Greenhouse Effect Explained

What Is The Greenhouse Effect?

In climate science, the so-called “Greenhouse Effect” is a natural climatic mechanism, occurring in the lower atmosphere, in which greenhouse gases like water vapor and carbon dioxide, trap heat energy being radiated outwards by the Earth. Part of the trapped energy is then radiated back down to Earth by the gases, warming the surface of the planet. 1 The gases serve as a layer of insulation, which conserves the planet’s heat and helps to maintain a cosy temperature.

Unfortunately, over the past two centuries, man-made greenhouse gas emissions from fossil fuels and cement manufacture, indirectly, from land use change, have massively overburdened this natural greenhouse effect, causing the climate crisis which now threatens to engulf us. 2

See below for exactly how the greenhouse effect works, what causes it, and how it contributes to climate change.

Where Does The Greenhouse Effect Happen?

Greenhouse gases accumulate in the lowest layer of Earth’s atmosphere, known as the troposphere. The atmosphere as a whole is a 300-mile high multi-layered mixture of gas that encircles Planet Earth. It is kept in place by the pull of Earth’s gravity. Oxygen in the atmosphere allows us to breathe, ozone protects us from harmful UV radiation, while gases like water vapor and carbon dioxide function like an insulation blanket, reducing the loss of heat into space.

The two most influential layers of the atmosphere are the troposphere (up to 12km/7 miles), and the stratosphere (up to 50km/30 miles). The troposphere is the densest part of the atmosphere, containing some 90 percent of the actual matter of the atmosphere, such as clouds, water vapor, and fossil fuel gases like CO2 and sulphur dioxide. This is where the “greenhouse effect” happens.

At the top of the troposphere is a mini-layer of cold air called the tropopause. This acts like an invisible barrier to the upward movement of water vapor and other greenhouse gases. The stratosphere contains the so-called ozone layer, situated 15-35 km above the Earth’s surface, which shields us from the Sun’s ultra-violet (UV) radiation.

What Is The Natural Greenhouse Effect?

Simple Diagram: How Greenhouse Effect Works
How the Natural Greenhouse Effect Works. Source: US EPA

As stated in the Introduction, the greenhouse effect is a natural mechanism whereby greenhouse gases trap some of the thermal energy given off by the Earth that would otherwise escape into space. Without the benefit of this natural heat-trapping mechanism, Earth’s temperature would be a chilly minus 18°C (zero degrees Fahrenheit) – that’s roughly the temperature of the North Pole – instead of the comfortable 15°C (59°F) that we’re used to.

The greenhouse effect works in a similar way to the process that goes on in a real garden greenhouse. In a greenhouse, sunlight (shortwave radiation) passes through the panes of glass and is absorbed by things in the greenhouse before being re-radiated outwards as infrared heat energy (longwave radiation). This longwave radiation is reflected by the glass, so it remains trapped inside the greenhouse causing the temperature inside the greenhouse to rise.

In the same way, most sunlight passes through the atmosphere and reaches the surface of the planet. As the latter becomes warmed by the sunlight, it radiates part of this energy back into the sky as infrared heat. This heat, unlike visible light, is absorbed by clouds and greenhouse gases, after which some of it is re-radiated back toward Earth’s surface. In effect, the greenhouse effect mechanism creates a second source of warmth for the planet, making it much warmer than it otherwise would be.

What Is The Enhanced Greenhouse Effect?

The enhanced greenhouse effect is simply that part of the greenhouse effect that is caused by recent human actions (like burning fossil fuels), as opposed to age-old natural actions (like plant respiration). For the full picture, let’s start with some background information. 3

Coal Power Station China
Coal-fired power station in China. China is the largest coal producer in the world, and despite national commitments to stop building new coal-fired stations, a number of regional authorities have used legal loopholes to ignore the order. Photo: pixabay

Global temperatures have been static for at least 5,000 years. In fact, during this time – judging by rock samples taken from sea beds – temperatures were in a slight decline (falling about 0.5°C in total). Then, in the late 19th century, they stopped declining and started rising. (See also: When Did Global Warming Start?) This upward trend has continued, broadly speaking, ever since, although in recent years the rate of warming has accelerated.

