names of Jupiter's outer satellites, but none were universally accepted until 1975 when the International Astronomical Union's (IAU) Task Group for Outer Solar System Nomenclature granted names to satellites V–XIII, and provided for a formal naming process for future satellites still to be discovered. The practice was to name newly discovered moons of Jupiter after lovers and favorites of the god Jupiter (Zeus) and, since 2004, also after their descendants. All of Jupiter's satellites from XXXIV (Euporie) onward are named after descendants of Jupiter or Zeus, except LIII (Dia), named after a lover of Jupiter. Names ending with "a" or "o" are used for prograde irregular satellites (the latter for highly inclined satellites), and names ending with "e" are used for retrograde irregulars. With the discovery of smaller, kilometre-sized moons around Jupiter, the IAU has established an additional convention to limit the naming of small moons with absolute magnitudes greater than 18 or diameters smaller than 1 km (0.62 mi). Some of the most recently confirmed moons have not received names.

Some asteroids share the same names as moons of Jupiter: 9 Metis, 38 Leda, 52 Europa, 85 Io, 113 Amalthea, 239 Adrastea. Two more asteroids previously shared the names of Jovian moons until spelling differences were made permanent by the IAU: Ganymede and asteroid 1036 Ganymed; and Callisto and asteroid 204 Kallisto.

Groups

The orbits of Jupiter's irregular satellites, and how they cluster into groups: by semi-major axis (the horizontal axis in Gm); by orbital inclination (the vertical axis); and orbital eccentricity (the yellow lines). The relative sizes are indicated by the circles.

Regular satellites

These have prograde and nearly circular orbits of low inclination and are split into two groups:

• Inner satellites or Amalthea group: Metis, Adrastea, Amalthea, and Thebe. These orbit very close to Jupiter; the innermost two orbit in less than a Jovian day. The latter two are respectively the fifth and seventh largest moons in the Jovian system. Observations suggest that at least the largest member, Amalthea, did not form on its present orbit, but farther from the planet, or that it is a captured Solar System body. These moons, along with a number of seen and as-yet-unseen inner moonlets (see Amalthea moonlets), replenish and maintain Jupiter's faint ring system. Metis and Adrastea help to maintain Jupiter's main ring, whereas Amalthea and Thebe each maintain their own faint outer rings.
• Main group or Galilean moons: Io, Europa, Ganymede and Callisto. They are some of the largest objects in the Solar System outside the Sun and the eight planets in terms of mass, larger than any known dwarf planet. Ganymede exceeds (and Callisto nearly equals) even the planet Mercury in diameter, though they are less massive. They are respectively the fourth-, sixth-, first-, and third-largest natural satellites in the Solar System, containing approximately 99.997% of the total mass in orbit around Jupiter, while Jupiter is almost 5,000 times more massive than the Galilean moons. The inner moons are in a 1:2:4 orbital resonance. Models suggest that they formed by slow accretion in the low-density Jovian subnebula—a disc of the gas and dust that existed around Jupiter after its formation—which lasted up to 10 million years in the case of Callisto. Europa, Ganymede, and Callisto are suspected of having subsurface water oceans, and Io may have a subsurface magma ocean.

Irregular satellites

Orbits and positions of Jupiter's irregular satellites as of 1 January 2021. Prograde orbits are colored blue while retrograde orbits are colored red.

 

Inclinations (°) vs. eccentricities of Jupiter's irregular satellites, with the major groups identified. Data as of 2021.

 

Main article: Irregular satellite

The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity) and composition; it is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field. These families bear the names of their largest members. The identification of satellite families is tentative, but the following are typically listed:

