Carbon dioxide in the atmosphere of Earth

In the atmosphere of Earth, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis, and oceanic carbon cycle. It is one of three main greenhouse gases in the atmosphere of Earth. The concentration of carbon dioxide (CO₂) in the atmosphere reached 427 ppm (0.0427%) on a molar basis in 2024, representing 3341 gigatonnes of CO₂. This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century. The increase is due to human activity.

The current increase in CO₂ concentrations is primarily driven by the burning of fossil fuels. Other significant human activities that emit CO₂ include cement production, deforestation, and biomass burning. The increase in atmospheric concentrations of CO₂ and other long-lived greenhouse gases such as methane increase the absorption and emission of infrared radiation by the atmosphere. This has led to a rise in average global temperature and ocean acidification. Another direct effect is the CO₂ fertilization effect. The increase in atmospheric concentrations of CO₂ causes a range of further effects of climate change on the environment and human living conditions.

Carbon dioxide is a greenhouse gas. It absorbs and emits infrared radiation at its two infrared-active vibrational frequencies. The two wavelengths are 4.26 μm (2,347 cm⁻¹) (antisymmetric stretching vibrational mode) and 14.99 μm (667 cm⁻¹) (bending vibrational mode). CO₂ plays a significant role in influencing Earth's surface temperature through the greenhouse effect. Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm⁻¹, as opposed to light emission from the much hotter Sun which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric CO₂ traps energy near the surface, warming the surface of Earth and its lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.

The present atmospheric concentration of CO₂ is the highest for 14 million years. Concentrations of CO₂ in the atmosphere were as high as 4,000 ppm during the Cambrian period about 500 million years ago, and 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 approximately 2,000 ppm. This peak happened during the Devonian period (400 million years ago). Another peak occurred in the Triassic period (220–200 million years ago).

Current concentration and future trends

Current situation

Since the start of the Industrial Revolution, atmospheric CO₂ concentration has been increasing, causing global warming and ocean acidification. In October 2023 the average level of CO₂ in Earth's atmosphere, adjusted for seasonal variation, was 422.17 parts per million by volume (ppm). Figures are published monthly by the National Oceanic & Atmospheric Administration (NOAA). The value had been about 280 ppm during the 10,000 years up to the mid-18th century.

Each part per million of CO₂ in the atmosphere represents approximately 2.13 gigatonnes of carbon, or 7.82 gigatonnes of CO₂.

It was pointed out in 2021 that "the current rates of increase of the concentration of the major greenhouse gases (carbon dioxide, methane and nitrous oxide) are unprecedented over at least the last 800,000 years".^: 515

As of 2024,^[update] it is estimated that 2,650 gigatonnes of CO₂ have been emitted by human activity since 1850, with annual emissions of 42 gigatonnes per year. About 1,050 gigatonnes remain in the atmosphere following absorption by oceans and land.

Some fraction (a projected 20–35%) of the fossil carbon transferred thus far will persist in the atmosphere as elevated CO₂ levels for many thousands of years after these carbon transfer activities begin to subside.

Annual and regional fluctuations

Atmospheric CO₂ concentrations fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay. 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 in mid-latitudes (30-60 degrees) 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.

Concentrations also vary on a regional basis, most strongly near the ground with much smaller variations aloft. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels.

Measurements and predictions made in the recent past

Data from 2009 found that the global mean CO₂ concentration was rising at a rate of approximately 2 ppm/year and accelerating.
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. Data from 2013 showed 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."
As of 2018, CO₂ concentrations were measured to be 410 ppm.

Measurement techniques

The concentrations of carbon dioxide in the atmosphere are expressed as parts per million by volume (abbreviated as ppmv, or ppm(v), or just ppm). To convert from the usual ppmv units to ppm mass (abbreviated as ppmm, or ppm(m)), multiply by the ratio of the molar mass of CO₂ to that of air, i.e. times 1.52 (44.01 divided by 28.96).

The first reproducibly accurate measurements of atmospheric CO₂ were from flask sample measurements made by Dave Keeling at Caltech in the 1950s. Measurements at Mauna Loa have been ongoing since 1958. Additionally, measurements are also made at many other sites around the world. Many measurement sites are part of larger global networks. Global network data are often made publicly available.

Data networks

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.

Analytical methods to investigate sources of CO₂

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.
There are 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.

Causes of the current increase

Anthropogenic CO₂ emissions

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. Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks. Thus carbon dioxide has gradually accumulated in the atmosphere and, as of May 2022, its concentration is 50% above pre-industrial levels.

The extraction and burning of fossil fuels, releasing carbon that has been underground for many millions of years, has increased the atmospheric concentration of CO₂. 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. 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.

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 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.

The International Energy Agency estimates that the top 1% of emitters globally each had carbon footprints of over 50 tonnes of CO₂ in 2021, more than 1,000 times greater than those of the bottom 1% of emitters. The global average energy-related carbon footprint is around 4.7 tonnes of CO₂ per person.

