What Are Greenhouse Gases?

We explain all you need to know about the main greenhouse gases, including water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), the fluorinated gases (CFCs, HCFCs, HFCs, PFCs) and ozone (O3).
Coal-fired power plant emits high level of greenhouses gases
Power station emits greenhouse gases. Photo: Ivabalk/Pixabay

In climate science, the term “greenhouse gases” (GHGs) describes those gases that contribute to the “greenhouse effect” in the troposphere, by trapping infrared heat energy emitted from the surface of the planet that would otherwise escape into space. By trapping this heat, GHGs help to maintain the Earth’s temperature at a cosy 15°C (59°F), instead of the minus 18°C (zero degrees Fahrenheit) it would otherwise be.

Unfortunately, in our drive for development and prosperity since the Industrial Revolution (c.1780-1840), we have caused our power plants, factories and motor vehicles to spew out such a volume of greenhouse gas emissions, that the Earth’s climate system has been unable to cope.

As a result, the planet has started heating up – a process now known as global warming.

The most abundant GHG is water vapor, while the most common man-made GHGs are carbon dioxide, methane, nitrous oxide and ozone.

Climate Change for Beginners
Climate Change for Students

What’s The Problem With Greenhouse Gases?

Although it’s true that the greenhouse effect is a natural component of the Earth’s climate system, recent human activities have caused a surge in greenhouse gas emissions. This massive, overproduction of GHGs has unbalanced the whole system and now threatens to raise global temperature to ecologically disastrous levels. What’s more, we can’t just dial down the heat, because most of the GHGs have relatively long lifespans. For instance, one fifth of all carbon dioxide emissions remain in the atmosphere for many thousands of years. 1

What Do Scientists Say About Greenhouse Gases?

The Intergovernmental Panel on Climate Change (IPCC), successor to the smaller Advisory Group on Greenhouse Gases, is the main U.N. agency responsible for providing authoritative scientific advice to the United Nations Framework Convention on Climate Change (UNFCCC).

Ever since the 1990s, the IPCC has identified man-made greenhouse gases in the atmosphere (including carbon dioxide, methane, nitrous oxide and water vapor) as the cause of global warming. Which is why it continues to issue urgent warnings that we must dramatically cut our GHG emissions, mostly by switching from fossil fuels to low-emission renewable energy.

Global Greenhouse Gas Emissions Scenarios
Potential future scenarios of global greenhouse gas emissions (measured in gigatonnes of carbon dioxide equivalents). A. No climate policies. B. Current policies. C. National pledges within the Paris Agreement. D. Scenarios with 2°C warming outcome. E. Scenarios with 1.5°C warming outcome. High, medium and low pathways represent ranges for a given scenario. Temperature figures represent the estimated average global temperature increase from pre-industrial, by the year 2100. Source: Climate Action Tracker (CAT)

In its Fifth Assessment Report (2014), the IPCC warned that in a business-as-usual scenario, global warming could reach 4.1°C to 4.8°C (above 1850-1900 levels) by the year 2100 – equivalent to ecological meltdown.

The following year, signatories to the Paris Climate Agreement agreed to limit global warming to well below 2°C and to pursue efforts to limit the rise in temperature even further to 1.5°C.

Four years later, in their Special Report on Global Warming of 1.5°C, the IPCC again recommended urgent action to limit warming to 1.5°C, but stated that emissions of carbon dioxide would need to fall by “about 45 percent from 2010 levels by 2030, reaching net zero around 2050.”

Incidentally, if you think there’s no real difference between 1.5°C and 2°C , please read our article: Why Does a Half-Degree Rise in Temperature Make Such a Difference to the Planet?

How Much Greenhouse Gas Is In The Atmosphere?

