The greenhouse effect happens when gases in a planet's atmosphere trap heat, preventing the planet from losing heat to space. This raises the planet's surface temperature. Surface heating can come from an internal heat source, like Jupiter, or from an external source, such as a star. On Earth, the Sun sends out shortwave radiation (sunlight), which passes through greenhouse gases to warm Earth's surface. In return, Earth's surface releases longwave radiation, which is mostly absorbed by greenhouse gases. This slows how quickly Earth can cool down.
Without the greenhouse effect, Earth's average surface temperature would be about −18 °C (−0.4 °F). This is much colder than the 20th century average of about 14 °C (57 °F). In addition to naturally occurring greenhouse gases, burning fossil fuels has increased carbon dioxide and methane in the atmosphere. This has caused global warming of about 1.2 °C (2.2 °F) since the Industrial Revolution. Since 1981, Earth's average surface temperature has risen by 0.18 °C (0.32 °F) each decade.
All objects with a temperature above absolute zero emit thermal radiation. The wavelengths of this radiation differ because the Sun and Earth have different surface temperatures. The Sun's surface is 5,500 °C (9,900 °F), so it sends out most of its energy as shortwave radiation in near-infrared and visible wavelengths (sunlight). Earth's surface is much cooler, so it emits longwave radiation in mid- and far-infrared wavelengths. A gas is a greenhouse gas if it absorbs longwave radiation. Earth's atmosphere absorbs 23% of incoming shortwave radiation but absorbs 90% of longwave radiation from the surface. This causes the atmosphere to store energy and warm Earth's surface.
The greenhouse effect was first proposed in 1824 by Joseph Fourier. Later, Claude Pouillet in 1827 and 1838 provided more evidence. In 1856, Eunice Newton Foote showed that air with water vapor warms more than dry air, and the warming effect is even stronger with carbon dioxide. The term "greenhouse" was first used to describe this phenomenon by Nils Gustaf Ekholm in 1901.
Definition
The greenhouse effect on Earth is the process where certain substances in the atmosphere, such as greenhouse gases, clouds, and some aerosols, absorb heat that is radiated from Earth's surface and other parts of the atmosphere. This absorption of heat helps regulate Earth's temperature.
The enhanced greenhouse effect refers to the increase in this natural process caused by higher levels of greenhouse gases in the atmosphere. These increased levels are the result of human activities, such as burning fossil fuels and cutting down forests.
Terminology
The term greenhouse effect is similar to how greenhouses work. Both greenhouses and the greenhouse effect keep heat from sunlight, but they do so in different ways. Greenhouses keep heat mainly by stopping the movement of air. In contrast, the greenhouse effect keeps heat by limiting how heat moves as radiation through the air and slowing how much heat escapes into space.
History of discovery and investigation
The greenhouse effect was first discussed in 1824 by Joseph Fourier, even though it wasn’t called that at the time. In 1827 and 1838, Claude Pouillet added more evidence to support the idea. In 1856, Eunice Newton Foote showed that air with water vapor warms more than dry air, and air with carbon dioxide warms even more. She concluded that an atmosphere with carbon dioxide would make Earth very hot.
John Tyndall was the first to measure how gases absorb and release heat. Starting in 1859, he found that only a small part of the atmosphere causes the warming effect, mostly due to water vapor. Small amounts of hydrocarbons and carbon dioxide also contributed. In 1896, Svante Arrhenius made the first calculation about how much Earth’s temperature might rise if carbon dioxide levels doubled. The term "greenhouse effect" was first used by Nils Gustaf Ekholm in 1901.
In 1896, Arrhenius used observations about how the Moon’s light interacts with the atmosphere to estimate how a decrease in carbon dioxide might cool Earth. He noted that cooler air would hold less water vapor, another greenhouse gas, and that this would cause more snow and ice, which reflect sunlight and further cool Earth. He calculated that cutting carbon dioxide levels in half could lead to an ice age, while doubling them might raise Earth’s temperature by 5–6 degrees Celsius.