Roles in natural processes on Earth

Greenhouse effect

On Earth, carbon dioxide is the most relevant, direct greenhouse gas that is influenced by human activities. Water is responsible for most (about 36–70%) of the total greenhouse effect, and the role of water vapor as a greenhouse gas depends on temperature. Carbon dioxide is often mentioned in the context of its increased influence as a greenhouse gas since the pre-industrial (1750) era. In 2013, 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%).

Earth's natural greenhouse effect makes life as we know it possible, and carbon dioxide in the atmosphere plays a significant role in providing for the relatively high temperature on Earth. 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.

The concept of more 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.

Carbon cycle

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, as well as some industrial processes such as cement making.

Natural sources of CO₂ are more or less balanced by natural carbon sinks, in the form of chemical and biological processes which remove CO₂ from the atmosphere. 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.

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.

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:

f(t)=0.217+0.259\exp(-t/172.9)+0.338\exp(-t/18,51)+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).

Oceanic carbon cycle

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.

From 1850 until 2022, the ocean has absorbed 26% of total anthropogenic emissions. 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.

A study published in Science Advances in 2025 concluded that faster flow of the Antarctic Circumpolar Current (ACC) at higher latitudes causes upwelling of isotopically light deep waters around Antarctica, likely increasing atmospheric carbon dioxide levels and thereby potentially constituting a critical positive feedback for future warming.

Effects of current increase

Direct effects

Direct effects of increasing CO₂ concentrations in the atmosphere include increasing global temperatures, ocean acidification and a CO₂ fertilization effect on plants and crops.

Temperature rise on land

Changes in global temperatures over the past century provide evidence for the effects of increasing greenhouse gases. When the climate system reacts to such changes, climate change follows. Measurement of the GST is one of the many lines of evidence supporting the scientific consensus on climate change, which is that humans are causing warming of Earth's climate system.

The global average and combined land and ocean surface temperature, show a warming of 1.09 °C (range: 0.95 to 1.20 °C) from 1850–1900 to 2011–2020, based on multiple independently produced datasets.^: 5 The trend is faster since the 1970s than in any other 50-year period over at least the last 2000 years.^: 8

Temperature rise in oceans

It is clear that the ocean is warming as a result of climate change, and this rate of warming is increasing.^: 9 The global ocean was the warmest it had ever been recorded by humans in 2022. This is determined by the ocean heat content, which exceeded the previous 2021 maximum in 2022. The steady rise in ocean temperatures is an unavoidable result of the Earth's energy imbalance, which is primarily caused by rising levels of greenhouse gases. Between pre-industrial times and the 2011–2020 decade, the ocean's surface has heated between 0.68 and 1.01 °C.^: 1214

The majority of ocean heat gain occurs in the Southern Ocean. For example, between the 1950s and the 1980s, the temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F), nearly twice the rate of the global ocean.

Ocean acidification

Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO₂) levels exceeding 422 ppm (as of 2024^[update]). CO₂ from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid (H₂CO₃) which dissociates into a bicarbonate ion (HCO−3) and a hydrogen ion (H⁺). The presence of free hydrogen ions (H⁺) lowers the pH of the ocean, increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.

A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more CO₂. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. There are several other factors that influence the atmosphere-ocean CO₂ exchange, and thus local ocean acidification. These include ocean currents and upwelling zones, proximity to large continental rivers, sea ice coverage, and atmospheric exchange with nitrogen and sulfur from fossil fuel burning and agriculture.

CO₂ fertilization effect

The CO₂ fertilization effect or carbon fertilization effect causes an increased rate of photosynthesis while limiting leaf transpiration in plants. Both processes result from increased levels of atmospheric carbon dioxide (CO₂). The carbon fertilization effect varies depending on plant species, air and soil temperature, and availability of water and nutrients. Net primary productivity (NPP) might positively respond to the carbon fertilization effect, although evidence shows that enhanced rates of photosynthesis in plants due to CO₂ fertilization do not directly enhance all plant growth, and thus carbon storage. The carbon fertilization effect has been reported to be the cause of 44% of gross primary productivity (GPP) increase since the 2000s. Earth System Models, Land System Models and Dynamic Global Vegetation Models are used to investigate and interpret vegetation trends related to increasing levels of atmospheric CO₂. However, the ecosystem processes associated with the CO₂ fertilization effect remain uncertain and therefore are challenging to model.

Terrestrial ecosystems have reduced atmospheric CO₂ concentrations and have partially mitigated climate change effects. The response by plants to the carbon fertilization effect is unlikely to significantly reduce atmospheric CO₂ concentration over the next century due to the increasing anthropogenic influences on atmospheric CO₂. Earth's vegetated lands have shown significant greening since the early 1980s largely due to rising levels of atmospheric CO₂.

Theory predicts the tropics to have the largest uptake due to the carbon fertilization effect, but this has not been observed. The amount of CO₂ uptake from CO₂ fertilization also depends on how forests respond to climate change, and if they are protected from deforestation.

Other direct effects

CO₂ emissions have also led to the stratosphere contracting by 400 meters since 1980, which could affect satellite operations, GPS systems and radio communications.