• Prograde satellites:
  • Themisto is the innermost irregular moon and is not part of a known family.
  • The Himalia group is spread over barely 1.4 Gm in semi-major axes, 1.6° in inclination (27.5 ± 0.8°), and eccentricities between 0.11 and 0.25. It has been suggested that the group could be a remnant of the break-up of an asteroid from the asteroid belt.
  • Carpo is another prograde moon and is not part of a known family. It has the highest inclination of all of the prograde moons.
  • Valetudo is the outermost prograde moon and is not part of a known family. Its prograde orbit crosses paths with several moons that have retrograde orbits and may in the future collide with them.
• Retrograde satellites:
  • The Carme group is spread over only 1.2 Gm in semi-major axis, 1.6° in inclination (165.7 ± 0.8°), and eccentricities between 0.23 and 0.27. It is very homogeneous in color (light red) and is believed to have originated from a D-type asteroid progenitor, possibly a Jupiter trojan.
  • The Ananke group has a relatively wider spread than the previous groups, over 2.4 Gm in semi-major axis, 8.1° in inclination (between 145.7° and 154.8°), and eccentricities between 0.02 and 0.28. Most of the members appear gray, and are believed to have formed from the breakup of a captured asteroid.
  • The Pasiphae group is quite dispersed, with a spread over 1.3 Gm, inclinations between 144.5° and 158.3°, and eccentricities between 0.25 and 0.43. The colors also vary significantly, from red to grey, which might be the result of multiple collisions. Sinope, sometimes included in the Pasiphae group, is red and, given the difference in inclination, it could have been captured independently; Pasiphae and Sinope are also trapped in secular resonances with Jupiter.

List

The moons of Jupiter are listed below by orbital period. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in bold. These are the four Galilean moons, which are comparable in size to the Moon. The other moons are much smaller, with the least massive Galilean moon being more than 7,000 times more massive than the most massive of the other moons. The irregular captured moons are shaded light gray when prograde and dark gray when retrograde. The orbits and mean distances of the irregular moons are strongly variable over short timescales due to frequent planetary and solar perturbations, therefore the listed orbital elements of all irregular moons are averaged over a 400-year numerical integration. Their orbital elements are all based on the epoch of 1 January 2000. As of 2021, S/2003 J 10 is the only moon of Jupiter considered lost due to its uncertain orbit. A number of other moons have only been observed for a year or two, but have decent enough orbits to be easily measurable at present.

Exploration

Main articles: Exploration of Jupiter, Ganymede (moon) § Exploration, Europa (moon) § Exploration, Callisto (moon) § Exploration, and Io (moon) § Observational history

 

The orbit and motion of the Galilean moons around Jupiter, as captured by JunoCam aboard the Juno spacecraft.

Nine spacecraft have visited Jupiter. The first were Pioneer 10 in 1973, and Pioneer 11 a year later, taking low-resolution images of the four Galilean moons and returning data on their atmospheres and radiation belts. The Voyager 1 and Voyager 2 probes visited Jupiter in 1979, discovering the volcanic activity on Io and the presence of water ice on the surface of Europa. Ulysses further studied Jupiter's magnetosphere in 1992 and then again in 2000.

The Galileo spacecraft was the first to enter orbit around Jupiter, arriving in 1995 and studying it until 2003. During this period, Galileo gathered a large amount of information about the Jovian system, making close approaches to all of the Galilean moons and finding evidence for thin atmospheres on three of them, as well as the possibility of liquid water beneath the surfaces of Europa, Ganymede, and Callisto. It also discovered a magnetic field around Ganymede.

Then the Cassini probe to Saturn flew by Jupiter in 2000 and collected data on interactions of the Galilean moons with Jupiter's extended atmosphere. The New Horizons spacecraft flew by Jupiter in 2007 and made improved measurements of its satellites' orbital parameters.

Ganymede taken by Juno during its 34th perijove.

In 2016, the Juno spacecraft imaged the Galilean moons from above their orbital plane as it approached Jupiter orbit insertion, creating a time-lapse movie of their motion.

Carbon dioxide in Earth's atmosphere

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_atmosphere 

 

CO₂ concentrations over the last 800,000 years

Carbon dioxide (CO₂) is an important trace gas in Earth's atmosphere. It is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds. CO₂ absorbs and emits infrared radiation at wavelengths of 4.26 μm (2347 cm⁻¹) (asymmetric stretching vibrational mode) and 14.99 μm (667 cm⁻¹) (bending vibrational mode) and consequently is a greenhouse gas that plays a significant role in influencing Earth's surface temperature through the greenhouse effect.