Average levels of carbon dioxide in the atmosphere have risen by more than 45 percent since the start of the Industrial Revolution. For 10,000 years up to the late-18th century, it was roughly 280 ppm (parts per million), but then rose to 412 ppm by November 2020. 2

Historical Greenhouse Gas Concentrations
Graph: Concentrations of atmospheric 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. The graph shows how greenhouse gases are upseting Earth’s energy balance, causing a rise in temperature. Source: IPCC Fourth Assessment Report

The total concentration of all GHGs reached 449 ppm (CO2 equivalent) in 2016, an increase of more than 4 ppm compared with 2015, and 33 ppm more than 10 years ago. 3 From 2014 to 2016, global emissions of carbon dioxide were essentially stable during modest economic growth. But in 2017 – despite increased climate action – they went up by 1.2 percent, stimulated by higher GDP. 4 In 2018 the level of CO2 reached 407.8 parts per million (ppm), up from 405.5 ppm in 2017 – an increase of 147 percent on the “pre-industrial” level (1850-1900).

What Are The Latest Greenhouse Gas Statistics?

According to the UN Emissions Gap Report 2020, the GHG emissions totalled 59.1 billion tons of CO2 equivalent (GtCO2e) which – subject to the effects of the COVID-19 pandemic – puts the Earth on course to experience a temperature rise of 3.2°C by the end of this century.

Sources of Greenhouse Gases

Greenhouse Gas Emissions 2020
Total 59.1 billion tons of CO2 equivalent. 38.0 billion tons (72.6%) came from fossil fuels and industry; 9.8 billion tons came from man-made methane emissions (18.7%); 2.8 billion tons came from man-made nitrous oxide emissions (5.4%); and 1.7 billion tons came from fluorinated gases (3.3%). A further 6.8 billion tons came from deforestation and land use. Total: 59.1 billion tons. Source: UN Emissions Gap Report 2020

Of the total of 59.1 billion tons of CO2 equivalent, 38.0 billion tons came from fossil fuels and industry; 6.8 billion tons came from deforestation and land use; 9.8 billion tons came from man-made methane emissions; 2.8 billion tons came from man-made nitrous oxide emissions; and 1.7 billion tons came from fluorinated gases. (Note: like all emissions statistics, these are approximate figures only.) 5 See also: Greenhouse Gas Statistics Not Consistent.

Do All Greenhouse Gases Have The Same Effect?

No. The strength of a greenhouse gas and its consequent impact on global warming depends on three factors: its abundance in the atmosphere, its active lifespan and its capacity to absorb energy and re-radiate it (radiative ability). Based on these factors, each gas is given a Global Warming Potential (GWP) rating, which is a measure of how much energy 1 ton of a particular greenhouse gas will absorb, over a given period of time, compared to 1 ton of carbon dioxide (CO2). So CO2 will always have a GWP of 1.

Global warming potential (GWP) of various greenhouse gases
Global Warming Potential of Various Greenhouse Gases. Source: IPCC. Fifth Assessment Report. 2014

Example: over 100 years, one ton of sulphur hexafluoride has the same effect on global warming as 23,500 tons of carbon dioxide. Fluorinated gases – including the HFCs, HCFCs, CFCs, PFCs, and sulphur hexafluoride are notorious for being high-GWP gases since, pound for pound, they absorb significantly more heat than carbon dioxide.

So which is the worst greenhouse gas?
The greenhouse gas with the biggest impact on climate change is carbon dioxide. Pound for pound it may not trap as much heat as nitrous oxide or sulphur hexafluoride, but there’s a lot more of it in the atmosphere and – most importantly – a sizeable chunk of it remains active for literally thousands of years.

Which Are The Most Important Greenhouse Gases?

We look at six greenhouse gases: water vapor, carbon dioxide, methane, nitrous oxide, the fluorocarbon family and ozone.

Water Vapor (H2O)

Water vapor is the most prevalent greenhouse gas, although its concentration levels in the atmosphere are totally dependent on air-temperature and remain highly variable: anything from less than 0.01 percent in cold regions to as much as 3 percent at about 32 °C.