As of 2019, the planet is 1°C warmer than it was in 1900, and looks set to heat up even more. According to the Special Report on Global Warming of 1.5°C issued by the IPCC in 2018, Earth’s temperature is likely to be 3°C higher by 2100, than it was at the end of the 19th century.

What’s the reason for this sudden increase of 1°C? Why are scientists forecasting another increase of 2°C by the end of the century?

Answer: Because over the past two hundred years, man-made greenhouse gases – caused mainly by the burning of fossil fuels in power plants, factories, homes and car engines – has led to huge discharges of greenhouse gases (like CO2) into the atmosphere. These huge emissions have ramped up the greenhouse effect, leading to a rapid increase in the temperature of the planet. 4

And if you don’t think one degree is much to worry about, see: Why Does A Half-Degree Rise in Temperature Make such a Difference to the Planet?

Unfortunately, this rapid warming can’t be dialled down very easily, because a significant part of the CO2 emitted since 1900, is forecast to remain active in the atmosphere for hundreds, if not thousands of years, after emissions cease. 5 It is this excessive heat-trapping activity, caused by the surge in man-made emissions and other activities (like deforestation) which is sometimes referred to as the enhanced greenhouse effect.

WANT TO MINIMIZE YOUR IMPACT ON THE GREENHOUSE EFFECT?
Read our article: How To Reduce Your Carbon Footprint.

What Are The Latest Greenhouse Gas Emissions Statistics?

The latest figures on global greenhouse gas emissions are as follows: total emissions for the year 2018 amounted to 55.3 GtCO2e (billion tons of CO2 equivalent). Of these, 37.5 GtCO2 came from fossil fuels and industry; 5.13 GtCO2 came from deforestation and land use; 8.8 GtCO2e came from man-made methane emissions; 3 GtCO2e came from man-made nitrous oxide; 1 GtCO2e came from fluorinated gases. [Note: like all statistics on climate change, these are approximate figures only. Sources include: “Greenhouse Gas Concentrations Report” World Meteorological Organization (2019); “Global Carbon Budget 2019”; and institutions like the United Nations. Ses also: Greenhouse Gas Statistics Lack Consistency.]

Who Discovered The “Greenhouse Effect”?

Who discovered the climate mechanism known as the “Greenhouse Effect”? The first possible candidate is the French mathematician Joseph Fourier (1768-1830) who calculated that the Earth was much warmer than it should be. He believed that the discrepancy was due to the planet’s atmosphere acting like an insulator. 6 However, he never used the term greenhouse effect and he didn’t realize that atmospheric gases were responsible.

Next up is Irish physicist John Tyndall (1820-93). Around 1860, he started measuring and comparing the radiative properties of a range of atmospheric gases and discovered it was carbon dioxide and water vapor, that produced the greenhouse effect, rather than nitrogen or oxygen. 7

Meanwhile, across the Atlantic, the American scientist Eunice Newton Foote (1819-88) had already become the first person to discover both the role of CO2 and the mechanism of the greenhouse effect, as outlined in her paper “Circumstances affecting the heat of the sun’s rays” delivered to the American Association for the Advancement of Science conference in 1856. 8

However, Foote didn’t use the term greenhouse effect, and neither did our next candidate, the Swedish chemist Svante Arrhenius (1859-1927). Arrhenius was the first to predict the precise effect of atmospheric carbon dioxide on the Earth’s temperature. He concluded that a doubling of CO2 raises the average global temperature by 5-6°C – an estimate not very far from present-day calculations. Arrhenius wrote about his “hot-house theory” of how gases in the atmosphere trap heat 9

The final candidate is the Swedish meteorologist Nils Gustaf Ekholm (1848-1923) who did indeed use the word “greenhouse” in a 1901 paper on atmospheric warming. However, he didn’t use the full term. What’s more, he lost the plot by thinking that heating the atmosphere would allow us to save the planet from the next Ice Age.

