Carbon Dioxide: The No 1. Greenhouse Gas

Carbon dioxide (CO2) is the most predominant and longest living greenhouse gas in the atmosphere. We explain its global warming potential, its main sources, its impact on the greenhouse effect and climate change, and its movement through the fast and slow carbon cycles. We also compare current levels of atmospheric CO2 with historical levels.
Coal Fired Power Station
Coal power plant in Datteln (Germany). Photo: © Arnold Paul (CC BY-3.0)

Carbon dioxide (CO2) is the most damaging of all the greenhouse gases in the atmosphere, due to its abundance and long active life, and its impact on global warming is profound. Man-made CO2 emissions derive mainly from fossil fuels, but land use and cement production are also sources. The reduction of CO2 levels, which are unprecedented in hundreds of thousands of years, is absolutely essential if we are to solve our climate crisis and rescue Planet Earth from irreversible damage.

Carbon dioxide (chemical formula CO2) is a colorless gas which is present in Earth’s atmosphere in tiny quantities (about 0.04 percent) compared to nitrogen (78 percent) and oxygen (21 percent). Despite its relative scarcity, it is the most versatile and important source of available carbon, which makes it the primary carbon source for life on Earth. For example, it plays a major role in the carbon cycle, notably in photosynthesis, the energy-producing mechanism essential for all plant life. 1 As noted, carbon dioxide is also a key contributor to the greenhouse effect, which prevents some of the planet’s radiant energy from being returned to space, thus keeping Earth’s temperature at a cosy 15 °C (59 °F), instead of the minus 18°C (0°F) it would otherwise be. CO2 has performed this warming role for millions of years, as part of Earth’s overall climate system – a system designed to maintain a stable global temperature.

Carbon dioxide is produced when any type of carbon, or almost any carbon compound, is burned in the presence of oxygen. So ever since the Industrial Revolution (c.1750-1850), when humans first started to burn large amounts of fossil fuels, like coal, oil and natural gas, huge amounts of CO2 have been released into the atmosphere from engines, boilers, factory chimneys, fireplaces and the like, until it reached the point when the climate system was unable to cope, and climate change began. For more on this topic, see: When Did Global Warming Start?

What Is The Role Of CO2 In The Carbon Cycle?

The carbon cycle is a continuous process which takes place in the air (troposphere), the rivers and oceans (hydrosphere), the soil (pedosphere), and the rocks (lithosphere). During this process, carbon atoms move around the Earth, being used and re-used in countless chemical reactions (like photosynthesis) that help to sustain life. It actually consists of two cycles – a fast one and a slow one – which combine to regulate the distribution of carbon so that the biosphere (including its temperature) is kept stable. CO2 is the most common form of carbon in both cycles. 

How Does The Fast Carbon Cycle Work?

In the fast cycle, CO2 moves from the atmosphere to plants and back again. If it is washed away into rivers and the ocean, it may diffuse back up into the air from the surface of the ocean. All this happens within a relatively short time scale, usually no more than 10 years.

How Is CO2 Used By Plants? 

The foundation of the fast cycle is photosynthesis. During this metabolic process, plants combine water (from their roots), carbon dioxide (from the air) and light energy (from the sun) to create fuel (glucose and other sugars) in order to grow and develop. Without photosynthesis (that is, without CO2 and sunlight), plants could not survive, and without plants, the earth’s biological food chain would collapse. So photosynthesis is utterly essential for life. See also: Is more CO2 good for plants?

The Photosynthesis Formula

Carbon dioxide + water + sunlight -> carbohydrate + oxygen
CO2 + H2O + sunlight -> CH2O + O2  

How Do Plants Release CO2 Back Into The Air?

The carbon dioxide used by plants during photosynthesis is released back into the atmosphere in one of three ways: (1) By respiration. When plants breathe (respire) they exhale CO2. (2) By decomposition. When plants die, tiny organisms called decomposers break down their remains, releasing CO2 (or methane) in the process. (3) By transfer. If a plant is eaten, the CO2 inside it moves up the food chain and is released into the atmosphere as described in (1) or (2). 

How Does CO2 Move From The Ocean Into The Air?