Because of its close correlation with temperature, water vapor creates what climate scientists call a “positive feedback loop.” The point is, as the temperature rises, more water evaporates and becomes vapor. So as the Earth gets hotter more water evaporates and, since water vapor is a greenhouse gas, it causes the temperature to go up even further. In any event, we all know the warming effect of clouds. On a clear night, for instance, heat escapes and the temperature drops. But if clouds cover the sky, the heat is trapped and the temperature stays relatively mild. For more, see: How Do Clouds Affect Climate?

Scientists estimate that there’s enough water vapor in the atmosphere at any one time to cover the entire surface of the planet with a layer of liquid water some 1 inch (25 mm) deep. In total, water vapor is estimated to account for between 36 and 70 percent of the global greenhouse effect.

The water cycle ensures that the water content of the atmosphere is continuously being replenished by evaporation and depleted by precipitation. Evaporation takes place from oceans, lakes, rivers, reservoirs, wetlands and moist earth. For example, the once mighty 1,450-mile long Colorado River is disappearing fast. Over the last 100 years, its flow has declined by around 16 percent, even though annual rainfall showed a slight increase in the huge Upper Colorado River Basin.

A new study carried out by climate research scientists at Colorado State University, which was published in the journal Water Resources Research, claims that as much as half the flow loss might be the result of evaporation caused by the rise in regional temperatures. To give you an idea of how much water vapor we’re talking about, the river’s daily flow rate is roughly 6.5 billion gallons. If the evaporation rate is 8 percent, that comes to 500 million gallons of vapor rising into the atmosphere every day from one river.

Man-Made Water Vapor

As far as human involvement is concerned, there is a myth that we have little or no effect on the production of water vapor and its consequent contribution to global warming. In fact, humans play a huge part in generating extra water vapor. There are direct increases in the water vapor in the atmosphere from anthropogenic irrigation, landscape watering, and the burning of hydrocarbon fuels. 6

To begin with, gasoline (petrol) reacts with oxygen to produce carbon dioxide and water vapor. So, the 391.40 million gallons of gasoline burned daily in the United States alone 7, releases more than 400 million gallons of water vapor per day. In addition, significant amounts of H2O are discharged into the atmosphere by burning gasoline, natural gas and distillate fuel oil.

Irrigation and landscaping are another two anthropogenic activities that cause major evaporation of water. According to the U.S. Environmental Protection Agency (EPA), each day Americans use an average of 9 billion gallons of water mainly for watering lawns and landscapes. 8 The United States uses a total of 322 billion gallons of water a day. 9

Carbon Dioxide (CO2)

Carbon dioxide – the most prevalent of all man-made greenhouse gases, makes up less than 0.05 percent of our atmosphere, but plays an integral part role in the complex carbon cycle, in which carbon moves from the atmosphere into organisms and the Earth, and then back into the atmosphere. The oceans contain huge amounts of suspended carbon, as does the soil and land biomass. 10

Chart showing recent increase in atmospheric levels of CO2
Recent Increase in CO2 levels. Source: National Oceanic and Atmospheric Administration. NOAA

Something like 750 billion tons of carbon dioxide move around the carbon cycle. By comparison, the 29 billion tons of CO2 produced by mankind seem fairly insignificant. Except they’re not. Because this human-produced CO2 is upsetting the natural balance of the carbon cycle. Result? Atmospheric CO2 is at its highest level in 15 to 20 million years. 11 Carbon dioxide actually accounts for three quarters of all greenhouse gas emissions. 12

For hundreds of thousands of years up until the nineteenth century, levels of atmospheric CO2 remained fairly stable at about 280ppm. Then they started to rise. As of November 2020, levels are now 412ppm – a 45 percent increase since the beginning of industrialization. 13 Most worrying of all, although over half of all atmospheric CO2 usually disappears within a century, residues can remain in the atmosphere for thousands of years. 14

Human Sources Of CO2

Most carbon dioxide from human activities is released by burning wood and other organic materials, and fossil fuels such as coal, peat, petroleum and natural gas. As the name suggests, fossil fuels are made from the decayed remains of bacteria, plants or animals which are then subjected to high pressure and temperature. When fossil fuels are burned, the carbon molecules previously locked inside these organic remains are released back into the atmosphere as carbon dioxide.