Concentrations of CO₂ in the atmosphere were as high as 4,000 parts per million (ppm, on a molar basis) during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years. Reconstructed temperature records for the last 420 million years indicate that atmospheric CO₂ concentrations peaked at ~2000 ppm during the Devonian (~400 Myrs ago) period, and again in the Triassic (220–200 Myrs ago) period. Global annual mean CO₂ concentration has increased by 50% since the start of the Industrial Revolution, from 280 ppm during the 10,000 years up to the mid-18th century to 421 ppm as of May 2022.  The present concentration is the highest for 14 million years. The increase has been attributed to human activity, particularly deforestation and the burning of fossil fuels. This increase of CO₂ and other long-lived greenhouse gases in Earth's atmosphere has produced the current episode of global warming. Between 30% and 40% of the CO₂ released by humans into the atmosphere dissolves into the oceans, wherein it forms carbonic acid and effects changes in the oceanic pH balance.

Between 1850 and 2019 the Global Carbon Project estimates that about 2/3rds of excess carbon dioxide emissions have been caused by burning fossil fuels, and a little less than half of that has stayed in the atmosphere.

Current concentration

See also: Climate change, Climate variability and change, Atmospheric methane, Holocene climate, and Quaternary climate

 

2011 carbon dioxide mole fraction in the troposphere

 

The Keeling Curve of atmospheric CO₂ concentrations measured at Mauna Loa Observatory

Carbon dioxide concentrations have shown several cycles of variation from about 180 parts per million during the deep glaciations of the Holocene and Pleistocene to 280 parts per million during the interglacial periods. Following the start of the Industrial Revolution, atmospheric CO₂ concentration increased to over 400 parts per million and continues to increase, causing the phenomenon of global warming. As of May 2022, the average monthly level of CO₂ in Earth's atmosphere reached 421 parts per million. The daily average concentration of atmospheric CO₂ at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013 although this concentration had already been reached in the Arctic in June 2012. Each part per million by volume of CO₂ in the atmosphere represents approximately 2.13 gigatonnes of carbon, or 7.82 gigatonnes of CO₂. As of 2018, CO₂ constitutes about 0.041% by volume of the atmosphere, (equal to 410 ppm) which corresponds to approximately 3210 gigatonnes of CO₂, containing approximately 875 gigatonnes of carbon. The global mean CO₂ concentration is currently rising at a rate of approximately 2 ppm/year and accelerating. The current growth rate at Mauna Loa is 2.50 ± 0.26 ppm/year (mean ± 2 std dev). As seen in the graph to the right, there is an annual fluctuation – the level drops by about 6 or 7 ppm (about 50 Gt) from May to September during the Northern Hemisphere's growing season, and then goes up by about 8 or 9 ppm. The Northern Hemisphere dominates the annual cycle of CO₂ concentration because it has much greater land area and plant biomass than the Southern Hemisphere. Concentrations reach a peak in May as the Northern Hemisphere spring greenup begins, and decline to a minimum in October, near the end of the growing season.

Since global warming is attributed to increasing atmospheric concentrations of greenhouse gases such as CO₂ and methane, scientists closely monitor atmospheric CO₂ concentrations and their impact on the present-day biosphere. The National Geographic wrote that the concentration of carbon dioxide in the atmosphere is this high "for the first time in 55 years of measurement—and probably more than 3 million years of Earth history." The current concentration may be the highest in the last 20 million years.

Past concentration

See also: Paleoclimatology and Great Oxidation Event

 

Concentration of atmospheric CO₂ over the last 40,000 years, from the Last Glacial Maximum to the present day. The current rate of increase is much higher than at any point during the last deglaciation.

Carbon dioxide concentrations have varied widely over the Earth's 4.54 billion year history. It is believed to have been present in Earth's first atmosphere, shortly after Earth's formation. The second atmosphere, consisting largely of nitrogen and CO
₂ was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids. A major part of carbon dioxide emissions were soon dissolved in water and incorporated in carbonate sediments.

The production of free oxygen by cyanobacterial photosynthesis eventually led to the oxygen catastrophe that ended Earth's second atmosphere and brought about the Earth's third atmosphere (the modern atmosphere) 2.4 billion years before the present. Carbon dioxide concentrations dropped from 4,000 parts per million during the Cambrian period about 500 million years ago to as low as 180 parts per million during the Quaternary glaciation of the last two million years.

Drivers of ancient-Earth CO₂ concentration

See also: Biogeochemical cycle

On long timescales, atmospheric CO₂ concentration is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and volcanic degassing. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO₂. On a timescale of billions of years, such downward trend appears bound to continue indefinitely as occasional massive historical releases of buried carbon due to volcanism will become less frequent (as earth mantle cooling and progressive exhaustion of internal radioactive heat proceed further). The rates of these processes are extremely slow; hence they are of no relevance to the atmospheric CO₂ concentration over the next hundreds or thousands of years.