Phytoplankton in the marine Carbon Cycle
How phytoplankton operate within the carbon cycle in the hydrosphere. Source: IAEA

Part of the CO2 that is washed into the ocean is photosynthesized by tiny marine plants called phytoplankton, who then respire some of it back into the atmosphere. Any carbon which is left inside the body parts or other residues of phytoplankton, usually falls into the ocean depths from which it is unlikely to return for hundreds if not thousands of years. This is where the fast cycle connects to the slow cycle.

The rest of the CO2 that reaches the ocean dissolves into the surface water, and later diffuses into the air as part of the continuous gaseous exchange between atmosphere and ocean. This exchange of CO2 between the air and seawater is governed by the differences in concentration – which is itself regulated by temperature. Warmer water can hold more CO2, and so is likely to release CO2 into the air. Conversely, colder waters tend to absorb CO2 from the air.

How Does The Slow Carbon Cycle Work?

The slow carbon cycle typically unfolds over hundreds of thousands if not millions of years. During this cycle carbon dioxide is put into long-term storage in the lithosphere, where it remains for geological time periods, before (eventually) being released into the atmosphere.

How Does CO2 Cause Chemical Weathering?

If rainfall containing dissolved carbon dioxide lands on exposed rock (especially silicates or limestone), it causes microscopic chemical weathering of the rock surface (producing calcium and bicarbonate in the process), before being carried away by rainfall run-off to the ocean. Here, marine organisms use the calcium and bicarbonate to build shells and skeletons, releasing half the CO2 back into the atmosphere. The rest of the CO2 sinks to the ocean floor in the calcified residues of the marine organisms.

After millions of years on the ocean floor, chemical and other processes turn these crushed residues into sedimentary rock. Over the course of another 10 million years or more, geological forces melt it and expel it into the atmosphere through volcanic activity. (Note: On average, volcanoes spew out between 130 and 380 million tons of carbon dioxide into the atmosphere, each year.)

What Happens To CO2 In The Soil?

If CO2 remains in the soil – perhaps after a plant or plant-eating animal dies – it may not be broken down by decomposers, but simply absorbed into the soil where, after thousands, if not millions, of years of compaction and cementation, it is transformed into sedimentary rock. (You know the rest.) 

Generally speaking, carbon dioxide takes between 100-200 million years to move between atmosphere, soil, ocean, and rocks in the slow carbon cycle.

Are Fossil Fuels Part Of The Slow Carbon Cycle?

Yes. For instance, coal originated in huge primeval swamps and wetlands, in the form of partly decayed trees and other plants. By contrast, petroleum and also natural gas began life as partly decayed plankton and other organic residues on the bottom of the sea. As these ancient carbon-rich, decaying materials sank deeper into the earth, more sediment accumulated above them, subjecting them (over millions of years) to immense heat and crushing pressure, that transformed them into fossil fuels.

If humans had not taken them out of the ground, they would still be there – just like the underground deposits of limestone and other carbon-rich rocks that lie undisturbed around the globe.

Unfortunately, over the last two centuries or more, humans have exhumed these fossil fuels and burned them in power plants and vehicle engines. So instead of quietly waiting underground for another 20 million years, before being released into the atmosphere, this mass of carbon dioxide has been prematurely evicted from the slow carbon cycle and discharged into the atmosphere. See also: Which is the Largest Carbon Reservoir?

Does Renewable Energy Emit Carbon Dioxide?
Yes. But it’s only a fraction of the CO2 emitted by fossil fuels. The cleanest and most sustainable renewable sources, include: solar power, organic biomass and biofuels, geothermal energy, offshore and onshore wind energy, as well as wave power and tidal power. Hydropower releases more greenhouse gases but significantly less than natural gas, and it’s the most reliable source of renewable energy.

How Does CO2 Cause The Greenhouse Effect?

The greenhouse effect is a naturally occurring mechanism during which some of the infrared heat energy given off by the sun-warmed Earth is absorbed by certain gases (mostly water vapor and carbon dioxide) in the lower atmosphere. 2 This infrared heat, that would otherwise have escaped into space, is then re-radiated downwards to warm the surface of the planet.

NOTE: The reason why CO2 can absorb this infrared energy while nitrogen and oxygen can’t, is because CO2 molecules can vibrate in ways that simpler nitrogen and oxygen molecules can’t. This property allows CO2 molecules to absorb and then re-radiate the energy of the infrared particles at wavelengths of between 4.26 microns (µm) and 14.99 µm.

