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Thursday, February 5, 2015

Gerard K. O'Neill


From Wikipedia, the free encyclopedia

Gerard K. O'Neill
Gerard Kitchen ONeill.GIF
Gerard K. O'Neill in 1977
Born (1927-02-06)February 6, 1927
Brooklyn, New York, US
Died April 27, 1992(1992-04-27) (aged 65)
Redwood City, California, US
Nationality American
Fields Physicist
Alma mater Cornell University
Known for Particle physics
Space Studies Institute
O'Neill cylinder

Gerard Kitchen O'Neill (February 6, 1927 – April 27, 1992) was an American physicist and space activist. As a faculty member of Princeton University, he invented a device called the particle storage ring for high-energy physics experiments.[1] Later, he invented a magnetic launcher called the mass driver.[2] In the 1970s, he developed a plan to build human settlements in outer space, including a space habitat design known as the O'Neill cylinder. He founded the Space Studies Institute, an organization devoted to funding research into space manufacturing and colonization.

O'Neill began researching high-energy particle physics at Princeton in 1954, after he received his doctorate from Cornell University. Two years later, he published his theory for a particle storage ring. This invention allowed particle physics experiments at much higher energies than had previously been possible. In 1965 at Stanford University, he performed the first colliding beam physics experiment.[3]

While teaching physics at Princeton, O'Neill became interested in the possibility that humans could survive and live in outer space. He researched and proposed a futuristic idea for human settlement in space, the O'Neill cylinder, in "The Colonization of Space", his first paper on the subject. He held a conference on space manufacturing at Princeton in 1975. Many who became post-Apollo-era space activists attended. O'Neill built his first mass driver prototype with professor Henry Kolm in 1976. He considered mass drivers critical for extracting the mineral resources of the Moon and asteroids. His award-winning book The High Frontier: Human Colonies in Space inspired a generation of space exploration advocates. He died of leukemia in 1992.

Birth, education, and family life

O'Neill was born in Brooklyn, New York on February 6, 1927 to Edward Gerard O'Neill, a lawyer, and Dorothy Lewis O'Neill (née Kitchen).[4][5][6] He had no siblings. His family moved to Speculator, New York when his father temporarily retired for health reasons.[6] For high school, O'Neill attended Newburgh Free Academy in Newburgh, New York. While he was a student there he edited the school newspaper and took a job as a news broadcaster at a local radio station.[4] He graduated in 1944, during World War II, and enlisted in the United States Navy on his 17th birthday.[4][7] The Navy trained him as a radar technician, which sparked his interest in science.[4]

After he was honorably discharged in 1946, O'Neill studied for an undergraduate degree in physics and mathematics at Swarthmore College.[7][8] As a child he had discussed the possibilities of humans in space with his parents, and in college he enjoyed working on rocket equations. However, he did not see space science as an option for a career path in physics, choosing instead to pursue high-energy physics.[9] In 1950 he graduated with Phi Beta Kappa honors. O'Neill performed his graduate studies at Cornell University with the help of an Atomic Energy Commission fellowship, and was awarded a Ph.D. in physics in 1954.[7]

O'Neill married Sylvia Turlington, also a Swarthmore graduate, in June 1950.[4][10] They had a son, Roger, and two daughters, Janet and Eleanor, before their marriage ended in divorce in 1966.[4][6]
One of O'Neill's favorite activities was flying. He held instrument certifications in both powered and sailplane flight and held the FAI Diamond Badge, a gliding award.[6][11] During his first cross-country glider flight in April 1973, he was assisted on the ground by Renate "Tasha" Steffen. He had met Tasha, who was 21 years younger than him, previously through the YMCA International Club. They were married the day after his flight.[5][6] They had a son, Edward O'Neill.[12]

High-energy physics research

After graduating from Cornell, O'Neill accepted a position as an instructor at Princeton University.[7] There he started his research into high-energy particle physics. In 1956, his second year of teaching, he published a two-page article that theorized that the particles produced by a particle accelerator could be stored for a few seconds in a storage ring.[1] The stored particles could then be directed to collide with another particle beam. This would increase the energy of the particle collision over the previous method, which directed the beam at a fixed target.[3] His ideas were not immediately accepted by the physics community.[4]

O'Neill became an assistant professor at Princeton in 1956, and was promoted to associate professor in 1959.[4][5] He visited Stanford University in 1957 to meet with Professor Wolfgang K. H. Panofsky.[13] This resulted in a collaboration between Princeton and Stanford to build the Colliding Beam Experiment (CBX).[14] With a US$800,000 grant from the Office of Naval Research, construction on the first particle storage rings began in 1958 at the Stanford High-Energy Physics Laboratory.[15][16] He figured out how to capture the particles and, by pumping the air out to produce a vacuum, store them long enough to experiment on them.[17][18] CBX stored its first beam on March 28, 1962. O'Neill became a full professor of physics in 1965.[3]

The two-mile-long Stanford Linear Accelerator tunnel

In collaboration with Burton Richter, O'Neill performed the first colliding beam physics experiment in 1965. In this experiment, particle beams from the Stanford Linear Accelerator were collected in his storage rings and then directed to collide at an energy of 600 MeV. At the time, this was the highest energy involved in a particle collision. The results proved that the charge of an electron is contained in a volume less than 100 attometers across. O'Neill considered his device to be capable of only seconds of storage, but, by creating an even stronger vacuum, others were able to increase this to hours.[3] In 1979, he, with physicist David C. Cheng, wrote the graduate-level textbook Elementary Particle Physics: An Introduction.[5] He retired from teaching in 1985, but remained associated with Princeton as professor emeritus until his death.[3]

Space colonization

Origin of the idea (1969)


NASA envisioned an ambitious scientific exploration of the Moon.

O'Neill saw great potential in the United States space program, especially the Apollo missions. He applied to the Astronaut Corps after NASA opened it up to civilian scientists in 1966. Later, when asked why he wanted to go on the Moon missions, he said, "to be alive now and not take part in it seemed terribly myopic".[6] He was put through NASA's rigorous mental and physical examinations. During this time he met Brian O'Leary, also a scientist-astronaut candidate, who became his good friend.[19] O'Leary was selected for Astronaut Group 6 but O'Neill was not.[20]

O'Neill became interested in the idea of space colonization in 1969 while he was teaching freshman physics at Princeton University.[3][21] His students were growing cynical about the benefits of science to humanity because of the controversy surrounding the Vietnam War.[22][23] To give them something relevant to study, he began using examples from the Apollo program as applications of elementary physics.[3][6] O'Neill posed the question during an extra seminar he gave to a few of his students: "Is the surface of a planet really the right place for an expanding technological civilization?"[21] His students' research convinced him that the answer was no.[21]

Bernal sphere, an "inside-out planet"

O'Neill was inspired by the papers written by his students. He began to work out the details of a program to build self-supporting space habitats in free space.[3][7] Among the details was how to provide the inhabitants of a space colony with an Earth-like environment. His students had designed giant pressurized structures, spun up to approximate Earth gravity by centrifugal force. With the population of the colony living on the inner surface of a sphere or cylinder, these structures resembled "inside-out planets". He found that pairing counter-rotating cylinders would eliminate the need to spin them using rockets.[21] This configuration has since been known as the O'Neill cylinder.