Studies by scientists like Tyndall, Lecher, Pretner, Keller, Roentgen, and Arrhenius showed that carbon dioxide and water vapor in the atmosphere strongly absorb and hold heat. Oxygen, nitrogen, and argon absorb and hold much less heat. This means that carbon dioxide and water vapor act like a layer that traps heat around Earth. The overall effects of having more or less carbon dioxide and water vapor in the atmosphere can be summarized as follows:
Measurement
Matter releases thermal radiation at a rate that increases rapidly as its temperature rises. Some of the radiation from Earth's surface is absorbed by greenhouse gases and clouds. If this absorption did not happen, Earth's surface would have an average temperature of −18 °C (−0.4 °F). Instead, because some radiation is absorbed, Earth's average surface temperature is about 15 °C (59 °F). This means the greenhouse effect raises Earth's temperature by 33 °C (59 °F).
Thermal radiation is measured by how much energy it carries, usually in watts per square meter (W/m²). Scientists also study the greenhouse effect by comparing how much longwave thermal radiation leaves Earth's surface to how much reaches space. On average, 398 W/m² of longwave radiation leaves Earth's surface, but only 239 W/m² reaches space. This difference means the greenhouse effect reduces the amount of energy leaving Earth by 159 W/m². The greenhouse effect can also be described as 40% of the longwave radiation that leaves Earth's surface but does not escape into space.
Whether measured as a temperature change or as a change in energy flow, the greenhouse effect refers to the same process.
Role in climate change
The increased greenhouse effect caused by human activities adding more greenhouse gases is called the enhanced greenhouse effect. This increase in heat trapped in the atmosphere due to human actions has been observed through measurements from tools like ARGO and CERES over the 21st century. Scientists have directly confirmed this rise, which is mainly due to higher levels of carbon dioxide in the air.
Carbon dioxide is released through burning fossil fuels, cement production, and tropical deforestation. Data from the Mauna Loa Observatory shows that carbon dioxide levels rose from about 313 parts per million (ppm) in 1960 to over 400 ppm in 2013. Today’s carbon dioxide levels are higher than the highest levels ever recorded in Earth’s history, as shown by ice core data, which previously showed maximum levels of about 300 ppm.
Ice core records from the past 800,000 years show that carbon dioxide levels ranged between about 180 ppm and 270 ppm before industrial times. Scientists who study past climates believe changes in carbon dioxide levels are a key factor in climate changes over this time period.
Energy balance and temperature
Hotter objects emit radiation with shorter wavelengths. The Sun emits shortwave radiation as sunlight, which includes ultraviolet, visible light, and near-infrared radiation. The Earth and its atmosphere emit longwave radiation, also called thermal infrared or terrestrial radiation.
Sunlight is reflected and absorbed by Earth and its atmosphere. The atmosphere and clouds reflect about 23% and absorb 23%. The surface reflects 7% and absorbs 48%. Overall, Earth reflects about 30% of incoming sunlight and absorbs the rest (240 W/m²).
The Earth and its atmosphere emit longwave radiation, which is sometimes called thermal radiation. Outgoing longwave radiation (OLR) is the radiation from Earth and its atmosphere that passes through the atmosphere and into space.
The greenhouse effect can be seen in graphs showing Earth's outgoing longwave radiation at different frequencies. The area between the curve for radiation from Earth's surface and the curve for outgoing radiation shows the size of the greenhouse effect.
Different substances reduce the amount of radiation reaching space at different frequencies. Carbon dioxide is responsible for a dip in outgoing radiation at around 667 cm⁻¹ (equivalent to a wavelength of 15 microns).
Each layer of the atmosphere with greenhouse gases absorbs some longwave radiation from lower layers. It also emits longwave radiation in all directions, both upward and downward, in balance with the radiation it absorbs. This reduces heat loss and increases warmth below. More greenhouse gases increase absorption and emission, trapping more heat at the surface and in lower layers.
The power of outgoing longwave radiation from a planet matches its effective temperature. The effective temperature is the temperature a planet would need to have to radiate the same amount of energy as a blackbody (a perfect emitter).
This concept compares Earth's outgoing radiation to its surface radiation:
– Emissions to space: Earth's effective temperature is −18 °C (0 °F), based on its outgoing longwave radiation.