What produces the most carbon dioxide?
Burning coal produces more CO2 emissions than any other fossil fuel. What’s more, because it was the first fossil fuel to be used commercially in modern times, and because it was found throughout the world, coal has become the leading source of greenhouse gas emissions.

As of 2019, an estimated 2,425 coal-fired power plants in the world, emit approximately 15 billion tonnes of carbon dioxide. Fortunately for global warming and the environment, global electricity production from coal is set to fall by about 3 percent in 2019, the largest drop on record. [“Analysis: Global coal power set for record fall in 2019.” CarbonBrief] In the EU coal power output could fall by up to 23 percent during 2019.

After fossil fuel burning, the next biggest anthropogenic source of CO2 emissions is the cement industry. For details, see: See: Cement Emissions are Bad For Climate Change.

Greenhouse Effect Unbalanced By Man-Made Emissions

For thousands of years, the amount of greenhouse gas in the atmosphere was just enough to maintain a stable, cosy temperature. Unfortunately, the ever-increasing use of fossil fuels since the Industrial Revolution has led to massive extra amounts of CO2 (and other GHGs) being emitted into the atmosphere, giving an unnaturally large boost to the greenhouse effect and unbalancing the climate system in the process. The result has been a gradually accelerating rise in global temperature.

This anthropogenic rise in greenhouse gas emissions mostly involves CO2 but others include Methane (CH4) Nitrous Oxide (N20), and the families of Chlorofluorocarbons (CFCs) and Hydrofluorocarbons (HCFCs and HFCs) and other fluorinated gases.

Greenhouse Gas Emissions By Type

Carbon dioxide 82%
Nitrous Oxide6%
HCFCs and HFCs3%

As you can see, carbon dioxide is far and away the most abundant GHG. 3 See also: Greenhouse Gas Statistics Lack Consistency.

Read our article: How To Reduce Your Carbon Footprint.

Isn’t Water Vapor An Important Greenhouse Gas?

Yes. But it is much less critical for two reasons. First, it is natural rather than man-made and has been a greenhouse gas for thousands, if not millions, of years without having any effect on global warming. Second, although water is responsible for about 36-70 percent of the greenhouse effect, its role as a greenhouse gas largely depends on temperature. (The warmer the air, the more water vapor it holds.) Since carbon dioxide is the most abundant GHG created by humans, it is this gas that is the driving influence on global warming.

In fact, the IPCC’s Fifth Assessment Report concluded that the increase in CO2 was responsible for about 70 percent of the climate forcing on Earth. 4 Nonetheless, water vapor remains one of the most important climate feedbacks because of its ability to amplify CO2’s warming effect. (See also: What is the Water Cycle?)

Other Types Of Man-Made Emissions Of CO2

In addition to creating CO2 emissions from fossil fuel combustion and numerous industrial processes, humans are also responsible for various forms of environmentally damaging land use, which add to the growing amount of CO2 in the atmosphere. Deforestation, for instance – the deliberate cutting down and burning of trees to create extra land for livestock grazing or crops – accounts for a growing percentage of global greenhouse gas emissions. 5 6

Is CO2 The Most Powerful Greenhouse Gas?

No. The impact of a greenhouse gas on global warming depends on three things: its abundance in the atmosphere, its active lifespan and its capacity to trap and re-radiate infrared energy. Based on these criteria, each gas is given a Global Warming Potential (GWP) rating, which is a measure of how much energy a particular greenhouse gas will absorb, compared to carbon dioxide.

100-Year GWP Ratings 7

Carbon Dioxide (CO2) 1
Methane (CH4) 28
Nitrous Oxide (N20) 265
Sulfur Hexafluoride (SF6)23,500

The ratings show that some F-gases are far more potent than carbon dioxide (CO2). HFC-23, for example, is 12,400 times more powerful than CO2, while Sulfur Hexafluoride is 23,500 times more powerful.

Are CO2 Levels Higher Than They Used To Be?

Yes. According to paleoclimatologists, the last time carbon dioxide levels exceeded those of today was 3 million years ago, not long after the formation of the polar ice caps. Here’s a brief outline of the historical record.

Around 34 million years ago, following a 40 percent decrease in carbon dioxide – possibly triggered by the uplift of the Himalayas and their subsequent chemical weathering – ice sheets formed at the Antarctic, creating the basis for our modern climate.