In billion-year timescales, it is predicted that plant, and therefore animal, life on land will die off altogether, since by that time most of the remaining carbon in the atmosphere will be sequestered underground, and natural releases of CO₂ by radioactivity-driven tectonic activity will have continued to slow down. The loss of plant life would also result in the eventual loss of oxygen. Some microbes are capable of photosynthesis at concentrations of CO₂ of a few parts per million and so the last life forms would probably disappear finally due to the rising temperatures and loss of the atmosphere when the sun becomes a red giant some four billion years from now.

Measuring ancient-Earth CO₂ concentration

See also: Climate reconstruction proxies

 

Graph of CO₂ (green), reconstructed temperature (blue) and dust (red) from the Vostok ice core for the past 420,000 years

 

Correspondence between temperature and atmospheric CO₂ during the last 800,000 years

The most direct method for measuring atmospheric carbon dioxide concentrations for periods before instrumental sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice sheets. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO₂ concentrations were about 260–280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years. The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years. During this time, the atmospheric carbon dioxide concentration has varied between 180 and 210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacials. The beginning of human agriculture during the current Holocene epoch may have been strongly connected to the atmospheric CO₂ increase after the last ice age ended, a fertilization effect raising plant biomass growth and reducing stomatal conductance requirements for CO₂ intake, consequently reducing transpiration water losses and increasing water usage efficiency.

Various proxy measurements have been used to attempt to determine atmospheric carbon dioxide concentrations millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves.

Phytane is a type of diterpenoid alkane. It is a breakdown product of chlorophyll and is now used to estimate ancient CO₂ levels. Phytane gives both a continuous record of CO₂ concentrations but it also can overlap a break in the CO₂ record of over 500 million years.

400 to 600 M yrs ago

There is evidence for high CO₂ concentrations between 200 and 150 million years ago of over 3,000 ppm, and between 600 and 400 million years ago of over 6,000 ppm.

5 to 60 M yrs ago

In more recent times, atmospheric CO₂ concentration continued to fall after about 60 million years ago. About 34 million years ago, the time of the Eocene–Oligocene extinction event and when the Antarctic ice sheet started to take its current form, CO₂ was about 760 ppm, and there is geochemical evidence that concentrations were less than 300 ppm by about 20 million years ago. Decreasing CO₂ concentration, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation. Low CO₂ concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 million years ago.

CO2 possibly higher 7,000 - 10,000 yrs ago

Based on an analysis of fossil leaves, Wagner et al. argued that atmospheric CO₂ concentrations during the last 7,000–10,000 year period were significantly higher than 300 ppm and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO₂. Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO₂ values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g. H.J. Smith et al.) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust concentrations in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between measurements of Antarctic and Greenland CO₂ concentrations.

Atmospheric CO₂ and the greenhouse effect

Main articles: Greenhouse effect and Radiative forcing

See also: Faint young Sun paradox and Climate change

 

A pictogram of the greenhouse effect

Earth's natural greenhouse effect makes life as we know it possible and carbon dioxide plays a significant role in providing for the relatively high temperature that the planet enjoys. The greenhouse effect is a process by which thermal radiation from a planetary atmosphere warms the planet's surface beyond the temperature it would have in the absence of its atmosphere. Without the greenhouse effect, the Earth's average surface temperature would be about −18 °C (−0.4 °F) compared to Earth's actual average surface temperature of approximately 14 °C (57.2 °F).

Carbon dioxide is believed to have played an important effect in regulating Earth's temperature throughout its 4.7 billion year history. Early in the Earth's life, scientists have found evidence of liquid water indicating a warm world even though the Sun's output is believed to have only been 70% of what it is today. It has been suggested by scientists that higher carbon dioxide concentrations in the early Earth's atmosphere might help explain this faint young sun paradox. When Earth first formed, Earth's atmosphere may have contained more greenhouse gases and CO₂ concentrations may have been higher, with estimated partial pressure as large as 1,000 kPa (10 bar), because there was no bacterial photosynthesis to reduce the gas to carbon compounds and oxygen. Methane, a very active greenhouse gas which reacts with oxygen to produce CO₂ and water vapor, may have been more prevalent as well, with a mixing ratio of 10⁻⁴ (100 parts per million by volume).