Deforestation (and forest degradation) is another anthropogenic source of CO2, accounting for roughly 10 percent of global greenhouse gas pollution. 15

Emissions of CO2 are also produced during many industrial and chemical processes. Some 5 million tons per year are produced as a bi-product during the manufacture of acrylic, used in the diaper, plastics, adhesives, floor polish, paint, personal care products and detergent industries. Cement industry CO2 emissions are also huge, accounting for roughly 8 percent of the world’s total.

See our article: How To Reduce Your Carbon Footprint.

What Energy Sources Emit the Least Greenhouse Gas?

Most renewable energies are extremely low in GHG emissions when compared to fossil fuels. Renewables that emit the least GHG include: solar power, offshore and onshore wind power and geothermal energy.

Next generation very low-emission sources include tidal power and wave energy, as well as hydrogen energy. Organic biomass and biofuels emit more GHG because they embrace a wider, more complex set of production processes involving many things from crop residues to human sewage. The renewable energy source with the highest emissions (though still low comparatively) is hydropower (hydroelectricity).

Methane (CH4)

Methane (CH4) is a powerful greenhouse gas that absorbs 84 times more longwave heat energy over 20 years, than carbon dioxide, although its lifetime of 12.4 years is far shorter. In 2010, climatologists in the Arctic recorded methane levels at 1850 parts per billion (ppb) compared with 722 parts per billion (ppb) in pre-industrial times. It was the highest methane recording for 800,000 years. In fact, it is close to the RCP8.5 trajectory identified by the IPCC as leading to global warming of between 4.1°C and 4.8°C. 16

Chart showing recent increase in CH4 levels
Recent Increase in CH4 levels. Source: NOAA

In 2020, the level of methane rose to 1876 ppb 17 of which roughly 55-60 percent was due to human activity. An estimated 18.7 percent of man-made global warming is caused by methane emissions. 18 19

The majority of naturally occurring methane emissions are caused by decomposition of animal and plant matter in the absence of oxygen, or by microorganisms. Wetlands and rice paddies between them furnish almost 35 percent of total emissions. Anthropogenic emissions are caused mostly by seepage from oil and natural gas supply chains, enteric fermentation in cattle, and expansion of water-logged rice growing areas.