First paper (1970–1974)

Looking for an outlet for his ideas, O'Neill wrote a paper titled "The Colonization of Space", and for four years attempted to have it published.[24] He submitted it to several journals and magazines, including Scientific American and Science, only to have it rejected by the reviewers. During this time
O'Neill gave lectures on space colonization at Hampshire College, Princeton, and other schools. Many students and staff attending the lectures became enthusiastic about the possibility of living in space.[21] Another outlet for O'Neill to explore his ideas was with his children; on walks in the forest they speculated about life in a space colony.[25] His paper finally appeared in the September 1974 issue of Physics Today. In it, he argued that building space colonies would solve several important problems:
It is important to realize the enormous power of the space-colonization technique. If we begin to use it soon enough, and if we employ it wisely, at least five of the most serious problems now facing the world can be solved without recourse to repression: bringing every human being up to a living standard now enjoyed only by the most fortunate; protecting the biosphere from damage caused by transportation and industrial pollution; finding high quality living space for a world population that is doubling every 35 years; finding clean, practical energy sources; preventing overload of Earth's heat balance.
—Gerard K. O'Neill, "The Colonization of Space"[26]

Diagram of the Lagrange points in the Earth-Moon system

He even explored the possibilities of flying gliders inside a space colony, finding that the enormous volume could support atmospheric thermals.[27] He calculated that humanity could expand on this man-made frontier to 20,000 times its population.[28] The initial colonies would be built at the Earth-Moon L4 and L5 Lagrange points.[29] L4 and L5 are stable points in the Solar System where a spacecraft can maintain its position without expending energy. The paper was well received, but many who would begin work on the project had already been introduced to his ideas before it was even published.[21] The paper received a few critical responses. Some questioned the practicality of lifting tens of thousands of people into orbit and his estimates for the production output of initial colonies.[30]

While he was waiting for his paper to be published, O'Neill organized a small two-day conference in May 1974 at Princeton to discuss the possibility of colonizing outer space.[21] The conference, titled First Conference on Space Colonization, was funded by Stewart Brand's Point Foundation and Princeton University.[31] Among those who attended were Eric Drexler (at the time a freshman at MIT), scientist-astronaut Joe Allen (from Astronaut Group 6), Freeman Dyson, and science reporter Walter Sullivan.[21] Representatives from NASA also attended and brought estimates of launch costs expected on the planned Space Shuttle.[21] O'Neill thought of the attendees as "a band of daring radicals".[32] Sullivan's article on the conference was published on the front page of The New York Times on May 13, 1974.[33] As media coverage grew, O'Neill was inundated with letters from people who were excited about living in space.[34] To stay in touch with them, O'Neill began keeping a mailing list and started sending out updates on his progress.[21][35] A few months later he heard Peter Glaser speak about solar power satellites at NASA's Goddard Space Flight Center. O'Neill realized that, by building these satellites, his space colonies could quickly recover the cost of their construction.[36] According to O'Neill, "the profound difference between this and everything else done in space is the potential of generating large amounts of new wealth".[6]

NASA studies (1975–1977)

O'Neill held a much larger conference the following May titled Princeton University Conference on Space Manufacturing.[37] At this conference more than two dozen speakers presented papers, including Keith and Carolyn Henson from Tucson, Arizona.[38][39]

After the conference Carolyn Henson arranged a meeting between O'Neill and Arizona Congressman Morris Udall. Udall wrote a letter of support, which he asked the Hensons to publicize, for O'Neill's work.[38] The Hensons included his letter in the first issue of the L-5 Society newsletter, sent to everyone on O'Neill's mailing list and those who had signed up at the conference.[38][40]

O'Neill testifying before the Senate Subcommittee on January 19, 1976

In June 1975, O'Neill led a ten-week study of permanent space habitats at NASA Ames. During the study he was called away to testify on July 23 to the House Subcommittee on Space Science and Applications.[41] On January 19, 1976, he also appeared before the Senate Subcommittee on Aerospace Technology and National Needs. In a presentation titled Solar Power from Satellites, he laid out his case for an Apollo-style program for building power plants in space.[42] He returned to Ames in June 1976 and 1977 to lead studies on space manufacturing.[43] In these studies, NASA developed detailed plans to establish bases on the Moon where space-suited workers would mine the mineral resources needed to build space colonies and solar power satellites.[44]

Private funding (1977–1978)

Although NASA was supporting his work with grants of up to $500,000 per year, O'Neill became frustrated by the bureaucracy and politics inherent in government funded research.[4][23] He thought that small privately funded groups could develop space technology faster than government agencies.[3] In 1977, O'Neill and his wife Tasha founded the Space Studies Institute, a non-profit organization, at Princeton University.[7][45] SSI received initial funding of almost $100,000 from private donors, and in early 1978 began to support basic research into technologies needed for space manufacturing and settlement.[46]

Kolm (left) and O'Neill (center) with mass driver

One of SSI's first grants funded the development of the mass driver, a device first proposed by O'Neill in 1974.[47][48] Mass drivers are based on the coilgun design, adapted to accelerate a non-magnetic object.[49] One application O'Neill proposed for mass drivers was to throw baseball-sized chunks of ore mined from the surface of the Moon into space.[50][51] Once in space, the ore could be used as raw material for building space colonies and solar power satellites. He took a sabbatical from Princeton to work on mass drivers at MIT. There he served as the Hunsaker Visiting Professor of Aerospace during the 1976–77 academic year.[52] At MIT, he, Henry H. Kolm, and a group of student volunteers built their first mass driver prototype.[32][43][47] The eight-foot (2.5 m) long prototype could apply 33 g (320 m/s2) of acceleration to an object inserted into it.[32][43][50] With financial assistance from SSI, later prototypes improved this to 1,800 g (18,000 m/s2), enough acceleration that a mass driver only 520 feet (160 m) long could launch material off the surface of the Moon.[43]

Opposition (1977–1985)

In 1977, O'Neill saw the peak of interest in space colonization, along with the publication of his first book, The High Frontier.[38] He and his wife were flying between meetings, interviews, and hearings.[6] On October 9, the CBS program 60 Minutes ran a segment about space colonies. Later they aired responses from the viewers, which included one from Senator William Proxmire, chairman of the Senate Subcommittee responsible for NASA's budget. His response was, "it's the best argument yet for chopping NASA's funding to the bone .... I say not a penny for this nutty fantasy".[53] He successfully eliminated spending on space colonization research from the budget.[54] In 1978, Paul Werbos wrote for the L-5 newsletter, "no one expects Congress to commit us to O'Neill's concept of large-scale space habitats; people in NASA are almost paranoid about the public relations aspects of the idea".[55] When it became clear that a government funded colonization effort was politically impossible, popular support for O'Neill's ideas started to evaporate.[38]

Other pressures on O'Neill's colonization plan were the high cost of access to Earth orbit and the declining cost of energy. Building solar power stations in space was economically attractive when energy prices spiked during the 1979 oil crisis. When prices dropped in the early 1980s, funding for space solar power research dried up.[56] His plan had also been based on NASA's estimates for the flight rate and launch cost of the Space Shuttle, numbers that turned out to have been wildly optimistic. His 1977 book quoted a Space Shuttle launch cost of $10 million, but in 1981 the subsidized price given to commercial customers started at $38 million.[57][58] Eventual accounting of the full cost of a launch in 1985 raised this as high as $180 million per flight.[59]

O'Neill was appointed by United States President Ronald Reagan to the National Commission on Space in 1985.[7] The commission, led by former NASA administrator Thomas Paine, proposed that the government commit to opening the inner Solar System for human settlement within 50 years.[60] Their report was released in May 1986, four months after the Space Shuttle Challenger broke up on ascent.[60]

Writing career


O'Neill cylinders as illustrated in The High Frontier

O'Neill's popular science book The High Frontier: Human Colonies in Space (1977) combined fictional accounts of space settlers with an explanation of his plan to build space colonies. Its publication established him as the spokesman for the space colonization movement.[3] It won the Phi Beta Kappa Award in Science that year, and prompted Swarthmore College to grant him an honorary doctorate.[5][61] The High Frontier has been translated into five languages and remained in print as of 2008.[43]

His 1981 book 2081: A Hopeful View of the Human Future was an exercise in futurology. O'Neill narrated it as a visitor to Earth from a space colony beyond Pluto.[62] The book explored the effects of technologies he called "drivers of change" on the coming century. Some technologies he described were space colonies, solar power satellites, anti-aging drugs, hydrogen-propelled cars, climate control, and underground magnetic trains. He left the social structure of the 1980s intact, assuming that humanity would remain unchanged even as it expanded into the Solar System. Reviews of 2081 were mixed. New York Times reviewer John Noble Wilford found the book "imagination-stirring", but Charles Nicol thought the technologies described were unacceptably far-fetched.[5]