During the 100 million years leading up to the cooling, Earth’s environment was warm and wet. Then, over a very short period of about 100,000 years, global temperature fell dramatically, many marine and aquatic creatures became extinct, ice formed over Antarctica and sea levels fell.

All this was the focus of a recent study of geochemical remnants of ancient algae from seabed cores collected by drilling in deep-ocean sediments, which found that the climate tipping point in atmospheric CO2 levels that initiates ice sheet formation is about 600 parts per million – a sobering thought considering that the world is on course to reach levels between 550 and 1,000 parts per million by 2100. 8

Following the formation of Antarctic ice, levels of atmospheric carbon dioxide continued to fall, falling below 400 ppm (our present level) about 3 million years ago. 9

Since then, according to measurements of trapped air taken from ice cores in East Antarctica, measuring several kilometres in length 10, CO2 concentrations have varied between 180–210 ppm during ice ages, rising to 280–300 ppm during warmer interglacials. 11

Further ice core samples show that by 10,000 BC, levels were stable at around 260–280 ppm, and did not vary again until the Industrial Revolution of 1750 AD. 12

How Much CO2 Is In The Atmosphere Now?

Atmospheric levels of greenhouse gases over the past 2,000 years.
Graph: Atmospheric concentrations of long-lived greenhouse gases over the past 2,000 years. Increases since about 1750 are attributed to man-made activities in the industrial era. PPM represents concentration units parts per million or parts per billion (ppb). This shows the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. Source: IPCC Fourth Assessment Report

By the time methodical thermometer-based records began in 1850, global temperatures and CO2 levels had already started climbing. However, 1850-1900 is the traditional baseline for the pre-industrial period, against which all subsequent developments are measured. 13

Since then global annual mean CO2 concentration has risen by almost 50 percent. That is, from 280 ppm around the pre-industrial baseline, to 414.8 ppm in May 2019. 14

Atmospheric Levels of CO2 over past 40 years
Source: National Oceanic and Atmospheric Administration

Research into historical levels of atmospheric carbon dioxide is ongoing, so statistics may change. For example, according to one study which presented a CO2 record spanning the past 40 million years – based on core samples from a single marine locality – Ocean Drilling Program Site 925, in the western equatorial Atlantic Ocean – today’s level of 415ppm is the highest for 14 million years. 15

Why Has Carbon Dioxide Increased In The Atmosphere?

Because we humans have burned and are continuing to burn fossil fuels, like coal, oil and natural gas, resulting in the release of ever-increasing quantities of CO2 and other (even more potent) greenhouse gases into the atmosphere. During the period 1750-1900, about 44 billion tons of CO2 were released into the atmosphere through the burning of fossil fuels, whereas from 1901 to 2013 the figure was about 1.4 trillion tons. Almost 32 times the earlier total. As of 2020, according to three recent studies, it looks like CO2 emissions will increase again in 2019. 16 17 18

Researchers have calculated that about 70 percent of anthropogenic CO2 emissions will be absorbed in the first 100 years. Absorption by carbon sinks slumps after this, with only an additional 10 percent or so being absorbed after 300 years. The remaining 20 percent remains active in the atmosphere for tens if not hundreds of thousands of years before being removed, with a mean lifetime of between 30,000 and 35,000 years. 19

Why Are Atmospheric CO2 Levels So Important?

Because carbon dioxide in the atmosphere boosts the greenhouse effect and leads directly to a rise in global temperature. And the bad news is that even if man-made CO2 emissions were to cease completely, atmospheric temperatures will not decrease significantly for thousands of years. 20

Carbon Capture and Storage

Atmospheric carbon dioxide is so damaging to our climate that the IPCC is recommending the adoption of carbon capture and storage (CCS) technologies (including “carbon capture, utilization and storage” CCUS, and “bioenergy with carbon capture and storage” BECCS) – as well as cuts in fossil fuel use – in order to slow down the rise in global temperature. Many countries are already including CCS in their climate change mitigation plans, although the technology behind it remains unproven on a large scale. Undaunted, the IPCC’s Special Report on Global Warming of 1.5°C (2018) states that all fossil fuel usage after 2050 will be neutralized through carbon capture and storage.


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