Radiative forcing drivers of climate change in year 2011, relative to pre-industrial (1750).

Though water is responsible for most (about 36-70%) of the total greenhouse effect, the role of water vapor as a greenhouse gas depends on temperature. On Earth, carbon dioxide is the most relevant, direct anthropologically influenced greenhouse gas. Carbon dioxide is often mentioned in the context of its increased influence as a greenhouse gas since the pre-industrial (1750) era. In the IPCC Fifth Assessment Report the increase in CO₂ was estimated to be responsible for 1.82 W m⁻² of the 2.63 W m⁻² change in radiative forcing on Earth (about 70%).

The concept of atmospheric CO₂ increasing ground temperature was first published by Svante Arrhenius in 1896. The increased radiative forcing due to increased CO₂ in the Earth's atmosphere is based on the physical properties of CO₂ and the non-saturated absorption windows where CO₂ absorbs outgoing long-wave energy. The increased forcing drives further changes in Earth's energy balance and, over the longer term, in Earth's climate.

Atmospheric CO₂ and the carbon cycle

Main articles: Carbon cycle and Atmospheric carbon cycle

 

This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans in billions of metric tons of carbon per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon.

Atmospheric carbon dioxide plays an integral role in the Earth's carbon cycle whereby CO₂ is removed from the atmosphere by some natural processes such as photosynthesis and deposition of carbonates, to form limestones for example, and added back to the atmosphere by other natural processes such as respiration and the acid dissolution of carbonate deposits. There are two broad carbon cycles on Earth: the fast carbon cycle and the slow carbon cycle. The fast carbon cycle refers to movements of carbon between the environment and living things in the biosphere whereas the slow carbon cycle involves the movement of carbon between the atmosphere, oceans, soil, rocks, and volcanism. Both cycles are intrinsically interconnected and atmospheric CO₂ facilitates the linkage.

Natural sources of atmospheric CO₂ include volcanic outgassing, the combustion of organic matter, wildfires and the respiration processes of living aerobic organisms. Man-made sources of CO₂ include the burning of fossil fuels for heating, power generation and transport, as well as some industrial processes such as cement making. It is also produced by various microorganisms from fermentation and cellular respiration. Plants, algae and cyanobacteria convert carbon dioxide to carbohydrates by a process called photosynthesis. They gain the energy needed for this reaction from absorption of sunlight by chlorophyll and other pigments. Oxygen, produced as a by-product of photosynthesis, is released into the atmosphere and subsequently used for respiration by heterotrophic organisms and other plants, forming a cycle with carbon.

Annual CO₂ flows from anthropogenic sources (left) into Earth's atmosphere, land, and ocean sinks (right) since year 1960. Units in equivalent gigatonnes carbon per year.

Most sources of CO₂ emissions are natural, and are balanced to various degrees by similar CO₂ sinks. For example, the decay of organic material in forests, grasslands, and other land vegetation - including forest fires - results in the release of about 436 gigatonnes of CO₂ (containing 119 gigatonnes carbon) every year, while CO₂ uptake by new growth on land counteracts these releases, absorbing 451 Gt (123 Gt C). Although much CO₂ in the early atmosphere of the young Earth was produced by volcanic activity, modern volcanic activity releases only 130 to 230 megatonnes of CO₂ each year. Natural sources are more or less balanced by natural sinks, in the form of chemical and biological processes which remove CO₂ from the atmosphere. By contrast, as of year 2019 the extraction and burning of geologic fossil carbon by humans releases over 30 gigatonnes of CO₂ (9 billion tonnes carbon) each year. This larger disruption to the natural balance is responsible for recent growth in the atmospheric CO₂ concentration.

Overall, there is a large natural flux of atmospheric CO₂ into and out of the biosphere, both on land and in the oceans. In the pre-industrial era, each of these fluxes were in balance to such a degree that little net CO₂ flowed between the land and ocean reservoirs of carbon, and little change resulted in the atmospheric concentration. From the human pre-industrial era to 1940, the terrestrial biosphere represented a net source of atmospheric CO₂ (driven largely by land-use changes), but subsequently switched to a net sink with growing fossil carbon emissions. In 2012, about 57% of human-emitted CO₂, mostly from the burning of fossil carbon, was taken up by land and ocean sinks.