The truth is, the first person to coin the expression “greenhouse effect” was the little-known English physicist John Henry Poynting (1852-1914). 10 He used the term “greenhouse effect” in an Autumn 1909 edition of the Philosophical Magazine, when he commented on an earlier article by the American mathematician, and astronomer Percival Lowell. He wraps the term in quotation marks throughout the paper, suggesting the term is a new one. His exact words were: “Prof. Lowell’s paper in the July number of the Philosophical Magazine marks an important advance in the evaluation of planetary temperatures… [but] he pays hardly any attention to the “blanketing effect” or, as I prefer to call it, the “greenhouse effect” of the atmosphere.” 11

What Are The Main Greenhouse Gases?

Gases that contribute the most to the greenhouse effect do not include the two most abundant gases in the atmosphere: namely, nitrogen (78 percent of the atmosphere) and oxygen (21 percent). These two gases are transparent both to incoming solar radiation (sunlight) and outgoing infrared radiation (heat), and have little greenhouse effect.

The two main greenhouse gases are water vapor (1-4 percent of the atmosphere) and carbon dioxide (0.037 percent).

Greenhouse gases generally allow sunlight through to the Earth but trap heat being given off by the Earth’s surface. They can be entirely natural (water vapor) or produced by a variety of industrial processes (HCFCs), or both (CO2). The main greenhouse gases are:

– Water vapor (H2O)
– Carbon dioxide (CO2)
– Methane (CH4)
– Nitrous oxide (N2O)
– CFC/HCFC/HFC family
– Ozone (O3)

Water Vapor (H2O) 

Water comes from oceans, rivers, reservoirs and wetlands on the surface of the Earth, where – in keeping with the water cycle – it evaporates into a gas and then rises on warm air into the troposphere.

It’s the most prevalent greenhouse gas, and its concentration levels are largely controlled by the temperature of the atmosphere. The warmer the air, the more water it can hold. But the more water vapor in the air, the more heat is absorbed as it escapes from the Earth’s surface, thus further warming the atmosphere. But the warmer the air, the more water it can hold, and so on.

As you can see, although water vapor is entirely natural, its concentration is going to be strongly affected by the rise in other greenhouse gases. Because as these gases trap more heat in the atmosphere, the air becomes warmer and able to hold more vapor. Scientists call this type of amplification a climate feedback.

More than 99 percent of all atmospheric water vapor is found in the troposphere, and accounts for roughly 36–70 percent of the global greenhouse effect.

Carbon Dioxide (CO2)

Carbon dioxide is a naturally occurring greenhouse gas whose production (by the terrestrial biosphere) and absorption (by the oceans) has been going on since time immemorial. Unfortunately, mankind has corrupted the natural carbon cycle by burning coal, peat, petroleum, wood and natural gas in ever increasing quantities. Before the Industrial Revolution atmospheric levels of CO2 were relatively stable at 280ppm (parts per million). In Feb 2020, they stood at 413ppm – a whopping 45 percent increase. 12 13 Paleoclimatologists (scientists who study prehistoric climate) believe that fluctuations in CO2 levels are a fundamental factor influencing climate change. 14

The effect of combustion-produced CO2 on global temperature is known as the Callendar effect, after the English steam engineer and inventor Guy Stewart Callendar (1898-1964). One of the problems about CO2 is that it has a long atmospheric lifetime, ranging from hundreds to thousands of years. 15

Where Does Carbon Dioxide Come From?

Carbon dioxide comes from natural sources as well as man-made activities. It is produced naturally by the respiration of plants and animals, as well as the decay of their remains, and forest fires. However, most atmospheric CO2 (85.5 percent) results from the combustion of fossil fuels such as: oil, coal and natural gas. The biggest culprits are power plants and transportation, as well as domestic heating and cooking appliances.

Carbon Cycle Diagram
Where Carbon Dioxide Comes From. Illustration of the Carbon Cycle showing the various carbon sinks, sources and fluxes of carbon dioxide. Yellow numbers are natural sources, and red are man-made contributions in billions of tons of carbon per year. White numbers indicate stored carbon. Image: © Earthobservatory.nasa.gov

On the negative emissions side, plants and trees absorb carbon dioxide through the mechanism of photosynthesis. This beneficial mechanism has been seriously undermined in recent years by deforestation as large areas of land have been cleared of trees and vegetation to make space for housing and agricultural activities.