In his book The Technology Edge, published in 1983, O'Neill wrote about economic competition with Japan.[63] He argued that the United States had to develop six industries to compete: microengineering, robotics, genetic engineering, magnetic flight, family aircraft, and space science.[51] He also thought that industrial development was suffering from short-sighted executives, self-interested unions, high taxes, and poor education of Americans. According to reviewer Henry Weil, O'Neill's detailed explanations of emerging technologies differentiated the book from others on the subject.[63]

Entrepreneurial efforts


Design for the satellite position determination system

O'Neill founded Geostar Corporation to develop a satellite position determination system for which he was granted a patent in 1982.[64] The system, primarily intended to track aircraft, was called Radio Determination Satellite Service (RDSS).[51] In April 1983 Geostar applied to the FCC for a license to broadcast from three satellites, which would cover the entire United States. Geostar launched GSTAR-2 into geosynchronous orbit in 1986. Its transmitter package permanently failed two months later, so Geostar began tests of RDSS by transmitting from other satellites.[65] With his health failing, O'Neill became less involved with the company at the same time it started to run into trouble.[66] In February 1991 Geostar filed for bankruptcy and its licenses were sold to Motorola for the Iridium satellite constellation project.[67] Although the system was eventually replaced by GPS, O'Neill made significant advances in the field of position determination.[66]

O'Neill founded O'Neill Communications in Princeton in 1986. He introduced his Local Area Wireless Networking, or LAWN, system at the PC Expo in New York in 1989.[68] The LAWN system allowed two computers to exchange messages over a range of a couple hundred feet at a cost of about $500 per node.[69] O'Neill Communications went out of business in 1993; the LAWN technology was sold to Omnispread Communications. As of 2008, Omnispread continued to sell a variant of O'Neill's LAWN system.[70]

On November 18, 1991, O'Neill filed a patent application for a high-speed train system. He called the company he wanted to form VSE International, for velocity, silence, and efficiency.[3] However, the concept itself he called Magnetic Flight. The vehicles, instead of running on a pair of tracks, would be elevated using electromagnetic force by a single track within a tube (permanent magnets in the track, with variable magnets on the vehicle), and propelled by electromagnetic forces through tunnels. He estimated the trains could reach speeds of up to 2,500 mph (4,000 km/h) — about five times faster than a jet airliner — if the air was evacuated from the tunnels.[7] To obtain such speeds, the vehicle would accelerate for the first half of the trip, and then decelerate for the second half of the trip. The acceleration was planned to be a maximum of about one-half of the force of gravity. O'Neill planned to build a network of stations connected by these tunnels, but he died two years before his first patent on it was granted.[3]

Death and legacy


O'Neill's ashes were carried on the upper stage of an Orbital Sciences Pegasus

O'Neill was diagnosed with leukemia in 1985.[66] He died on April 27, 1992, from complications of the disease at the Sequoia Hospital in Redwood City, California.[7][8] He was survived by his wife Tasha, his ex-wife Sylvia, and his four children.[7][71] A sample of his incinerated remains was buried in space.[72] The vial containing his ashes was attached to a Pegasus XL rocket and launched into Earth orbit on April 21, 1997.[72][73] It re-entered the atmosphere in May 2002.[74]

O'Neill directed his Space Studies Institute to continue their efforts "until people are living and working in space".[75] After his death, management of SSI was passed to his son Roger and colleague Freeman Dyson.[43] SSI continued to hold conferences every other year to bring together scientists studying space colonization until 2001.[76]

Henry Kolm went on to start Magplane Technology in the 1990s to develop the magnetic transportation technology that O'Neill had written about. In 2007, Magplane demonstrated a working magnetic pipeline system to transport phosphate ore in Florida. The system ran at a speed of 40 mph (65 km/h), far slower than the high-speed trains O'Neill envisioned.[77][78]

All three of the founders of the Space Frontier Foundation, an organization dedicated to opening the space frontier to human settlement, were supporters of O'Neill's ideas and had worked with him in various capacities at the Space Studies Institute.[79] One of them, Rick Tumlinson, describes three men as models for space advocacy: Wernher von Braun, Gerard K. O'Neill, and Carl Sagan. Von Braun pushed for "projects that ordinary people can be proud of but not participate in".[80] Sagan wanted to explore the universe from a distance. O'Neill, with his grand scheme for settlement of the Solar System, emphasized moving ordinary people off the Earth "en masse".[80]

The National Space Society (NSS) gives the Gerard K. O'Neill Memorial Award for Space Settlement Advocacy to individuals noted for their contributions in the area of space settlement. Their contributions can be scientific, legislative, and educational. The award is a trophy cast in the shape of a Bernal sphere. The NSS first bestowed the award in 2007 on lunar entrepreneur and former astronaut Harrison Schmitt. In 2008, it was given to physicist John Marburger.[81]

In fiction, the protagonist of Stephen Baxter's Manifold:Time names his spaceship the Gerard K. O'Neill.

As of November, 2013, Gerard O'Neill's papers and work are now located in the archives at the Smithsonian National Air and Space Museum, Steven F. Udvar-Hazy Center.

Publications

Books

Papers

Patents

O'Neill was granted six patents in total (two posthumously) in the areas of global position determination and magnetic levitation.
  • US 4359733  Satellite-based vehicle position determining system, granted November 16, 1982
  • US 4744083  Satellite-based position determining and message transfer system with monitoring of link quality, granted May 10, 1988
  • US 4839656  Position determination and message transfer system employing satellites and stored terrain map, granted June 13, 1989
  • US 4965586  Position determination and message transfer system employing satellites and stored terrain map, granted October 23, 1990
  • US 5282424  High speed transport system, granted February 1, 1994
  • US 5433155  High speed transport system, granted July 18, 1995

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Greenhouse gas


From Wikipedia, the free encyclopedia

refer to caption and image description
Greenhouse effect schematic showing energy flows between space, the atmosphere, and Earth's surface. Energy influx and emittance are expressed in watts per square meter (W/m2).

A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect.[1] The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth's surface would average about 33 °C colder, which is about 59 °F below the present average of 14 °C (57 °F).[2][3][4]

Since the beginning of the Industrial Revolution (taken as the year 1750), the burning of fossil fuels and extensive clearing of native forests has contributed to a 40% increase in the atmospheric concentration of carbon dioxide, from 280 ppm in 1750 to 392.6 ppm in 2012.[5][6] It has now reached 400 ppm in the northern hemisphere. This increase has occurred despite the uptake of a large portion of the emissions by various natural "sinks" involved in the carbon cycle.[7][8] Anthropogenic carbon dioxide (CO
2) emissions (i.e., emissions produced by human activities) come from combustion of carbon-based fuels, principally wood, coal, oil, and natural gas.[9] Under ongoing greenhouse gas emissions, available Earth System Models project that the Earth's surface temperature could exceed historical analogs as early as 2047 affecting most ecosystems on Earth and the livelihoods of over 3 billion people worldwide.[10] Greenhouse gases also trigger[clarification needed] ocean bio-geochemical changes with broad ramifications in marine systems.[11]

In the Solar System, the atmospheres of Venus, Mars, and Titan also contain gases that cause a greenhouse effect, though Titan's atmosphere has an anti-greenhouse effect that reduces the warming.

Gases in Earth's atmosphere

Main articles: Greenhouse effect, Global warming and Carbon dioxide in Earth's atmosphere

Greenhouse gases

refer to caption and adjacent text
Atmospheric absorption and scattering at different wavelengths of electromagnetic waves. The largest absorption band of carbon dioxide is in the infrared.