The ratio of the increase in atmospheric CO₂ to emitted CO₂ is known as the airborne fraction (Keeling et al., 1995). This ratio varies in the short-term and is typically about 45% over longer (5-year) periods. Estimated carbon in global terrestrial vegetation increased from approximately 740 gigatonnes in 1910 to 780 gigatonnes in 1990. By 2009, oceanic neutralization had decreased the pH of seawater by 0.11 due to uptake of emitted CO₂.

Atmospheric CO₂ and photosynthesis

Main article: Terrestrial biological carbon cycle

See also: photosynthesis, Carbon fixation, C4 carbon fixation, and C3 carbon fixation

 

Photosynthesis changes sunlight into chemical energy, splits water to liberate O₂, and fixes CO₂ into sugar.

Carbon dioxide in the Earth's atmosphere is essential to life and to most of the planetary biosphere. Over the course of Earth's geologic history CO₂ concentrations have played a role in biological evolution. The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide as sources of electrons, rather than water. Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible. In recent geologic times, low CO₂ concentrations below 600 parts per million might have been the stimulus that favored the evolution of C4 plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficient C3 metabolic pathway. At current atmospheric pressures photosynthesis shuts down when atmospheric CO₂ concentrations fall below 150 ppm and 200 ppm although some microbes can extract carbon from the air at much lower concentrations. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about six times larger than the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 billion metric tonnes of carbon into biomass per year.

Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from CO₂ and water using energy from light. However, not all organisms that use light as a source of energy carry out photosynthesis, since photoheterotrophs use organic compounds, rather than CO₂, as a source of carbon. In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes CO₂ but does not release oxygen.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction, so photosynthesis needs to supply both the source of energy to drive this process and the electrons needed to convert CO₂ into a carbohydrate. This addition of the electrons is a reduction reaction. In general outline and in effect, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds are oxidized to produce CO₂ and water, and to release exothermic chemical energy to drive the organism's metabolism. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.

Most organisms that utilize photosynthesis to produce oxygen use visible light to do so, although at least three use shortwave infrared or, more specifically, far-red radiation.

Effects of increased CO₂ on plants and crops

See also: CO2 fertilization effect

A 1993 review of scientific greenhouse studies found that a doubling of CO₂ concentration would stimulate the growth of 156 different plant species by an average of 37%. Response varied significantly by species, with some showing much greater gains and a few showing a loss. For example, a 1979 greenhouse study found that with doubled CO₂ concentration the dry weight of 40-day-old cotton plants doubled, but the dry weight of 30-day-old maize plants increased by only 20%.

In addition to greenhouse studies, field and satellite measurements attempt to understand the effect of increased CO₂ in more natural environments. In free-air carbon dioxide enrichment (FACE) experiments plants are grown in field plots and the CO₂ concentration of the surrounding air is artificially elevated. These experiments generally use lower CO₂ levels than the greenhouse studies. They show lower gains in growth than greenhouse studies, with the gains depending heavily on the species under study. A 2005 review of 12 experiments at 475–600 ppm showed an average gain of 17% in crop yield, with legumes typically showing a greater response than other species and C4 plants generally showing less. The review also stated that the experiments have their own limitations. The studied CO₂ levels were lower, and most of the experiments were carried out in temperate regions. Satellite measurements found increasing leaf area index for 25% to 50% of Earth's vegetated area over the past 35 years (i.e., a greening of the planet), providing evidence for a positive CO₂ fertilization effect.

A 2017 Politico article states that increased CO₂ levels may have a negative impact on the nutritional quality of various human food crops, by increasing the levels of carbohydrates, such as glucose, while decreasing the levels of important nutrients such as protein, iron, and zinc. Crops experiencing a decrease in protein include rice, wheat, barley and potatoes.

Atmospheric CO₂ and the oceanic carbon cycle

Main article: Oceanic carbon cycle

See also: Solubility pump and Ocean acidification

 

Air-sea exchange of CO₂

The Earth's oceans contain a large amount of CO₂ in the form of bicarbonate and carbonate ions—much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate:

CaCO
₃ + CO₂ + H
₂O ⇌ Ca²⁺
+ 2 HCO⁻
₃

Reactions like this tend to buffer changes in atmospheric CO₂. Since the right side of the reaction produces an acidic compound, adding CO₂ on the left side decreases the pH of seawater, a process which has been termed ocean acidification (pH of the ocean becomes more acidic although the pH value remains in the alkaline range). Reactions between CO₂ and non-carbonate rocks also add bicarbonate to the seas. This can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO₂. Over hundreds of millions of years, this has produced huge quantities of carbonate rocks.