This is a triple whammy for global warming, because the CO2-absorption benefits of forest photosynthesis are typically replaced by CO2-emitting chimneys and methane-emitting livestock. What’s more, when forests are cut down, not only does carbon absorption stop, but also the carbon stored in the trees is instantly discharged into the atmosphere as carbon dioxide during the process of wood burning (slash-and-burn). An estimated 9-10 percent of global greenhouse gas emissions are caused by deforestation and forest degradation. 16

Can We Suck CO2 Directly Out Of The Atmosphere?

Yes. Prototype direct air capture machines in Switzerland, manufactured by Climeworks, are already sucking in air and absorbing its CO2 content. However, the technology remains relatively small-scale at present. The CO2 removal plant is roughly one thousand times more efficient than photosynthesis. 17

Methane (CH4)

Methane (CH4) absorbs 28 times more longwave heat energy, over 100 years, than carbon dioxide but 84 times as much over 20 years. Fortunately, it is present in smaller concentrations, so its contribution to the greenhouse effect is nothing like as great. Methane is also quite short-lived, being active in the atmosphere for only about 10-12 years. 18

As of 2019, methane levels are 259 percent higher than pre-industrial levels. Methane accounts for 8.8 billion tons of CO2 equivalent per annum – that’s roughly 17.5 percent of global greenhouse gas emissions. 19

Where Does Methane Come From?

CH4 is produced as cattle digest food, and also during the decomposition of organic plant and animal matter in wetlands and similar locations. Global warming increases atmospheric methane levels through an important feedback loop, since a rise in temperature boosts plant decay.

For instance, scientists are especially worried about the possible emission of large quantities of methane caused by the decomposition of the northern permafrost, where thawing is currently under way. Man-made emissions of methane come from the burning of biomass, as well as seepage from coal mines, gas pipelines and landfill sites. See also: Why Are Methane Levels Rising?

How Much Methane Do Cows Produce?

One cow produces an average of 70-120 kilos of methane per year. With about 1 billion cattle in the world, this means a total output of 700,000 and 1.2 million tonnes of methane, per year.

Nitrous Oxide (N2O)

Nitrous oxide (N2O) is another long-lived gas, having an average atmospheric lifetime of some 110 years. In addition, it has 265-298 times more heat-trapping ability than carbon dioxide.

Where Does Nitrous Oxide Come From?

According to statistics from the IPCC (2014), an estimated 29.5 million tonnes of nitrous oxide (N20) enter the atmosphere each year 20 of which around 62-64 percent stem from natural sources and 36-38 percent from human activities. Natural emissions come from: microbiological processes in the soil (60 percent), denitrification in marine sediments (35 percent) and atmospheric chemical reactions (5 percent). Man-made emissions of nitrous oxide come from: agricultural fertilisers and livestock manure (42 percent), drainage of fertilisers (25 percent), biomass burning (10 percent), fossil fuel-fired power plants and vehicle emissions (10 percent), atmospheric chemical reactions (9 percent) and human waste from sewage plants (5 percent). 21

According to the IPCC, “atmospheric concentrations of carbon dioxide, methane and nitrous oxide are unprecedented in at least the last 800,000 years.” 22

The CFC/HCFC/HFC Family

These man-made chemical compounds (halocarbons) are composed of carbon, chlorine, fluorine, and hydrogen molecules. Volatile derivatives of methane, ethane, and propane, they include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). CFC gases are the most prevalent halocarbons in the atmosphere and the most powerful. CFCs can absorb 10,000 times more heat than carbon dioxide. HFCs are between 1,000 and 3,000 times more potent. Because these chemicals cause ozone depletion in the upper atmosphere, their use is being phased out under the Montreal Protocol (1987). CFCs are already banned, while HCFCs are due to be phased out by 2030 and HFCs by 2050.

Where Do CFCs, HCFCs And HFCs Come From?