Greenhouse gases are those that can absorb and emit infrared radiation,[1] but not radiation in or near the visible spectrum. In order, the most abundant greenhouse gases in Earth's atmosphere are:
Atmospheric concentrations of greenhouse gases are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound).[12] The proportion of an emission remaining in the atmosphere after a specified time is the "airborne fraction" (AF). More precisely, the annual AF is the ratio of the atmospheric increase in a given year to that year's total emissions. For CO
2 the AF over the last 50 years (1956–2006) has been increasing at 0.25 ± 0.21%/year.[13]

Non-greenhouse gases

Although contributing to many other physical and chemical reactions, the major atmospheric constituents, nitrogen (N
2), oxygen (O
2), and argon (Ar), are not greenhouse gases. This is because molecules containing two atoms of the same element such as N
2 and O
2 and monatomic molecules such as argon (Ar) have no net change in their dipole moment when they vibrate and hence are almost totally unaffected by infrared radiation. Although molecules containing two atoms of different elements such as carbon monoxide (CO) or hydrogen chloride (HCl) absorb IR, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. Because they do not contribute significantly to the greenhouse effect, they are usually omitted when discussing greenhouse gases.

Indirect radiative effects

world map of carbon monoxide concentrations in the lower atmosphere
The false colors in this image represent levels of carbon monoxide in the lower atmosphere, ranging from about 390 parts per billion (dark brown pixels), to 220 parts per billion (red pixels), to 50 parts per billion (blue pixels).[14]

Some gases have indirect radiative effects (whether or not they are a greenhouse gas themselves). This happens in two main ways. One way is that when they break down in the atmosphere they produce another greenhouse gas. For example, methane and carbon monoxide (CO) are oxidized to give carbon dioxide (and methane oxidation also produces water vapor; that will be considered below). Oxidation of CO to CO
2 directly produces an unambiguous increase in radiative forcing although the reason is subtle. The peak of the thermal IR emission from the Earth's surface is very close to a strong vibrational absorption band of CO
2 (667 cm−1). On the other hand, the single CO vibrational band only absorbs IR at much higher frequencies (2145 cm−1), where the ~300 K thermal emission of the surface is at least a factor of ten lower. On the other hand, oxidation of methane to CO
2, which requires reactions with the OH radical, produces an instantaneous reduction, since CO
2 is a weaker greenhouse gas than methane; but it has a longer lifetime. As described below this is not the whole story, since the oxidations of CO and CH
4 are intertwined by both consuming OH radicals. In any case, the calculation of the total radiative effect needs to include both the direct and indirect forcing.

A second type of indirect effect happens when chemical reactions in the atmosphere involving these gases change the concentrations of greenhouse gases. For example, the destruction of non-methane volatile organic compounds (NMVOC) in the atmosphere can produce ozone. The size of the indirect effect can depend strongly on where and when the gas is emitted.[15]

Methane has a number of indirect effects in addition to forming CO
2. Firstly, the main chemical that destroys methane in the atmosphere is the hydroxyl radical (OH). Methane reacts with OH and so more methane means that the concentration of OH goes down. Effectively, methane increases its own atmospheric lifetime and therefore its overall radiative effect. The second effect is that the oxidation of methane can produce ozone. Thirdly, as well as making CO
2 the oxidation of methane produces water; this is a major source of water vapor in the stratosphere, which is otherwise very dry. CO and NMVOC also produce CO
2 when they are oxidized. They remove OH from the atmosphere and this leads to higher concentrations of methane. The surprising effect of this is that the global warming potential of CO is three times that of CO
2.[16] The same process that converts NMVOC to carbon dioxide can also lead to the formation of tropospheric ozone. Halocarbons have an indirect effect because they destroy stratospheric ozone. Finally hydrogen can lead to ozone production and CH
4 increases as well as producing water vapor in the stratosphere.[15]

Contribution of clouds to Earth's greenhouse effect

The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the greenhouse gases. Clouds are water droplets or ice crystals suspended in the atmosphere.[17][18]

Impacts on the overall greenhouse effect

refer to caption and adjacent text
Schmidt et al. (2010)[19] analysed how individual components of the atmosphere contribute to the total greenhouse effect. They estimated that water vapor accounts for about 50% of the Earth's greenhouse effect, with clouds contributing 25%, carbon dioxide 20%, and the minor greenhouse gases and aerosols accounting for the remaining 5%. In the study, the reference model atmosphere is for 1980 conditions. Image credit: NASA.[20]

The contribution of each gas to the greenhouse effect is affected by the characteristics of that gas, its abundance, and any indirect effects it may cause. For example, the direct radiative effect of a mass of methane is about 72 times stronger than the same mass of carbon dioxide over a 20-year time frame[21] but it is present in much smaller concentrations so that its total direct radiative effect is smaller, in part due to its shorter atmospheric lifetime. On the other hand, in addition to its direct radiative impact, methane has a large, indirect radiative effect because it contributes to ozone formation. Shindell et al. (2005)[22] argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect.[23]

When ranked by their direct contribution to the greenhouse effect, the most important are:[17]
Compound
 Formula
 Contribution
(%)
Water vapor and clouds H
2O		 36 – 72%  
Carbon dioxide CO
2		 9 – 26%
Methane CH
4		 4–9%  
Ozone O
3		 3–7%  
In addition to the main greenhouse gases listed above, other greenhouse gases include sulfur hexafluoride, hydrofluorocarbons and perfluorocarbons (see IPCC list of greenhouse gases). Some greenhouse gases are not often listed. For example, nitrogen trifluoride has a high global warming potential (GWP) but is only present in very small quantities.[24]

Proportion of direct effects at a given moment

It is not possible to state that a certain gas causes an exact percentage of the greenhouse effect. This is because some of the gases absorb and emit radiation at the same frequencies as others, so that the total greenhouse effect is not simply the sum of the influence of each gas. The higher ends of the ranges quoted are for each gas alone; the lower ends account for overlaps with the other gases.[17][18]
In addition, some gases such as methane are known to have large indirect effects that are still being quantified.[25]

Atmospheric lifetime

Aside from water vapor, which has a residence time of about nine days,[26] major greenhouse gases are well-mixed, and take many years to leave the atmosphere.[27] Although it is not easy to know with precision how long it takes greenhouse gases to leave the atmosphere, there are estimates for the principal greenhouse gases. Jacob (1999)[28] defines the lifetime \tau of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically \tau can be defined as the ratio of the mass m (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box (F_{out}), chemical loss of X (L), and deposition of X (D) (all in kg/s): \tau = \frac{m}{F_{out}+L+D}.[28] If one stopped pouring any of this gas into the box, then after a time \tau, its concentration would be about halved.

The atmospheric lifetime of a species therefore measures the time required to restore equilibrium following a sudden increase or decrease in its concentration in the atmosphere. Individual atoms or molecules may be lost or deposited to sinks such as the soil, the oceans and other waters, or vegetation and other biological systems, reducing the excess to background concentrations. The average time taken to achieve this is the mean lifetime.

Carbon dioxide has a variable atmospheric lifetime, and cannot be specified precisely.[29] The atmospheric lifetime of CO
2 is estimated of the order of 30–95 years.[30] This figure accounts for CO
2 molecules being removed from the atmosphere by mixing into the ocean, photosynthesis, and other processes. However, this excludes the balancing fluxes of CO
2 into the atmosphere from the geological reservoirs, which have slower characteristic rates.[31] While more than half of the CO
2 emitted is removed from the atmosphere within a century, some fraction (about 20%) of emitted CO
2 remains in the atmosphere for many thousands of years.[32][33][34] Similar issues apply to other greenhouse gases, many of which have longer mean lifetimes than CO
2. E.g., N2O has a mean atmospheric lifetime of 114 years.[21]

Radiative forcing

The Earth absorbs some of the radiant energy received from the sun, reflects some of it as light and reflects or radiates the rest back to space as heat.[35] The Earth's surface temperature depends on this balance between incoming and outgoing energy.[35] If this energy balance is shifted, the Earth's surface could become warmer or cooler, leading to a variety of changes in global climate.[35]

A number of natural and man-made mechanisms can affect the global energy balance and force changes in the Earth's climate.[35] Greenhouse gases are one such mechanism.[35] Greenhouse gases in the atmosphere absorb and re-emit some of the outgoing energy radiated from the Earth's surface, causing that heat to be retained in the lower atmosphere.[35] As explained above, some greenhouse gases remain in the atmosphere for decades or even centuries, and therefore can affect the Earth's energy balance over a long time period.[35] Factors that influence Earth's energy balance can be quantified in terms of "radiative climate forcing."[35] Positive radiative forcing indicates warming (for example, by increasing incoming energy or decreasing the amount of energy that escapes to space), while negative forcing is associated with cooling.[35]

Global warming potential

The global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO
2 and evaluated for a specific timescale. Thus, if a gas has a high (positive) radiative forcing but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO
2 its GWP will increase with the timescale considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.