Ultimately, most of the CO₂ emitted by human activities will dissolve in the ocean; however, the rate at which the ocean will take it up in the future is less certain. Even if equilibrium is reached, including dissolution of carbonate minerals, the increased concentration of bicarbonate and decreased or unchanged concentration of carbonate ion will give rise to a higher concentration of un-ionized carbonic acid and dissolved CO₂. This higher concentration in the seas, along with higher temperatures, would mean a higher equilibrium concentration of CO₂ in the air.

Carbon dioxide has unique long-term effects on climate change that are nearly "irreversible" for a thousand years after emissions stop (zero further emissions). The greenhouse gases methane and nitrous oxide do not persist over time in the same way as carbon dioxide. Even if human carbon dioxide emissions were to completely cease, atmospheric temperatures are not expected to decrease significantly in the short term. This is because the air temperature is determined by a balance between heating, due to greenhouse gases, and cooling due to heat transfer to the ocean. If emissions were to stop, CO₂ levels and the heating effect would slowly decrease, but simultaneously the cooling due to heat transfer would diminish (because sea temperatures would get closer to the air temperature), with the result that the air temperature would decrease only slowly. Sea temperatures would continue to rise, causing thermal expansion and some sea level rise. Lowering global temperatures more rapidly would require carbon sequestration or geoengineering.

Carbon moves between the atmosphere, vegetation (dead and alive), the soil, the surface layer of the ocean, and the deep ocean. A detailed model has been developed by Fortunat Joos in Bern and colleagues, called the Bern model. A simpler model based on it gives the fraction of CO₂ remaining in the atmosphere as a function of the number of years after it is emitted into the atmosphere:

${\displaystyle f(t)=0.217+0.338\exp(-t/18.51)+0.259\exp(-t/172.9)+0.186\exp(-t/1.186)}$

According to this model, 21.7% of the carbon dioxide released into the air stays there forever, but of course this is not true if carbon-containing material is removed from the cycle (and stored) in ways that are not operative at present (artificial sequestration).

Anthropogenic CO₂ emissions

See also: Greenhouse gas emissions, List of countries by carbon dioxide emissions, and Radiative forcing

 

The US, China and Russia have cumulatively contributed the greatest amounts of CO₂ since 1850.

 

CO₂ in Earth's atmosphere if half of anthropogenic CO₂ emissions are not absorbed. (NASA computer simulation)

While CO₂ absorption and release is always happening as a result of natural processes, the recent rise in CO₂ levels in the atmosphere is known to be mainly due to human (anthropogenic) activity. There are four ways human activity, especially fossil fuel burning, is known to have caused the rapid increase in atmospheric CO₂ over the last few centuries:

• Various national statistics accounting for fossil fuel consumption, combined with knowledge of how much atmospheric CO₂ is produced per unit of fossil fuel (e.g. liter of gasoline).
• By examining the ratio of various carbon isotopes in the atmosphere. The burning of long-buried fossil fuels releases CO₂ containing carbon of different isotopic ratios to those of living plants, enabling distinction between natural and human-caused contributions to CO₂ concentration.
• Higher atmospheric CO₂ concentrations in the Northern Hemisphere, where most of the world's population lives (and emissions originate from), compared to the southern hemisphere. This difference has increased as anthropogenic emissions have increased.
• Atmospheric O₂ levels are decreasing in Earth's atmosphere as it reacts with the carbon in fossil fuels to form CO₂.

Burning fossil fuels such as coal, petroleum, and natural gas is the leading cause of increased anthropogenic CO₂; deforestation is the second major cause. In 2010, 9.14 gigatonnes of carbon (GtC, equivalent to 33.5 gigatonnes of CO₂ or about 4.3 ppm in Earth's atmosphere) were released from fossil fuels and cement production worldwide, compared to 6.15 GtC in 1990. In addition, land use change contributed 0.87 GtC in 2010, compared to 1.45 GtC in 1990. In 1997, human-caused Indonesian peat fires were estimated to have released between 13% and 40% of the average annual global carbon emissions caused by the burning of fossil fuels. In the period 1751 to 1900, about 12 GtC were released as CO₂ to the atmosphere from burning of fossil fuels, whereas from 1901 to 2013 the figure was about 380 GtC.