Chlorofluorocarbons (CFCs) do not exist naturally. They were created in laboratories for such diverse uses as aerosol propellants, refrigerants and cleaning solvents. CFCs used to account for about a quarter of the enhanced greenhouse effect before they were banned in the late 1990s. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are also entirely synthetic compounds. Developed in the 1980s as replacements for CFCs (brands include Suva refrigerants and Dymel propellants), they are used as blowing agents for foams, as solvents, degreasing agents and cleaning agents.

Ozone (O3)

Ozone in the Stratosphere – which accounts for 90 percent of the total- is formed naturally by chemical reactions involving sunlight and oxygen. It does not contribute to the greenhouse effect.

The remainder is ground level ozone which forms as sunlight reacts with volatile organic compounds (VOCs) and nitrogen oxides (NOx). This type of ozone is a pollutant rather than a greenhouse gas, causing eye, nose and respiratory problems in humans and animals. Not surprisingly, ground level ozone – along with hydrocarbons, NOx and carbon monoxide – is an important component of photochemical smog.

Despite its short lifetime, higher ozone levels are becoming evident during spring and summer in the northern hemisphere due to higher urbanization and greater industrial activity. It accounts for no more than 3–7 percent of the greenhouse effect.

Impact On Earth’s Energy Balance And Global Warming?

Earth's Energy Balance: Sun
How the sun heats up the planet. Source: NASA
  • The temperature of the Earth is regulated by the laws of physics, according to a mechanism popularly known as Earth’s Energy Balance (or Earth’s Energy Budget). Basically: if incoming energy = outgoing energy, then Earth’s temperature is maintained at about 15°C (59°F).
  • For example, Earth’s energy balance helps to prevent the planet from overheating due to incoming sunlight. It does this by constantly radiating solar energy outwards into space. How much energy is radiated? According to the Stefan-Boltzmann Law, the amount of heat radiated from an object is proportional to its temperature. For instance, if temperature doubles, radiated energy increases 16 times. This heat loss process – called “radiative cooling” – is how the planet stops itself from overheating. 23
  • The naturally-occurring greenhouse effect plays an important role in Earth’s energy balance. Without it, almost all the heat radiated by the planet would disappear into deep space and temperatures on the Earth’s surface would fall to minus 18°C (0°F). But with the greenhouse effect, enough of the heat given off by the Earth is trapped and redirected towards the ground, maintaining the Earth’s surface at a cosy 15°C temperature.
  • However, as we have seen, during the past 150 years or so, mankind’s insatiable appetite for industrial development and growth has destabilized Earth’s energy balance. How? By pumping huge amounts of greenhouse gas into the atmosphere, thus supercharging the greenhouse effect and causing an abnormal amount of heat to be retained by the Earth.
  • In line with Earth’s Energy Balance which stipulates that incoming energy must equal outgoing energy, in order to maintain a steady 15°C (59°F), the extra heat retained by the Earth causes a rise in the temperature of the planet. The average temperature of the planet is already 1°C warmer than it was in 1900, and might easily rise another two or three degrees Celsius before the end of the century, causing possibly irreversible damage to our biosphere.

Approximate Radiation Inflows and Outflows From Outer Space to Earth and Back

How Earth's climate system works: infographic
Image shows the various sources of incoming and outgoing radiation energy to and from Planet Earth. Source: IPCC 4th Assessment Report (2007), Working Group 1, The Physicals Science Basis.
Why do we use the term “average temperature”?

The idea of there being an average temperature for the entire globe may seem strange. After all, right now, the highest and lowest temperatures on Earth are likely to differ by more than 55°C (100°F). Also, temperatures fluctuate from night to day and in relation to the opposing seasons of the Northern and Southern Hemispheres. So, to talk of an “average” temperature, seems like nonsense. Even so, a global average temperature is a convenient tool for detecting and tracking changes in Earth’s climate system, so as to understand the radiative stresses on the hydrosphere and cryosphere, and the effect this may be having on the food web, animal habitats and biodiversity.

How Soon Before Temperatures Get Much Hotter?