– Emissions from surface: Earth's surface temperature is about 16 °C (61 °F), which is 34 °C (61 °F) warmer than its effective temperature.
Earth's surface temperature is often reported as the average near-surface air temperature, about 15 °C (59 °F), which is 33 °C (59 °F) warmer than its effective temperature.
Energy flux is the rate of energy flow per unit area, measured in W/m² (joules per square meter per second). Most climate discussions use global values, which are total energy flows divided by Earth's surface area (5.1 × 10¹⁴ m²).
Radiation fluxes are important because radiative transfer is the only process that exchanges energy between Earth and space.
A planet's temperature depends on the balance between incoming and outgoing radiation. If incoming radiation exceeds outgoing, the planet warms. If outgoing exceeds incoming, the planet cools. Earth tends toward radiative equilibrium, where outgoing radiation equals absorbed incoming radiation.
Earth's energy imbalance (EEI) is the difference between absorbed incoming sunlight and outgoing longwave radiation. This imbalance drives surface temperature changes. A UN report states the EEI is "the most critical number defining the prospects for global warming." One study says the EEI "represents the most fundamental metric for global climate change."
In 2015, Earth's EEI was about 0.7 W/m², showing Earth was gaining heat and warming. By 2024, the trend had doubled. Over 90% of retained energy warms the oceans, with smaller amounts heating land, atmosphere, and ice.
A simple model assumes steady conditions, but real-world factors like the day/night cycle, seasons, and weather create complexity. Solar heating occurs only during the day, but the atmosphere cools slightly at night. Thermal inertia in Earth's climate system resists rapid temperature changes. Diurnal temperature changes decrease with height in the atmosphere.
Effect of lapse rate
In the lower part of the atmosphere, called the troposphere, air temperature decreases as altitude increases. The speed at which temperature changes with height is known as the lapse rate.
On Earth, the average temperature decrease is about 6.5°C per kilometer (or 3.6°F per 1,000 feet), though this can vary. This temperature change happens because of convection. Warm air near Earth’s surface rises, expands, and cools. At the same time, cooler air above sinks, compresses, and warms. This movement creates a temperature difference between the surface and higher altitudes.
This temperature difference is important for the greenhouse effect. If the lapse rate were zero (meaning temperature stayed the same at all heights), the greenhouse effect would not exist.
Greenhouse gases make the air near Earth’s surface mostly unable to let longwave radiation pass through. However, higher in the atmosphere, where air is less dense and has less water vapor, longwave radiation can escape more easily.
For each wavelength of longwave radiation, the radiation that reaches space is emitted by a specific layer of the atmosphere. The strength of this radiation depends on the average temperature of that layer. This temperature is called the effective emission temperature.
Each wavelength of radiation also has an effective emission altitude, which is the average height of the layer that emits it. These temperatures and altitudes change depending on the wavelength. This can be seen in graphs showing radiation emitted to space.
Earth’s surface emits longwave radiation with wavelengths between 4 and 100 microns. Greenhouse gases, which allow sunlight to pass through, absorb some of these wavelengths more strongly.
Near Earth’s surface, the atmosphere blocks most longwave radiation, so heat loss from the surface mainly happens through evaporation and convection. However, higher in the atmosphere, radiative energy loss becomes more important because water vapor, a key greenhouse gas, is less abundant.
Instead of thinking of longwave radiation leaving Earth as coming directly from the surface, it is more accurate to think of it as being emitted by a layer in the middle of the troposphere. This layer is connected to the surface through the lapse rate. The temperature difference between the surface and this layer explains the difference between heat emitted by the surface and heat emitted to space, which is the greenhouse effect.
Infrared absorbing constituents in the atmosphere
A greenhouse gas (GHG) is a gas that helps trap heat by slowing the escape of longwave radiation from a planet’s atmosphere. These gases are the main reason for the greenhouse effect in Earth’s energy system.
Gases that can absorb and release longwave radiation are called infrared active and act as greenhouse gases.