Methane has an atmospheric lifetime of 12 ± 3 years. The 2007 IPCC report lists the GWP as 72 over a time scale of 20 years, 25 over 100 years and 7.6 over 500 years.[21] A 2014 analysis, however, states that although methane’s initial impact is about 100 times greater than that of CO
2, because of the shorter atmospheric lifetime, after six or seven decades, the impact of the two gases is about equal, and from then on methane’s relative role continues to decline.[36] The decrease in GWP at longer times is because methane is degraded to water and CO
2 through chemical reactions in the atmosphere.
Examples of the atmospheric lifetime and GWP relative to CO
2 for several greenhouse gases are given in the following table:[21]
Atmospheric lifetime and GWP relative to CO
2 at different time horizon for various greenhouse gases.
Gas name Chemical
formula		 Lifetime
(years)		 Global warming potential (GWP) for given time horizon
20-yr 100-yr 500-yr
Carbon dioxide CO
2		 See above 1 1 1
Methane CH
4		 12 72 25 7.6
Nitrous oxide N
2O		 114 289 298 153
CFC-12 CCl
2F
2		 100 11 000 10 900 5 200
HCFC-22 CHClF
2		 12 5 160 1 810 549
Tetrafluoromethane CF
4		 50 000 5 210 7 390 11 200
Hexafluoroethane C
2F
6		 10 000 8 630 12 200 18 200
Sulfur hexafluoride SF
6		 3 200 16 300 22 800 32 600
Nitrogen trifluoride NF
3		 740 12 300 17 200 20 700

The use of CFC-12 (except some essential uses) has been phased out due to its ozone depleting properties.[37] The phasing-out of less active HCFC-compounds will be completed in 2030.[38]

Natural and anthropogenic sources

refer to caption and article text
Top: Increasing atmospheric carbon dioxide levels as measured in the atmosphere and reflected in ice cores. Bottom: The amount of net carbon increase in the atmosphere, compared to carbon emissions from burning fossil fuel.
refer to caption and image description
This diagram shows a simplified representation of the contemporary global carbon cycle. Changes are measured in gigatons of carbon per year (GtC/y). Canadell et al. (2007) estimated the growth rate of global average atmospheric CO
2 for 2000–2006 as 1.93 parts-per-million per year (4.1 petagrams of carbon per year).[39]

Aside from purely human-produced synthetic halocarbons, most greenhouse gases have both natural and human-caused sources. During the pre-industrial Holocene, concentrations of existing gases were roughly constant. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.[40][41]

The 2007 Fourth Assessment Report compiled by the IPCC (AR4) noted that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century".[42] In AR4, "most of" is defined as more than 50%.

Abbreviations used in the two tables below: ppm = parts-per-million; ppb = parts-per-billion; ppt = parts-per-trillion; W/m2 = watts per square metre
Current greenhouse gas concentrations[5]
Gas Pre-1750
tropospheric
concentration[43]		 Recent
tropospheric
concentration[44]		 Absolute increase
since 1750		 Percentage
increase
since 1750		 Increased
radiative forcing
(W/m2)[45]
Carbon dioxide (CO
2)		 280 ppm[46] 395.4 ppm[47] 115.4 ppm 41.2% 1.88
Methane (CH
4)		 700 ppb[48] 1893 ppb /[49]
1762 ppb[49]		 1193 ppb /
1062 ppb		 170.4% /
151.7%		 0.49
Nitrous oxide (N
2O)		 270 ppb[45][50] 326 ppb /[49]
324 ppb[49]		 56 ppb /
54 ppb		 20.7% /
20.0%		 0.17
Tropospheric
ozone (O
3)		 237 ppb[43] 337 ppb[43] 100 ppb 42% 0.4[51]
Relevant to radiative forcing and/or ozone depletion; all of the following have no natural sources and hence zero amounts pre-industrial[5]
Gas Recent
tropospheric
concentration		 Increased
radiative forcing
(W/m2)
CFC-11
(trichlorofluoromethane)
(CCl
3F)		 236 ppt /
234 ppt		 0.061
CFC-12 (CCl
2F
2)		 527 ppt /
527 ppt		 0.169
CFC-113 (Cl
2FC-CClF
2)		 74 ppt /
74 ppt		 0.022
HCFC-22 (CHClF
2)		 231 ppt /
210 ppt		 0.046
HCFC-141b (CH
3CCl
2F)		 24 ppt /
21 ppt		 0.0036
HCFC-142b (CH
3CClF
2)		 23 ppt /
21 ppt		 0.0042
Halon 1211 (CBrClF
2)		 4.1 ppt /
4.0 ppt		 0.0012
Halon 1301 (CBrClF
3)		 3.3 ppt /
3.3 ppt		 0.001
HFC-134a (CH
2FCF
3)		 75 ppt /
64 ppt		 0.0108
Carbon tetrachloride (CCl
4)		 85 ppt /
83 ppt		 0.0143
Sulfur hexafluoride (SF
6)		 7.79 ppt /[52]
7.39 ppt[52]		 0.0043
Other halocarbons Varies by
substance		 collectively
0.02
Halocarbons in total 0.3574
refer to caption and article text
400,000 years of ice core data

Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the following section). Both CO
2 and CH
4 vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates CO
2 mole fractions stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxies and modeling suggests larger variations in past epochs; 500 million years ago CO
2 levels were likely 10 times higher than now.[53] Indeed higher CO
2 concentrations are thought to have prevailed throughout most of the Phanerozoic eon, with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the Devonian period, about 400 Ma.[54][55][56] The spread of land plants is thought to have reduced CO
2 concentrations during the late Devonian, and plant activities as both sources and sinks of CO
2 have since been important in providing stabilising feedbacks.[57] Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator (Snowball Earth) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the CO
2 concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as limestone at the rate of about 1 mm per day.[58]
This episode marked the close of the Precambrian eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are only about 1% of emissions from human sources.[58][59][60]

Ice cores

Measurements from Antarctic ice cores show that before industrial emissions started atmospheric CO
2 mole fractions were about 280 parts per million (ppm), and stayed between 260 and 280 during the preceding ten thousand years.[61] Carbon dioxide mole fractions in the atmosphere have gone up by approximately 35 percent since the 1900s, rising from 280 parts per million by volume to 387 parts per million in 2009. One study using evidence from stomata of fossilized leaves suggests greater variability, with carbon dioxide mole fractions above 300 ppm during the period seven to ten thousand years ago,[62] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO
2 variability.[63][64] Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the firn) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels.

Changes since the Industrial Revolution

Refer to caption
Recent year-to-year increase of atmospheric CO
2.
Refer to caption
Major greenhouse gas trends.

Since the beginning of the Industrial Revolution, the concentrations of most of the greenhouse gases have increased. For example, the mole fraction of carbon dioxide has increased from 280 ppm by about 36% to 380 ppm, or 100 ppm over modern pre-industrial levels. The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973.[citation needed]; however the next 50 ppm increase took place in about 33 years, from 1973 to 2006.[65]

Recent data also shows that the concentration is increasing at a higher rate. In the 1960s, the average annual increase was only 37% of what it was in 2000 through 2007.[66]

Today, the stock of carbon in the atmosphere increases by more than 3 million tonnes per annum (0.04%) compared with the existing stock.[clarification needed] This increase is the result of human activities by burning fossil fuels, deforestation and forest degradation in tropical and boreal regions.[67]

The other greenhouse gases produced from human activity show similar increases in both amount and rate of increase. Many observations are available online in a variety of Atmospheric Chemistry Observational Databases.