Part of a series on the
Carbon cycle

By regions

Carbon dioxide In the atmosphere Ocean acidification Removal Satellite measurements

Forms of carbon

Metabolic pathways

Carbon respiration

Carbon pumps

Carbon sequestration

Methane

Biogeochemical

Other

The Integrated Carbon Observation System (ICOS) continuously releases data about CO₂ emissions, budget and concentration at individual observation stations.

CO₂ emissions

Year	Fossil fuels and industry Gt C	Land use change Gt C	Total Gt C	Total Gt CO2
2010	9.05	1.38	10.43	38.2
2011	9.35	1.34	10.69	39.2
2012	9.5	1.47	10.97	40.3
2013	9.54	1.52	11.06	40.6
2014	9.61	1.66	11.27	41.4
2015	9.62	1.7	11.32	41.5
2016	9.66	1.54	11.2	41.1
2017	9.77	1.47	11.24	41.3
2018	9.98	1.51	11.49	42.1
2019 (projection)	10.0	1.8	11.8	43.1

Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks. As a result, carbon dioxide has gradually accumulated in the atmosphere, and as of May 2022, its concentration is 50% above pre-industrial levels. Various techniques have been proposed for removing excess carbon dioxide from the atmosphere (see Carbon sink#Artificial sequestration). Currently about half of the carbon dioxide released from the burning of fossil fuels is not absorbed by vegetation and the oceans and remains in the atmosphere.

• 

  Global fossil carbon emissions 1800–2014

• 

  False-color image of smoke and ozone pollution from Indonesian fires, 1997

• 

  Biosphere CO₂ flux in the northern hemisphere winter (NOAA Carbon Tracker)

• 

  Biosphere CO₂ flux in the northern hemisphere summer (NOAA Carbon Tracker)

Ongoing measurements of atmospheric CO₂

See also: Total Carbon Column Observing Network and Space-based measurements of carbon dioxide

 

Carbon Dioxide observations from 2005 to 2014 showing the seasonal variations and the difference between northern and southern hemispheres

The first reproducibly accurate measurements of atmospheric CO₂ were from flask sample measurements made by Dave Keeling at Caltech in the 1950s. A few years later in March 1958 the first ongoing measurements were started by Keeling at Mauna Loa. Measurements at Mauna Loa have been ongoing since then. Now measurements are made at many sites globally. Additional measurement techniques are also used as well. Many measurement sites are part of larger global networks. Global network data are often made publicly available on the conditions of proper acknowledgment according to the respective data user policies.

There are several surface measurement (including flasks and continuous in situ) networks including NOAA/ERSL, WDCGG, and RAMCES. The NOAA/ESRL Baseline Observatory Network, and the Scripps Institution of Oceanography Network data are hosted at the CDIAC at ORNL. The World Data Centre for Greenhouse Gases (WDCGG), part of GAW, data are hosted by the JMA. The Reseau Atmospherique de Mesure des Composes an Effet de Serre database (RAMCES) is part of IPSL.

From these measurements, further products are made which integrate data from the various sources. These products also address issues such as data discontinuity and sparseness. GLOBALVIEW-CO2 is one of these products.

Ongoing ground-based total column measurements began more recently. Column measurements typically refer to an averaged column amount denoted X_CO2, rather than a surface only measurement. These measurements are made by the TCCON. These data are also hosted on the CDIAC, and made publicly available according to the data use policy.

Satellite measurements are also a recent addition to atmospheric X_CO2 measurements. SCIAMACHY aboard ESA's ENVISAT made global column X_CO2 measurements from 2002 to 2012. AIRS aboard NASA's Aqua satellite makes global X_CO2 measurements and was launched shortly after ENVISAT in 2012. More recent satellites have significantly improved the data density and precision of global measurements. Newer missions have higher spectral and spatial resolutions. JAXA's GOSAT was the first dedicated GHG monitoring satellite to successfully achieve orbit in 2009. NASA's OCO-2 launched in 2014 was the second. Various other satellites missions to measure atmospheric X_CO2 are planned.