    These moist, waterlogged environments (like peat bogs) are ideal for methane production. Wetland soils and plants account for roughly 20 percent of atmospheric methane. 20
    Methane in rice paddies is emitted by microscopic organisms like bacteria. Luckily for them, the water blocks oxygen from penetrating the soil, thus creating ideal conditions for their growth. Due to the burgeoning world population, rice fields have become one of the most significant anthropogenic sources of this greenhouse gas. With water-logged soil and increasingly warm weather, rice paddies act like wetlands, except that they are generated by humans in order to grow food. These paddies now account for roughly 15 percent, or more, of anthropogenic methane emissions. 21 22
    Significant amounts of methane are produced by methanogenesis – that is, by bacteriological microorganisms (methanogens) inside the multi-chambered stomachs of cows and other ruminants. This gas is released whenever the animal passes wind. In 2006, a U.N. report stated that livestock generated more greenhouse gas emissions (measured in CO2-equivalents) than the entire transportation sector. Livestock is responsible for 37 percent of anthropogenic methane, 65 percent of anthropogenic nitrous oxide and 9 percent of anthropogenic CO2. Henning Steinfeld, co-author of the study, said: “Livestock are one of the most significant contributors to today’s most serious environmental problems.” 23 24 25
    Like cattle, termites also contain methanogenic microorganisms in their gut. These organisms rely for their energy on an anaerobic process called methanogenesis. They account for roughly 4 percent of methane emissions.
    Decaying organic material and compressed anaerobic conditions make landfills a serious source of methane emissions. When waste is first deposited at a landfill site, there’s plenty of oxygen and the waste undergoes aerobic decomposition, with almost no CH14 produced. Usually however, after about 12 months, oxygen levels are exhausted and anaerobic conditions prevail, allowing methane-producing organisms (methanogens) to dominate the decomposition process. Even after the landfill is closed, these methanogens continue releasing methane for years.
    Landfill waste sites are the third largest source of atmospheric methane in the United States, and account for about 18 percent of global methane emissions in 2014. 26 The discharge of methane gas from landfill sites, caused by anaerobic decay of domestic waste, has become a major problem in the UK. For instance, about 90 percent of UK domestic waste ends up in landfill sites, accounting for roughly 29 percent of all man-made methane emissions in the country.
    Methane is a primary component of natural gas, and thus during the drilling, production, processing, storage, transmission, and distribution of natural gas, a significant amount of methane is lost into the atmosphere. In fact, U.S. methane emissions from natural gas and petroleum systems totalled a whopping 8.1 million tons in 2015. 27 Of this, 6.5 million tons were emitted by the natural gas system, while oil/petroleum installations emitted 1.6 million tons of methane – often from oil storage tank vents and other openings. 28
    A 2017-18 review of emissions studies indicated that the “EPA Inventory of Greenhouse Gas Emissions and Sinks: 1990–2015” report significantly underestimated 2015 methane emissions from the oil and gas industry whom, the review said, actually emitted 13 million tons of methane – roughly 60 percent more than the figure in the original EPA report. 29 See also: Why Are Methane Emissions Rising?
    The discovery of a methane cloud occupying about 2,500 square miles (6,500 km2) floating over the Four Corners region of the south-west United States, in 2014, highlighted yet another connection between “Big Coal” and global warming. The investigation that followed, concluded: “the source is likely to be from established gas, coal, and coalbed methane mining and processing.” The coal, oil and natural gas industries together account for at least one fifth of all methane emissions. Tell this to the climate change denial machine.
    As the planet warms up and temperatures rise, the frozen methane inside the northern permafrost – land that is frozen for more than two consecutive years – is slowly being released as the frozen peat layer thaws. Unfortunately, recent years (2000-2007) have witnessed unprecedented thawing of permafrost in both Alaska and Siberia up to five times greater than previously thought. 30 A type of Pleistocene-age permafrost known as yedoma (formed 1.8 million to 10,000 years ago), is a significant source of atmospheric methane (about 4 million tons of CH4 per year).

Nitrous Oxide (N2O)

The greenhouse gas nitrous oxide (N2O) has long been overlooked as a danger to the environment, yet it retains significant global warming potential. It has an average lifetime of some 121 years, and a GWP rating of 265. 31 This means it has 265 times more heat-trapping power than carbon dioxide. Its concentration in the troposphere has increased by 15 percent since about 1800.

In 2020, the level of N20 rose to 332 ppb 32

As of 2010, just under 30 million tons of nitrous oxide (N20) were being emitted into the atmosphere each year. Of this, 62-64 percent was from natural sources and 36-38 percent from human activities. 33

Graph showing recent increase in N2O levels.
Recent increase in N2O levels. Source: NOAA

Natural Sources Of N2O

The majority of the N2O emitted into the atmosphere, from natural sources, is produced by microorganisms in soils and the ocean, like bacteria and fungi. 34 See also: Nitrogen Cycle: How it Works. Soils under natural vegetation are a particularly important source of the gas, accounting for 60 percent of all naturally occurring emissions. Other natural sources include denitrification (a process whereby nitrate is converted to nitrogen gas by soil microbes in marine sediments) (35 percent) and atmospheric chemical reactions (5 percent). 35 See also: Why is Soil So Important to the Planet?