Raised levels of greenhouse gases don’t change the average surface temperature overnight. It may take decades or a century or two before the full effects become known. This time lag – known as “thermal inertia” – is due to the fact that most of the excess global heat is absorbed by the oceans. (For more on this, see: How Do Oceans Influence Climate? and also: Effects of Global Warming on Oceans.)

Thermal inertia slows things down, but it can’t prevent the warming from happening. According to the IPCC, the most likely scenario is that, by 2100, the temperature of Planet Earth will increase by 3°C above pre-industrial levels (1850-1900). But in the absence of a coherent climate change mitigation strategy, it might increase by twice this amount.

References

greenhouse effect, greenhouse gas, greenhouse gases, carbon dioxide, water vapor, nitrous oxide, methane

  1. What is the Greenhouse Effect? []
  2. Analysis: Why scientists think 100% of global warming is due to humans. []
  3. Enhanced Greenhouse Effect. Australian Academy of Science []
  4. IPCC Special Report on Global Warming of 1.5°C: Summary for policymakers[]
  5. Atmospheric Lifetime of Fossil Fuel Carbon Dioxide”. Annual Review of Earth and Planetary Sciences. 37 (1): 117–34. Archer, David; Eby, Michael; Brovkin, Victor; Ridgwell, Andy; Cao, Long; Mikolajewicz, Uwe; Caldeira, Ken; Matsumoto, Katsumi; Munhoven, Guy; Montenegro, Alvaro; Tokos, Kathy (2009). []
  6. “How Joseph Fourier discovered the greenhouse effect.” Peter Lynch. Irish Times March 21, 2019. []
  7. John Tyndall, Heat considered as a Mode of Motion. 1863, 1873. []
  8. “This Lady Scientist Defined the Greenhouse Effect but Didn’t Get the Credit, Because of Sexism”. Leila McNeill. Smithsonian. []
  9. “Worlds in the Making.” Svante Arrhenius. 1903.[]
  10. “Who first coined the term Greenhouse Effect?” Professor Steve Easterbrook, University of Toronto. Serendipity. August 18, 2015. []
  11. LXXIV. On Prof. Lowell’s method for evaluating the surface-temperatures of the planets; with an attempt to represent the effect of day and night on the temperature of the earth”. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 14 (84): 749–760. Poynting, J.H. (1907). []
  12. Climate Change: Atmospheric Carbon Dioxide. Climate.gov []
  13. Carbon dioxide levels hit record peak in May 2019.” NOAA.gov []
  14. The rise in global atmospheric CO2, surface temperature, and sea level from emissions traced to major carbon producers.” Climatic Change 144, 579–590 (2017). Ekwurzel, B., Boneham, J., Dalton, M.W. et al. []
  15. Greenhouse Gases: How Long Will They Last? Environmental Defense Fund[]
  16. What is the Relationship Between Deforestation and Climate Change?” Rainforest Alliance. August 12, 2018. See also: By the Numbers: The Value of Tropical Forests in the Climate Change Equation. WRI. []
  17. “This Machine Just Started Sucking CO2 Out Of The Air To Save Us From Climate Change.” FastCompany.com May 31, 2017. []
  18. IPCC Fourth Assessment Report. Climate Change 2007: Working Group I: The Physical Science Basis 2.3.2 Atmospheric Methane. []
  19.  See also: “Very Strong Atmospheric Methane Growth in the 4 Years 2014–2017: Implications for the Paris Agreement.” E. G. Nisbet M. R. Manning E. J. Dlugokencky R. E. Fisher D. Lowry S. E. Michel C. Lund Myhre S. M. Platt G. Allen P. Bousquet R. Brownlow M. Cain J. L. France. 05 February 2019 []
  20. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (2014), cited in “Global Greenhouse Gas Emissions Data” U.S. EPA. []
  21. “Nitrous Oxide Emissions”. U.S. Environmental Protection Agency (EPA). []
  22. IPCC Fifth Assessment Report. 2014.[]
  23. “Climate and Earth’s Energy Budget.” NASA Earth Observatory. Rebecca Lindsey. January 14, 2009. []
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