Anthropogenic greenhouse gases

This graph shows changes in the annual greenhouse gas index (AGGI) between 1979 and 2011.[68] The AGGI measures the levels of greenhouse gases in the atmosphere based on their ability to cause changes in the Earth's climate.[68]
This bar graph shows global greenhouse gas emissions by sector from 1990 to 2005, measured in carbon dioxide equivalents.[69]
Modern global CO2 emissions from the burning of fossil fuels.

Since about 1750 human activity has increased the concentration of carbon dioxide and other greenhouse gases. Measured atmospheric concentrations of carbon dioxide are currently 100 ppm higher than pre-industrial levels.[70] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,[71] but over periods longer than a few years natural sources are closely balanced by natural sinks, mainly photosynthesis of carbon compounds by plants and marine plankton. As a result of this balance, the atmospheric mole fraction of carbon dioxide remained between 260 and 280 parts per million for the 10,000 years between the end of the last glacial maximum and the start of the industrial era.[72]

It is likely that anthropogenic (i.e., human-induced) warming, such as that due to elevated greenhouse gas levels, has had a discernible influence on many physical and biological systems.[73] Future warming is projected to have a range of impacts, including sea level rise,[74] increased frequencies and severities of some extreme weather events,[74] loss of biodiversity,[75] and regional changes in agricultural productivity.[75]

The main sources of greenhouse gases due to human activity are:
  • burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations in the air. Land use change (mainly deforestation in the tropics) account for up to one third of total anthropogenic CO
    2     emissions.[72]
  • livestock enteric fermentation and manure management,[76] paddy rice farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are sources of atmospheric methane.
  • use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.
  • agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide (N
    2    O    ) concentrations.
The seven sources of CO
2 from fossil fuel combustion are (with percentage contributions for 2000–2004):[77]
Seven main fossil fuel
combustion sources		 Contribution
(%)
Liquid fuels (e.g., gasoline, fuel oil) 36%
Solid fuels (e.g., coal) 35%
Gaseous fuels (e.g., natural gas) 20%
Cement production  3 %
Flaring gas industrially and at wells < 1%  
Non-fuel hydrocarbons < 1%  
"International bunker fuels" of transport
not included in national inventories[78]		  4 %
Carbon dioxide, methane, nitrous oxide (N
2O) and three groups of fluorinated gases (sulfur hexafluoride (SF
6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs)) are the major anthropogenic greenhouse gases,[79]:147[80] and are regulated under the Kyoto Protocol international treaty, which came into force in 2005.[81] Emissions limitations specified in the Kyoto Protocol expire in 2012.[81] The Cancún agreement, agreed in 2010, includes voluntary pledges made by 76 countries to control emissions.[82] At the time of the agreement, these 76 countries were collectively responsible for 85% of annual global emissions.[82]

Although CFCs are greenhouse gases, they are regulated by the Montreal Protocol, which was motivated by CFCs' contribution to ozone depletion rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused in the media.

Sectors

Tourism
According to UNEP global tourism is closely linked to climate change. Tourism is a significant contributor to the increasing concentrations of greenhouse gases in the atmosphere. Tourism accounts for about 50% of traffic movements. Rapidly expanding air traffic contributes about 2.5% of the production of CO
2. The number of international travelers is expected to increase from 594 million in 1996 to 1.6 billion by 2020, adding greatly to the problem unless steps are taken to reduce emissions.[83]

Role of water vapor


Increasing water vapor in the stratosphere at Boulder, Colorado.

Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for clear sky conditions and between 66% and 85% when including clouds.[18] Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields. The atmospheric concentration of vapor is highly variable and depends largely on temperature, from less than 0.01% in extremely cold regions up to 3% by mass at in saturated air at about 32 °C.(see Relative humidity#other important facts) [84]

The average residence time of a water molecule in the atmosphere is only about nine days, compared to years or centuries for other greenhouse gases such as CH
4 and CO
2.[85] Thus, water vapor responds to and amplifies effects of the other greenhouse gases. The Clausius–Clapeyron relation establishes that more water vapor will be present per unit volume at elevated temperatures. This and other basic principles indicate that warming associated with increased concentrations of the other greenhouse gases also will increase the concentration of water vapor (assuming that the relative humidity remains approximately constant; modeling and observational studies find that this is indeed so). Because water vapor is a greenhouse gas, this results in further warming and so is a "positive feedback" that amplifies the original warming. Eventually other earth processes offset these positive feedbacks, stabilizing the global temperature at a new equilibrium and preventing the loss of Earth's water through a Venus-like runaway greenhouse effect.[86]

Direct greenhouse gas emissions

Between the period 1970 to 2004, GHG emissions (measured in CO
2-equivalent)[87] increased at an average rate of 1.6% per year, with CO
2 emissions from the use of fossil fuels growing at a rate of 1.9% per year.[88][89] Total anthropogenic emissions at the end of 2009 were estimated at 49.5 gigatonnes CO
2-equivalent.[90]:15 These emissions include CO
2 from fossil fuel use and from land use, as well as emissions of methane, nitrous oxide and other GHGs covered by the Kyoto Protocol.
At present, the primary source of CO
2 emissions is the burning of coal, natural gas, and petroleum for electricity and heat.[91]

Regional and national attribution of emissions

This figure shows the relative fraction of anthropogenic greenhouse gases coming from each of eight categories of sources, as estimated by the Emission Database for Global Atmospheric Research version 3.2, fast track 2000 project [1]. These values are intended to provide a snapshot of global annual greenhouse gas emissions in the year 2000. The top panel shows the sum over all anthropogenic greenhouse gases, weighted by their global warming potential over the next 100 years. This consists of 72% carbon dioxide, 18% methane, 8% nitrous oxide and 1% other gases. Lower panels show the comparable information for each of these three primary greenhouse gases, with the same coloring of sectors as used in the top chart. Segments with less than 1% fraction are not labeled.

There are several different ways of measuring GHG emissions, for example, see World Bank (2010)[92]:362 for tables of national emissions data. Some variables that have been reported[93] include:
  • Definition of measurement boundaries: Emissions can be attributed geographically, to the area where they were emitted (the territory principle) or by the activity principle to the territory produced the emissions. These two principles result in different totals when measuring, for example, electricity importation from one country to another, or emissions at an international airport.
  • Time horizon of different GHGs: Contribution of a given GHG is reported as a CO
    2     equivalent. The calculation to determine this takes into account how long that gas remains in the atmosphere. This is not always known accurately and calculations must be regularly updated to reflect new information.
  • What sectors are included in the calculation (e.g., energy industries, industrial processes, agriculture etc.): There is often a conflict between transparency and availability of data.
  • The measurement protocol itself: This may be via direct measurement or estimation. The four main methods are the emission factor-based method, mass balance method, predictive emissions monitoring systems, and continuous emissions monitoring systems. These methods differ in accuracy, cost, and usability.
These different measures are sometimes used by different countries to assert various policy/ethical positions on climate change (Banuri et al., 1996, p. 94).[94] This use of different measures leads to a lack of comparability, which is problematic when monitoring progress towards targets. There are arguments for the adoption of a common measurement tool, or at least the development of communication between different tools.[93]

Emissions may be measured over long time periods. This measurement type is called historical or cumulative emissions. Cumulative emissions give some indication of who is responsible for the build-up in the atmospheric concentration of GHGs (IEA, 2007, p. 199).[95]

The national accounts balance would be positively related to carbon emissions. The national accounts balance shows the difference between exports and imports. For many richer nations, such as the United States, the accounts balance is negative because more goods are imported than they are exported. This is mostly due to the fact that it is cheaper to produce goods outside of developed countries, leading the economies of developed countries to become increasingly dependent on services and not goods. We believed that a positive accounts balance would means that more production was occurring in a country, so more factories working would increase carbon emission levels.(Holtz-Eakin, 1995, pp.;85;101).[96]

Emissions may also be measured across shorter time periods. Emissions changes may, for example, be measured against a base year of 1990. 1990 was used in the United Nations Framework Convention on Climate Change (UNFCCC) as the base year for emissions, and is also used in the Kyoto Protocol (some gases are also measured from the year 1995).[79]:146,149 A country's emissions may also be reported as a proportion of global emissions for a particular year.