Most gases with molecules made of two different atoms (like carbon monoxide, CO) and all gases with three or more atoms (such as water vapor, H₂O, and carbon dioxide, CO₂) are infrared active. This happens because when these molecules vibrate, their structure changes in a way that allows them to interact with heat.
Gases with only one atom (like argon, Ar) or two identical atoms (like nitrogen, N₂, and oxygen, O₂) are not infrared active. These gases do not absorb or emit longwave radiation because their molecules are symmetrical and lack a dipole moment. These gases make up more than 99% of Earth’s dry atmosphere.
Greenhouse gases absorb and emit longwave radiation within specific wavelength ranges, called spectral lines or bands.
When greenhouse gases absorb radiation, they transfer the energy to the surrounding air as heat (the movement of gas molecules). This energy moves from greenhouse gas molecules to other molecules through collisions.
Greenhouse gases do not "re-emit" photons after absorbing them. Because gas molecules collide billions of times each second, any energy gained from absorbing a photon is shared with other molecules before a new photon can be emitted.
In a separate process, greenhouse gases emit longwave radiation at a rate determined by air temperature. This energy is either absorbed by other greenhouse gas molecules or leaves the atmosphere, cooling it.
Effect on air: Air is warmed by latent heat (heat released when water vapor condenses into droplets), thermals (warm air rising from the surface), and sunlight absorbed in the atmosphere. Air is cooled when greenhouse gases and clouds emit longwave radiation. In the troposphere, greenhouse gases usually cool the air more than they warm it, emitting more heat than they absorb. On average, warming and cooling balance each other, keeping the atmosphere’s temperature stable.
Effect on surface cooling: Longwave radiation moves both upward and downward in the atmosphere. These opposing flows reduce how much heat the surface can lose through radiation. Latent heat and thermals provide non-radiative cooling that partially offsets this reduction, but surface cooling is still reduced for a given surface temperature.
Effect on top-of-atmosphere (TOA) energy balance: Greenhouse gases reduce the amount of longwave radiation that escapes into space for a given surface temperature. This changes the energy balance at the top of the atmosphere (TOA). To balance incoming sunlight with outgoing energy, the surface temperature must be higher than Earth’s effective temperature (the temperature associated with emissions to space). When considering the warming effect of greenhouse gases, it is important to focus on the TOA energy budget, not the surface energy budget.
Clouds and aerosols: Clouds and aerosols both cool the planet by reflecting sunlight and warm it by trapping heat. On average, clouds have a strong cooling effect, but their overall impact depends on factors like type, height, and optical properties. For example, thin cirrus clouds can warm the atmosphere. Clouds can absorb and emit infrared radiation, influencing how heat moves through the atmosphere.
While the effect of greenhouse gases on energy balance is well understood, the impact of aerosols on energy balance is less certain. This uncertainty depends heavily on complex computer models that are difficult to test fully.
Basic formulas
A given amount of thermal radiation has an associated effective radiating temperature, also called the effective temperature. This is the temperature a black body (a perfect absorber and emitter of radiation) would need to have in order to emit that amount of thermal radiation. The overall effective temperature of a planet is calculated using the formula:
where OLR is the average flux (power per unit area) of outgoing longwave radiation emitted to space, and σ is the Stefan-Boltzmann constant. Similarly, the effective temperature of the surface is calculated using the formula:
where SLR is the average flux of longwave radiation emitted by the surface. (OLR is a standard abbreviation. SLR is used here to describe the flux of surface-emitted longwave radiation, though there is no official abbreviation for this term.)
The IPCC reports the greenhouse effect, G, as being 159 W/m². This value represents the difference between the flux of longwave thermal radiation that leaves the surface and the flux of outgoing longwave radiation that reaches space:
The greenhouse effect can also be described using the normalized greenhouse effect, g̃, defined as:
The normalized greenhouse effect is the fraction of thermal radiation emitted by the surface that does not reach space. Based on IPCC data, g̃ = 0.40. This means 40% less thermal radiation reaches space compared to the amount that leaves the surface.
Sometimes the greenhouse effect is measured as a temperature difference. This temperature difference is closely connected to the values described above.