Man-Made Sources Of N2O

Among industrial emissions, the production of nitric acid (used in the synthesis of nitrogen fertilizers) and adipic acid (a precursor to nylon and other synthetic clothing fibres) are the largest sources of nitrous oxide emissions. 36

About 42 percent of global nitrous oxide emissions are produced in agriculture, from the addition of nitrogen-based fertilizers to the soil roughly 79 percent of all nitrous oxide released in the United States comes from nitrogen fertilization), from rice cultivation 37 and from the breakdown of animal manure. An additional 25 percent is accounted for through the drainage of fertilisers, and a further 10 percent through biomass burning. About 10 percent of global nitrous oxide emissions are released during the burning of gasoline by cars or trucks. 38

The COVID-19 lockdowns during the spring and summer of 2020 led to dramatic falls in nitrous oxide emissions over China and elsewhere. However, the effect of COVID-19 on climate change is not likely to be permanent. Indeed, atmospheric levels of CO2 and some other greenhouse gases are higher than ever.

Depletion Of The Ozone Layer

It is now thought that Nitrous oxide is partly responsible for depleting the ozone layer. A new study suggests that nitrous oxide is the single most important ozone-depleting substance (ODS). 39

Synthetic Fluorinated Gases

Atmospheric Levels of F-Gases
Atmospheric Concentrations of Fluorinated Gases. Source: National Oceanic and Atmospheric Administration.

F-gases (fluorinated gases) comprise a cluster of man-made greenhouse gases that include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). They are synthetic compounds that did not exist in the atmosphere before the 20th century.

After evidence showed that CFCs damaged the ozone layer, they were banned by the Montreal Protocol on Substances that Deplete the Ozone Layer (1989). Subsequent agreements have been reached to phase out the replacements for CFCs, namely HCFCs and HFCs, by 2030 and 2050 respectively.

These synthetic greenhouse gases are notorious for having high ‘global warming potential’ ratings – HFC-23, for instance has a GWP rating of 12,400, while the perfluorinated compound sulphur hexafluoride (SF6) has a GWP of 23,500. Before they were banned in the late 1990s, CFCs accounted for about a quarter of the anthropogenic greenhouse effect. What’s more, while CFCs can last anywhere between 50 and 140 years, HFCs, PFCs, sulphur hexafluoride and nitrogen trifluoride can stay in the atmosphere for centuries.

The most common F-gases are HFCs. These were developed in the 1990s as substitutes for CFCs and HCFCs. The most important HFCs include: HFC-23, HFC-134a and HFC152a.

HFCs are used in a very wide variety of different applications including: industrial and domestic refrigeration, air-conditioning systems, heat pump equipment, fire extinguishers and suppressants, the semiconductor industry, aerosol propellants and solvents.

PFCs are also used in the refrigeration industry, but are especially widespread in the electronics, cosmetics, and pharmaceutical industries. In the past, PFCs were frequently used as fire extinguishers and can still be found in older fire protection systems.

Why Are F-Gases A Problem For Global Warming?

HFCs and PFCs present a problem because of their extremely widespread use and their high global warming potential (GWP). 40 41 42

Two HFCs that enjoy the widest application are HFC-23 and HFC-134a, both of which are becoming increasingly popular as replacements for ozone-depleting substances (ODS) across several industries. HFC-23 has a GWP rating of 12,400 (meaning it’s 12,400 times more potent as a greenhouse gas than carbon dioxide) and a lifetime of 270 years. 43 Its sister compound HFC-134a has a GWP of 1,300 and a lifetime of 14 years. Scientists worry that their continuing success as refrigerants (in particular) is already pushing up their atmospheric concentrations, posing a serious risk for climate change. Meantime, PFCs have a GWP rating over 100 years of 7.390-12,200 and a lifetime of between 2,600 and 50,000 years. 44

Ozone (O3)

Ozone is found in both the upper and lower atmosphere. In the Stratosphere, where almost 90 percent of ozone is found, it occurs naturally, being formed by reactions involving sunlight and oxygen. Its presence in the so-called “ozone layer” is beneficial, as it shields the surface of the Earth from damaging UV light from the sun. It is not seen as a significant greenhouse gas.

