The airborne fraction is a scaling factor defined as the ratio of the annual increase in atmospheric CO
2 to the CO
2 emissions from human sources. [1] It represents the proportion of human emitted CO2 that remains in the atmosphere. Observations over the past six decades show that the airborne fraction has remained relatively stable at around 45%. [2] This indicates that the land and ocean's capacity to absorb CO2 has kept up with the rise in human CO2 emissions, despite the occurrence of notable interannual and sub-decadal variability, which is predominantly driven by the land's ability to absorb CO2. There is some evidence for a recent increase in airborne fraction, which would imply a faster increase in atmospheric CO
2 for a given rate of human fossil-fuel burning. [3] Changes in carbon sinks can affect the airborne fraction as well.
Anthropogenic CO2 that is released into the atmosphere is partitioned into three components: approximately 45% remains in the atmosphere (referred to as the airborne fraction), while about 24% and 31% are absorbed by the oceans (ocean sink) and terrestrial biosphere (land sink), respectively. [4] If the airborne fraction increases, this indicates that a smaller amount of the CO2 released by humans is being absorbed by land and ocean sinks, due to factors such as warming oceans or thawing permafrost. As a result, a greater proportion of anthropogenic emissions remains in the atmosphere, thereby accelerating the rate of climate change. This has implications for future projections of atmospheric CO2 levels, which must be adjusted to account for this trend. [5] The question of whether the airborne fraction is rising, remaining steady at approximately 45%, or declining remains a matter of debate. Resolving this question is critical for comprehending the global carbon cycle and has relevance for policymakers and the general public.
The quantity “airborne fraction” is termed by Charles David Keeling in 1973, and studies conducted in the 1970s and 1980s defined airborne fraction from cumulative carbon inventory changes as, [5]
Or,
In which C is atmospheric carbon dioxide, t is time, FF is fossil-fuel emissions and LU is the emission to the atmosphere due to land use change.
At present, studies examining the trends in airborne fraction are producing contradictory outcomes, with emissions linked to land use and land cover change representing the most significant source of uncertainty. Some studies show that there is no statistical evidence of an increasing airborne fraction and calculated airborne fraction as, [6]
Where Gt is growth of atmospheric CO2 concentration, EFF is the fossil-fuel emissions flux, ELUC is the land use change emissions flux.
Another argument was presented that the airborne fraction of CO2 released by human activities, particularly through fossil-fuel emissions, cement production, and land-use changes, is on the rise. [7] Since 1959, the average CO2 airborne fraction has been 0.43, but it has shown an increase of approximately 0.2% per year over that period. [3]
On the other hand, the findings of another group suggest that the CO2 airborne fraction has declined by 0.014 ± 0.010 per decade since 1959. [8] This indicates that the combined land-ocean sink has expanded at a rate that is at least as rapid as anthropogenic emissions. The way they calculated the airborne fraction is:
Where, AF is airborne fraction and SF is sink fraction. ELULCC is the land use and land cover change emissions flux, EFF is the fossil-fuel emissions flux, and SO and SL are the ocean and land sinks, respectively.
The trend analyses of airborne fraction may be affected by external natural occurrences, such as the El Niño-Southern Oscillation (ENSO), volcanic eruptions, and other similar events. [9] It is possible that the methodologies used in these studies to analyze the trend of airborne fraction are not robust, and therefore, the conclusions drawn from them are not warranted.
The scientific community has been investigating the causes of climate change for decades. After thousands of studies, it came to a consensus, where it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." This consensus is supported by around 200 scientific organizations worldwide, The dominant role in this climate change has been played by the direct emissions of carbon dioxide from the burning of fossil fuels. Indirect CO2 emissions from land use change, and the emissions of methane, nitrous oxide and other greenhouse gases play major supporting roles.
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Trace gases are gases that are present in small amounts within an environment such as a planet's atmosphere. Trace gases in Earth's atmosphere are gases other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of its atmosphere.
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In oceanic biogeochemistry, the continental shelf pump is proposed to operate in the shallow waters of the continental shelves, acting as a mechanism to transport carbon from surface waters to the interior of the adjacent deep ocean.
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The Global Ocean Data Analysis Project (GLODAP) is a synthesis project bringing together oceanographic data, featuring two major releases as of 2018. The central goal of GLODAP is to generate a global climatology of the World Ocean's carbon cycle for use in studies of both its natural and anthropogenically forced states. GLODAP is funded by the National Oceanic and Atmospheric Administration, the U.S. Department of Energy, and the National Science Foundation.
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The Global Carbon Project (GCP) is an organisation that seeks to quantify global greenhouse gas emissions and their causes. Established in 2001, its projects include global budgets for three dominant greenhouse gases—carbon dioxide, methane, and nitrous oxide —and complementary efforts in urban, regional, cumulative, and negative emissions.
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Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).
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Climate change feedbacks are natural processes which impact how much global temperatures will increase for a given amount of greenhouse gas emissions. Positive feedbacks amplify global warming while negative feedbacks diminish it. Feedbacks influence both the amount of greenhouse gases in the atmosphere and the amount of temperature change that happens in response. While emissions are the forcing that causes climate change, feedbacks combine to control climate sensitivity to that forcing.
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The oceanic carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon and organic carbon. Part of the marine carbon cycle transforms carbon between non-living and living matter.
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