Another measurement is of per capita emissions. This divides a country's total annual emissions by its mid-year population.[92]:370 Per capita emissions may be based on historical or annual emissions (Banuri et al., 1996, pp. 106–107).[94]

Land-use change[edit]

Refer to caption.
Greenhouse gas emissions from agriculture, forestry and other land use, 1970-2010.

Land-use change, e.g., the clearing of forests for agricultural use, can affect the concentration of GHGs in the atmosphere by altering how much carbon flows out of the atmosphere into carbon sinks.[97] Accounting for land-use change can be understood as an attempt to measure "net" emissions, i.e., gross emissions from all GHG sources minus the removal of emissions from the atmosphere by carbon sinks (Banuri et al., 1996, pp. 92–93).[94]

There are substantial uncertainties in the measurement of net carbon emissions.[98] Additionally, there is controversy over how carbon sinks should be allocated between different regions and over time (Banuri et al., 1996, p. 93).[94] For instance, concentrating on more recent changes in carbon sinks is likely to favour those regions that have deforested earlier, e.g., Europe.

Greenhouse gas intensity

Refer to caption.
Greenhouse gas intensity in the year 2000, including land-use change.
Refer to caption.
Carbon intensity of GDP (using PPP) for different regions, 1982-2011.
Refer to caption.
Carbon intensity of GDP (using MER) for different regions, 1982-2011.

Greenhouse gas intensity is a ratio between greenhouse gas emissions and another metric, e.g., gross domestic product (GDP) or energy use. The terms "carbon intensity" and "emissions intensity" are also sometimes used.[99] GHG intensities may be calculated using market exchange rates (MER) or purchasing power parity (PPP) (Banuri et al., 1996, p. 96).[94] Calculations based on MER show large differences in intensities between developed and developing countries, whereas calculations based on PPP show smaller differences.

Cumulative and historical emissions

Cumulative energy-related CO
2 emissions between the years 1850–2005 grouped into low-income, middle-income, high-income, the EU-15, and the OECD countries.
Cumulative energy-related CO
2 emissions between the years 1850–2005 for individual countries.
Map of cumulative per capita anthropogenic atmospheric CO
2 emissions by country. Cumulative emissions include land use change, and are measured between the years 1950 and 2000.
Regional trends in annual CO
2 emissions from fuel combustion between 1971 and 2009.
Regional trends in annual per capita CO
2 emissions from fuel combustion between 1971 and 2009.
Cumulative anthropogenic (i.e., human-emitted) emissions of CO
2 from fossil fuel use are a major cause of global warming,[100] and give some indication of which countries have contributed most to human-induced climate change.[101]:15
Top-5 historic CO
2 contributors by region over the years 1800 to 1988 (in %)
Region Industrial
CO
2		 Total
CO
2
OECD North America 33.2 29.7
OECD Europe 26.1 16.6
Former USSR 14.1 12.5
China   5.5   6.0
Eastern Europe   5.5   4.8

The table above to the left is based on Banuri et al. (1996, p. 94).[94] Overall, developed countries accounted for 83.8% of industrial CO
2 emissions over this time period, and 67.8% of total CO
2 emissions. Developing countries accounted for industrial CO
2 emissions of 16.2% over this time period, and 32.2% of total CO
2 emissions. The estimate of total CO
2 emissions includes biotic carbon emissions, mainly from deforestation. Banuri et al. (1996, p. 94)[94] calculated per capita cumulative emissions based on then-current population. The ratio in per capita emissions between industrialized countries and developing countries was estimated at more than 10 to 1.

Including biotic emissions brings about the same controversy mentioned earlier regarding carbon sinks and land-use change (Banuri et al., 1996, pp. 93–94).[94] The actual calculation of net emissions is very complex, and is affected by how carbon sinks are allocated between regions and the dynamics of the climate system.
Non-OECD countries accounted for 42% of cumulative energy-related CO
2 emissions between 1890–2007.[102]:179–180 Over this time period, the US accounted for 28% of emissions; the EU, 23%; Russia, 11%; China, 9%; other OECD countries, 5%; Japan, 4%; India, 3%; and the rest of the world, 18%.[102]:179–180

Changes since a particular base year

Between 1970–2004, global growth in annual CO2 emissions was driven by North America, Asia, and the Middle East.[103] The sharp acceleration in CO2 emissions since 2000 to more than a 3% increase per year (more than 2 ppm per year) from 1.1% per year during the 1990s is attributable to the lapse of formerly declining trends in carbon intensity of both developing and developed nations. 
China was responsible for most of global growth in emissions during this period. Localised plummeting emissions associated with the collapse of the Soviet Union have been followed by slow emissions growth in this region due to more efficient energy use, made necessary by the increasing proportion of it that is exported.[77] In comparison, methane has not increased appreciably, and N
2O by 0.25% y−1.

Using different base years for measuring emissions has an effect on estimates of national contributions to global warming.[101]:17–18[104] This can be calculated by dividing a country's highest contribution to global warming starting from a particular base year, by that country's minimum contribution to global warming starting from a particular base year. Choosing between different base years of 1750, 1900, 1950, and 1990 has a significant effect for most countries.[101]:17–18 Within the G8 group of countries, it is most significant for the UK, France and Germany. These countries have a long history of CO
2 emissions (see the section on Cumulative and historical emissions).

Annual emissions


Per capita anthropogenic greenhouse gas emissions by country for the year 2000 including land-use change.

Annual per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries.[79]:144 Due to China's fast economic development, its annual per capita emissions are quickly approaching the levels of those in the Annex I group of the Kyoto Protocol (i.e., the developed countries excluding the USA).[105] Other countries with fast growing emissions are South Korea, Iran, and Australia. On the other hand, annual per capita emissions of the EU-15 and the USA are gradually decreasing over time.[105] Emissions in Russia and the Ukraine have decreased fastest since 1990 due to economic restructuring in these countries.[106]

Energy statistics for fast growing economies are less accurate than those for the industrialized countries. For China's annual emissions in 2008, the Netherlands Environmental Assessment Agency estimated an uncertainty range of about 10%.[105]

The GHG footprint, or greenhouse gas footprint, refers to the amount of GHG that are emitted during the creation of products or services. It is more comprehensive than the commonly used carbon footprint, which measures only carbon dioxide, one of many greenhouse gases.