When the greenhouse effect is expressed as a temperature difference, ΔT_GHE, it refers to the difference between the effective temperature associated with thermal radiation emissions from the surface and the effective temperature associated with emissions to space:
Informal discussions about the greenhouse effect often compare the actual surface temperature to the temperature the planet would have without greenhouse gases. However, in formal scientific discussions, the size of the greenhouse effect is usually measured using the formula above. This formula refers to the effective surface temperature rather than the actual surface temperature and compares the surface to the top of the atmosphere, not to a hypothetical situation.
The temperature difference, ΔT_GHE, shows how much warmer a planet's surface is compared to the planet's overall effective temperature.
Earth's top-of-atmosphere (TOA) energy imbalance (EEI) is the amount by which incoming radiation exceeds outgoing radiation:
where ASR is the average flux of absorbed solar radiation. ASR can be calculated as:
where A is the planet's albedo (reflectivity) and MSI is the mean solar irradiance at the top of the atmosphere.
The radiative equilibrium temperature of a planet can be expressed as:
A planet's temperature tends to move toward a state of radiative equilibrium, where the TOA energy imbalance is zero (EEI = 0). When this happens, the planet's overall effective temperature is given by:
This concept is important because it shows the effective temperature a planet will tend toward.
If we know the effective temperature, T_eff, and the greenhouse effect, we can determine the planet's average surface temperature.
This is why the greenhouse effect is important: it is one of the key factors that help determine a planet's average surface temperature.
Typically, a planet is close to radiative equilibrium, with incoming and outgoing energy being nearly balanced. Under these conditions, the planet's equilibrium temperature depends on the mean solar irradiance and the planet's albedo (how much sunlight is reflected back to space instead of being absorbed).
The greenhouse effect measures how much warmer the surface is compared to the planet's overall effective temperature. Using the definition of ΔT_GHE, the effective surface temperature, T_surface,eff, is:
The relationship between T_surface,eff and T_eff can also be expressed using G or g̃.
Thus, the principle that a larger greenhouse effect corresponds to a higher surface temperature, assuming other factors (like those determining T_eff) remain constant, is true by definition.
Note that the greenhouse effect influences the planet's overall temperature alongside its tendency to move toward radiative equilibrium.
Misconceptions
Sometimes people misunderstand how the greenhouse effect works and how it causes temperatures to rise.
A common mistake is called the surface budget fallacy. This mistake happens when someone believes that increasing carbon dioxide (CO₂) in the atmosphere can only cause warming by increasing the amount of heat radiated downward to Earth’s surface. This idea assumes that the atmosphere near the surface is already very thick at trapping heat, so more CO₂ would not make a difference. However, this focus on the surface is incorrect. What matters most is the balance of energy at the top of the atmosphere. When CO₂ increases, it reduces the amount of heat that escapes into space (called outgoing longwave radiation, or OLR). This creates an imbalance in the energy at the top of the atmosphere, which leads to warming. Scientists like Callendar (1938) and Plass (1959) studied the surface energy balance, but later research by Manabe in the 1960s showed that the top-of-atmosphere energy balance is the key to understanding warming.
Some people who do not believe in the greenhouse effect claim that it violates the second law of thermodynamics because they think greenhouse gases send heat from the cooler atmosphere to the warmer Earth’s surface. This is a misunderstanding. Heat transfer through radiation works by considering the net flow of energy after radiation moves in both directions. Normally, heat flows from the warmer surface to the cooler atmosphere and space. Greenhouse gases emit radiation downward to the surface, but this is part of the normal process of heat transfer. This downward radiation does not add heat to the surface; instead, it reduces the amount of heat that escapes upward, slowing the cooling process.
Simplified models
Simplified models are sometimes used to help explain how the greenhouse effect works and how it influences Earth's surface temperature.
In one simple model, the atmosphere is treated as a single layer that exchanges heat radiation with the ground and space. More complex models add more layers or include the movement of air, called convection.
One way to simplify the model is to assume all heat radiation leaving Earth comes from a height where the air temperature matches Earth's overall effective temperature for emissions, written as $ T_{mathrm{eff}} $. Some scientists call this height the effective radiating level (ERL). They explain that as carbon dioxide (CO₂) levels rise, the ERL must move higher to keep the same amount of CO₂ above that height.