In the troposphere, where it is found in relatively low concentrations (usually 20-30 parts per billion, but 100 ppb in polluted areas), ground level ozone is a serious pollutant. 45

Ground level ozone is not emitted into the air by anything. Instead, it is formed by the reaction of sunlight on certain chemicals including volatile organic compounds (VOCs) and nitrogen oxides (NOx) that are emitted into the air – mostly by vehicular engines. Ozone is a key component of urban smog around the world, especially in the Northern Hemisphere.

Levels of tropospheric ozone have risen about 38 percent since pre-industrial times, and this increase is due to atmospheric chemistry involving short-lived pollutants discharged from human sources.

The atmospheric lifetime of tropospheric ozone is just over 3 weeks, so its distribution is variable and its effect on global warming is relatively small. However, as stated, tropospheric ozone is a pollutant whose potency is caused by its strength as an oxidant. It is this characteristic that causes damage to respiratory tissues and makes ozone a serious breathing hazard near ground level.

Photochemical smog, of which ozone is a key ingredient, is present in all modern cities, but it is more prevalent in cities with sunny (chemically friendly), dry (rain disperses smog) climates and a large number of cars, trucks and motorbikes – the older the better! Examples include: Beijing, Delhi, Denver, Lahore, Los Angeles, Mexico City, Santiago, and Teheran, to name but a few. 46 It wasn’t until the early 1950s that the characteristics of smog were first understood, thanks to chemist Arie Haagen-Smit, who identified ozone as a key component.

Overall, tropospheric ozone is an unhealthy pollutant rather than a greenhouse gas, although on warm summer days it can have a dramatic effect on local weather conditions, chiefly in urban areas. For example, it can lead to spikes in daily temperature and occasional heat waves.