Top emitter countries

Bar graph of annual per capita CO
2 emissions from fuel combustion for 140 countries in 2009.
Bar graph of cumulative energy-related per capita CO
2 emissions between 1850–2008 for 185 countries.
See also: List of countries by carbon dioxide emissions, List of countries by carbon dioxide emissions per capita, List of countries by greenhouse gas emissions and List of countries by greenhouse gas emissions per capita

Annual

In 2009, the annual top ten emitting countries accounted for about two-thirds of the world's annual energy-related CO
2 emissions.[107]
Top-10 annual energy-related CO
2 emitters for the year 2009[108]
Country  % of global total
annual emissions		 Tonnes of GHG
per capita
People's Rep. of China 23.6 5.13
United States 17.9 16.9
India 5.5 1.37
Russian Federation 5.3 10.8
Japan 3.8 8.6
Germany 2.6 9.2
Islamic Rep. of Iran 1.8 7.3
Canada 1.8 15.4
Korea 1.8 10.6
United Kingdom 1.6 7.5

Cumulative

Top-10 cumulative energy-related CO
2 emitters between 1850–2008[109]
Country  % of world
total		 Metric tonnes
CO
2 per person
United States 28.5 1,132.7
China 9.36 85.4
Russian Federation 7.95 677.2
Germany 6.78 998.9
United Kingdom 5.73 1,127.8
Japan 3.88 367
France 2.73 514.9
India 2.52 26.7
Canada 2.17 789.2
Ukraine 2.13 556.4

Embedded emissions

One way of attributing greenhouse gas (GHG) emissions is to measure the embedded emissions (also referred to as "embodied emissions") of goods that are being consumed. Emissions are usually measured according to production, rather than consumption.[110] For example, in the main international treaty on climate change (the UNFCCC), countries report on emissions produced within their borders, e.g., the emissions produced from burning fossil fuels.[102]:179[111]:1 Under a production-based accounting of emissions, embedded emissions on imported goods are attributed to the exporting, rather than the importing, country. Under a consumption-based accounting of emissions, embedded emissions on imported goods are attributed to the importing country, rather than the exporting, country.

Davis and Caldeira (2010)[111]:4 found that a substantial proportion of CO
2 emissions are traded internationally. The net effect of trade was to export emissions from China and other emerging markets to consumers in the US, Japan, and Western Europe. Based on annual emissions data from the year 2004, and on a per-capita consumption basis, the top-5 emitting countries were found to be (in tCO
2 per person, per year): Luxembourg (34.7), the US (22.0), Singapore (20.2), Australia (16.7), and Canada (16.6).[111]:5 Carbon Trust research revealed that approximately 25% of all CO
2 emissions from human activities 'flow' (i.e. are imported or exported) from one country to another. Major developed economies were found to be typically net importers of embodied carbon emissions — with UK consumption emissions 34% higher than production emissions, and Germany (29%), Japan (19%) and the USA (13%) also significant net importers of embodied emissions.[112]

Effect of policy

Governments have taken action to reduce GHG emissions (climate change mitigation). Assessments of policy effectiveness have included work by the Intergovernmental Panel on Climate Change,[113] International Energy Agency,[114][115] and United Nations Environment Programme.[116] Policies implemented by governments have included[117][118][119] national and regional targets to reduce emissions, promoting energy efficiency, and support for renewable energy.
Countries and regions listed in Annex I of the United Nations Framework Convention on Climate Change (UNFCCC) (i.e., the OECD and former planned economies of the Soviet Union) are required to submit periodic assessments to the UNFCCC of actions they are taking to address climate change.[119]:3 Analysis by the UNFCCC (2011)[119]:8 suggested that policies and measures undertaken by Annex I Parties may have produced emission savings of 1.5 thousand Tg CO
2-eq in the year 2010, with most savings made in the energy sector. The projected emissions saving of 1.5 thousand Tg CO
2-eq is measured against a hypothetical "baseline" of Annex I emissions, i.e., projected Annex I emissions in the absence of policies and measures. The total projected Annex I saving of 1.5 thousand CO
2-eq does not include emissions savings in seven of the Annex I Parties.[119]:8

Projections

A wide range of projections of future GHG emissions have been produced.[120] Rogner et al. (2007)[121] assessed the scientific literature on GHG projections. Rogner et al. (2007)[88] concluded that unless energy policies changed substantially, the world would continue to depend on fossil fuels until 2025–2030. Projections suggest that more than 80% of the world's energy will come from fossil fuels. This conclusion was based on "much evidence" and "high agreement" in the literature.[88] Projected annual energy-related CO2 emissions in 2030 were 40–110% higher than in 2000, with two-thirds of the increase originating in developing countries.[88] Projected annual per capita emissions in developed country regions remained substantially lower (2.8–5.1 tonnes CO2) than those in developed country regions (9.6–15.1 tonnes CO2).[122] Projections consistently showed increase in annual world GHG emissions (the "Kyoto" gases,[123] measured in CO2-equivalent) of 25–90% by 2030, compared to 2000.[88]

Relative CO
2 emission from various fuels

One liter of gasoline, when used as a fuel, produces 2.32 kg (about 1300 liters or 1.3 cubic meters) of carbon dioxide, a greenhouse gas. One US gallon produces 19.4 lb (1,291.5 gallons or 172.65 cubic feet)[124][125][126]
Mass of carbon dioxide emitted per quantity of energy for various fuels[127]
Fuel name CO
2
emitted
(lbs/106 Btu)		 CO
2
emitted
(g/MJ)
Natural gas 117 50.30
Liquefied petroleum gas 139 59.76
Propane 139 59.76
Aviation gasoline 153 65.78
Automobile gasoline 156 67.07
Kerosene 159 68.36
Fuel oil 161 69.22
Tires/tire derived fuel 189 81.26
Wood and wood waste 195 83.83
Coal (bituminous) 205 88.13
Coal (sub-bituminous) 213 91.57
Coal (lignite) 215 92.43
Petroleum coke 225 96.73
Tar-sand Bitumen [citation needed] [citation needed]
Coal (anthracite) 227 97.59

Life-cycle greenhouse-gas emissions of energy sources

A literature review of numerous energy sources CO
2 emissions by the IPCC in 2011, found that, the CO
2 emission value, that fell within the 50th percentile of all total life cycle emissions studies conducted, was as follows.[128]
Lifecycle greenhouse gas emissions by electricity source.
Technology Description 50th percentile
(g CO
2/kWhe)
Hydroelectric reservoir 4
Ocean Energy wave and tidal 8
Wind onshore 12
Nuclear various generation II reactor types 16
Biomass various 18
Solar thermal parabolic trough 22
Geothermal hot dry rock 45
Solar PV Polycrystaline silicon 46
Natural gas various combined cycle turbines without scrubbing 469
Coal various generator types without scrubbing 1001

Removal from the atmosphere ("sinks")

Natural processes

Greenhouse gases can be removed from the atmosphere by various processes, as a consequence of:
  • a physical change (condensation and precipitation remove water vapor from the atmosphere).
  • a chemical reaction within the atmosphere. For example, methane is oxidized by reaction with naturally occurring hydroxyl radical, OH· and degraded to CO
    2     and water vapor (CO
    2     from the oxidation of methane is not included in the methane Global warming potential). Other chemical reactions include solution and solid phase chemistry occurring in atmospheric aerosols.
  • a physical exchange between the atmosphere and the other compartments of the planet. An example is the mixing of atmospheric gases into the oceans.
  • a chemical change at the interface between the atmosphere and the other compartments of the planet. This is the case for CO
    2    , which is reduced by photosynthesis of plants, and which, after dissolving in the oceans, reacts to form carbonic acid and bicarbonate and carbonate ions (see ocean acidification).
  • a photochemical change. Halocarbons are dissociated by UV light releasing Cl· and F· as free radicals in the stratosphere with harmful effects on ozone (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).

Negative emissions

A number of technologies remove greenhouse gases emissions from the atmosphere. Most widely analysed are those that remove carbon dioxide from the atmosphere, either to geologic formations such as bio-energy with carbon capture and storage[129][130][131] and carbon dioxide air capture,[131] or to the soil as in the case with biochar.[131] The IPCC has pointed out that many long-term climate scenario models require large scale manmade negative emissions to avoid serious climate change.[132]

History of scientific research

In the late 19th century scientists experimentally discovered that N
2 and O
2 do not absorb infrared radiation (called, at that time, "dark radiation"). On the contrary, water (both as true vapor and condensed in the form of microscopic droplets suspended in clouds) and CO
2 and other poly-atomic gaseous molecules do absorb infrared radiation. In the early 20th century researchers realized that greenhouse gases in the atmosphere made the Earth's overall temperature higher than it would be without them. During the late 20th century, a scientific consensus evolved that increasing concentrations of greenhouse gases in the atmosphere cause a substantial rise in global temperatures and changes to other parts of the climate system,[133] with consequences for the environment and for human health.

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