This method is less precise than considering how radiation wavelengths change with altitude. However, it can still help explain how the greenhouse effect becomes stronger as greenhouse gas levels increase.
Earth's overall equivalent emission altitude has been rising by about 23 meters (75 feet) every decade. This trend is linked to a global surface temperature increase of 0.12°C (0.22°F) per decade between 1979 and 2011.
Related effects on Earth
Scientists have noticed that sometimes a negative greenhouse effect happens in parts of Antarctica. When there is a strong temperature inversion, meaning the air above is warmer than the ground below, greenhouse gases can cause more heat to escape into space instead of being trapped. This means the amount of heat leaving Earth's surface is greater than the heat coming from the surface. In these cases, the greenhouse effect has a negative value.
Most scientists think that over time, a runaway greenhouse effect will happen because the Sun becomes brighter as it ages. In about one billion years, the Sun will be 10% brighter, causing Earth's surface temperature to rise to 47°C (117°F) unless Earth reflects more sunlight. This would lead to rapid heating, boiling oceans, and Earth becoming a planet with conditions similar to Venus, where the atmosphere is extremely hot and thick.
Bodies other than Earth
In the Solar System, besides Earth, at least two other planets and one moon also have a greenhouse effect.
The greenhouse effect on Venus is very strong. It makes the surface temperature as high as 735 K (462 °C; 863 °F). This happens because Venus has a very thick atmosphere made mostly of carbon dioxide, about 97%.
Even though Venus is about 30% closer to the Sun, it absorbs less sunlight than Earth. Venus reflects about 77% of sunlight, while Earth reflects about 30%. Without the greenhouse effect, Venus’s surface would be much colder, around 232 K (−41 °C; −42 °F). This shows that being closer to the Sun is not the reason Venus is warmer than Earth.
On Venus, the high pressure in the atmosphere causes carbon dioxide to absorb light across a wide range of colors, not just specific ranges as it does on Earth.
Scientists have long believed that Venus experienced a runaway greenhouse effect involving carbon dioxide and water vapor. This idea is still widely accepted. Venus’s atmosphere is now 96% carbon dioxide, and its surface pressure is about the same as being 900 meters underwater on Earth. Venus may have had oceans in the past, but they would have boiled away as the surface temperature rose to 735 K (462 °C; 863 °F).
Mars has about 70 times more carbon dioxide than Earth, but its greenhouse effect is small, only about 6 K (11 °F). This is because Mars has very little water vapor and a very thin atmosphere.
The same calculations that explain warming on Earth also explain Mars’s temperature, based on its atmosphere’s composition.
Saturn’s moon Titan has both a greenhouse effect and an anti-greenhouse effect. Gases like nitrogen, methane, and hydrogen in Titan’s atmosphere help trap heat, raising the surface temperature by 21 K (38 °F) compared to what it would be without these gases.
Normally, nitrogen and hydrogen do not absorb heat, but on Titan, they do because of high pressure, thick atmosphere, and long wavelengths of radiation from Titan’s cold surface.
A high-altitude haze on Titan absorbs sunlight but lets heat pass through, creating an anti-greenhouse effect that lowers the temperature by about 9 K (16 °F).
Together, these effects make Titan’s surface 12 K (22 °F) warmer than it would be without an atmosphere.
The size of the greenhouse effect on different planets cannot be predicted just by comparing the amount of greenhouse gases in their atmospheres. Other factors, like atmospheric pressure, also influence the greenhouse effect.
Atmospheric pressure affects how much heat greenhouse gases can trap. High pressure allows gases to absorb more heat, while low pressure reduces this ability.
This happens because higher pressure causes molecules to collide more often, which broadens the range of light they can absorb. On Earth, each air molecule near the surface collides about 7 billion times per second. This rate decreases at higher altitudes, where pressure and temperature are lower.
On other planets, high pressure (like on Venus) makes greenhouse gases more effective at trapping heat, while low pressure (like on Mars) makes them less effective.