  1. “Frequently Asked Question 10.3: If emissions of greenhouse gases are reduced, how quickly do their concentrations in the atmosphere decrease?”. Global Climate Projections. IPCC Fourth Assessment Report. 2007. []
  2. Trends in Atmospheric Carbon Dioxide”. Earth System Research Laboratory. NOAA. []
  3. Atmospheric greenhouse gas concentrations.” European Environment Agency (EEA). March 2019. []
  4. “Climate change: CO2 emissions rising.” Matt McGrath. BBC News. 27 November 2018. []
  5. Source: “UN Emissions Gap Report 2020.” UNEP. []
  6. “The Role of Anthropogenic Water Vapor in Earth’s Climate.” Thayer Watkins. San Jose State University. []
  7. U.S. Energy Information Administration, 2018. []
  8.  “Water Sense: Outdoor water use in the United States.” EPA. 2017. []
  9. “How much water is used by people in the United States?” USGS. 2015. []
  10. Scientists Quantify Global Volcanic CO2 Venting; Estimate Total Carbon on Earth.” Deep Carbon Observatory. Oct 1, 2019. []
  11. Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years.” Aradhna K. Tripati, Christopher D. Roberts, Robert A. Eagle. Science 04 Dec 2009. []
  12. Global Greenhouse Gas Emissions by Gas.” United States Environmental Protection Agency. []
  13. Carbon dioxide levels hit record peak in May 2019.” []
  14. GHGs: How Long Will They Last? – Environmental Defense Fund. []
  15. What is the Relationship Between Deforestation and Climate Change?” Rainforest Alliance. August 12, 2018. []
  16.  IPCC Fifth Assessment Report (2014). Working Group I: “Climate Change 2014: The Physical Science Basis – Summary for Policymakers. []
  17. Earth System Research Laboratory Global Monitoring Division, NOAA, August 2020. []
  18. See also: “Very Strong Atmospheric Methane Growth in the 4 Years 2014–2017: Implications for the Paris Agreement.” E. G. Nisbet et al. 05 February 2019 []
  19. See also: Advancing scientific understanding of the global methane budget in support of the Paris Agreement. Global Biogeochemical Cycles, 2019. Anita L. Ganesan et al. []
  20. An ecological perspective on methane emissions from northern wetlands”. Trends in Ecology and Evolution. Jill L Bubier; Tim R. Moore. (December 1994). []
  21. Acclimation of methane emissions from rice paddy fields to straw addition.” Yu Jiang, et al; Science Advances. 16 Jan 2019: Vol. 5, no. 1. []
  22. Investigations of methane emissions from rice cultivation in Indian context.” Environment international. 31. pp.469-82. Anand, Shalini & Dahiya, R. P. & Talyan, Vikash & Vrat, Prem. (2005). []
  23. “Livestock a major threat to environment”. United Nations Food and Agriculture Organization. 29 November 2006. []
  24. Revised methane emissions factors and spatially distributed annual carbon fluxes for global livestock.” Carbon Balance Manage 12, 16 (2017). Wolf, J., Asrar, G.R. & West, T.O. []
  25. Livestock and climate change: impact of livestock on climate and mitigation strategies.” Giampiero Grossi, Pietro Goglio, Andrea Vitali, Adrian G Williams. Animal Frontiers, Volume 9, Issue 1, January 2019, Pages 69–76. []
  26. “Greenhouse Gas Emissions”. United States Environmental Protection Agency. 2014. []
  27. EPA Inventory of U.S Greenhouse Gas Emissions and Sinks: 1990–2015 report. []
  28. Aerial Surveys of Elevated Hydrocarbon Emissions from Oil and Gas Production Sites.” David R. Lyon et al. Environ. Sci. Technol. 2016. April 5, 2016. []
  29. Assessment of methane emissions from the U.S. oil and gas supply chain”. Science. Alvarez, Ramon A. et al; (2018-07-13). []
  30. “Methane bubbles climate trouble”. BBC News. September 7, 2006. []
  31.  “Contribution of Working Group I.” IPCC. Fifth Assessment Report (AR5) (2013) []
  32. Earth System Research Laboratory Global Monitoring Division, NOAA, August 2020. []
  33. “Methane and Nitrous Oxide Emissions from Natural Source.” U.S. Environmental Protection Agency (2010). []
  34. Nitrogen Oxides Control Technology Fact Book. William Andrew. p. 6. ISBN 978-0-8155-1294-3. Sloss, Leslie L. (1992). []
  35. US Environmental Protection Agency (2010), “Methane and Nitrous Oxide Emissions from Natural Sources”. Report EPA 430-R-10-001. []
  36. “Couplings Between Changes in the Climate System and Biogeochemistry”. In IPCC’s Fourth Assessment Report. Cambridge University Press. K. L. Denman, G. Brasseur, et al. (2007). []
  37. High nitrous oxide fluxes from rice indicate the need to manage water for both long- and short-term climate impacts,” Kritee Kritee, et al; PNAS September 25, 2018 115 (39) 9720-9725; September 10, 2018. []
  38. “Nitrous Oxide Emissions”. U.S. Environmental Protection Agency (EPA). 23 December 2015. []
  39. “Laughing gas is biggest threat to ozone layer.” Lisa Grossman. New Scientist. August 28, 2009. []
  40. Fluorinated greenhouse gases.” EU Commission. []
  41. Evolution of uses and emissions of fluorinated gases, in particular those of hydro-fluorocarbon type (H-FC).” Green Facts. []
  42. Emission sources and mitigation of fluorinated Non-CO2 greenhouse gas in registered CDM projects.” Seung-Jae Lee, et al; 12 April 2017. []
  43. “Changes in Atmospheric Constituents and in Radiative Forcing.” (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Forster, P. et al; (2007). []
  44. U.S.Environmental Protection Agency. []
  45. “Health Aspects of Air Pollution with Particulate Matter, Ozone and Nitrogen Dioxide.” WHO-Europe report 13–15 January 2003. (PDF) []
  46. Living in the Environment: Principles, Connections, and Solutions (12th Edition). Belmont: The Thomson Corporation. p.423. Miller, Jr., George Tyler (2018). []
Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on email