Greenhouse effect

The greenhouse effect occurs when heat-trapping gases in a planet's atmosphere prevent the planet from losing heat to space, raising its surface temperature. On Earth, the Sun emits shortwave radiation (sunlight) that passes through these gases to warm the planet's surface. The Earth then emits longwave radiation, which is mostly absorbed by the greenhouse gases, slowing the cooling process.

Without the greenhouse effect, Earth would be much colder, with an average temperature of about −18 °C (−0.4 °F) instead of the 20th century average of 14 °C (57 °F). However, human activities, especially the burning of fossil fuels, have increased amounts of carbon dioxide and methane in the atmosphere. This has led to global warming of about 1.2 °C (2.2 °F) since the Industrial Revolution.

All objects above absolute zero give off heat as radiation. The Sun, being very hot, emits shortwave radiation, while Earth, being cooler, emits longwave radiation. Greenhouse gases specifically absorb longwave radiation, trapping heat and warming the planet. This natural process has been intensified by human actions, changing our climate.

Definition

The greenhouse effect is how certain gases and particles in Earth’s atmosphere trap heat. These gases, like those sometimes called greenhouse gases, along with clouds and some tiny particles called aerosols, catch the heat that Earth’s surface gives off and keep our planet warmer.

When people add more of these gases to the air, it makes the natural greenhouse effect even stronger. This is called the enhanced greenhouse effect, and it changes Earth’s climate.
Greenhouse gases clouds aerosols

Terminology

The term greenhouse effect is named after greenhouses. Both keep heat from the sunlight, but they do it in different ways. Greenhouses stay warm mostly by stopping air from moving. The greenhouse effect, however, keeps heat by slowing down how quickly it escapes into space.

History of discovery and investigation

See also: History of climate change science

Smart thinkers began to understand the greenhouse effect in the early 1800s. In 1824, Joseph Fourier suggested that certain gases in the air could trap heat. Later, Claude Pouillet and Eunice Newton Foote added more evidence, showing that gases like water vapor and carbon dioxide help keep Earth warmer.

John Tyndall measured how different gases trap heat and found that even a small amount of water vapor and carbon dioxide make a big difference. In 1896, Svante Arrhenius predicted how much warmer Earth would get if there was more carbon dioxide in the air. The idea was later named the "greenhouse effect" by Nils Gustaf Ekholm in 1901.

Measurement

The greenhouse effect can be measured by looking at how much warmer Earth is because of it. Without greenhouse gases, Earth's average temperature would be about −18 °C, but because these gases trap heat, the average temperature is around 15 °C—a difference of 33 °C.

Scientists also measure the greenhouse effect by looking at energy. Earth’s surface sends out about 398 watts of heat energy for every square meter, but only about 239 watts reach space. This shows that the greenhouse effect traps a lot of energy, about 159 watts per square meter.

Role in climate change

Main articles: Climate change and Earth's energy budget

The greenhouse effect becomes stronger when we add more gases to the atmosphere from human activities. This is called the enhanced greenhouse effect. Scientists have seen this happen by measuring changes in the amount of heat that stays on Earth. The main reason for this increase is more carbon dioxide (CO₂) in the air.

We make CO₂ when we burn fuels like coal and oil, and when we make things like cement or cut down forests. Since 1960, the amount of CO₂ has gone up a lot. It started at about 313 parts per million and passed 400 parts per million in 2013. This is more CO₂ than what we find in old ice records from the past 800,000 years. Changes in CO₂ levels have been a big part of how Earth’s climate has changed over time.

Energy balance and temperature

Sunlight, made of ultraviolet, visible light, and near-infrared radiation, warms the Earth. About 30% of this sunlight is reflected back into space, while the rest is absorbed, heating the planet.

The Earth also sends heat back into space as longwave radiation. Greenhouse gases in the atmosphere trap some of this heat, keeping the planet warmer. This is why Earth’s surface is much warmer than it would be without these gases. The balance between incoming and outgoing energy helps decide Earth’s temperature. When more energy comes in than goes out, the planet warms up.

Effect of lapse rate

Further information: Lapse rate

In the lower part of Earth's atmosphere, called the troposphere, the air gets colder as you go higher up. This change in temperature with height is known as the lapse rate.

The air near the ground gets warmed by the Earth. As it rises, it spreads out and cools down. At the same time, other air moves down, gets squeezed, and warms up. This movement creates a pattern where temperatures change with height, which is very important for the greenhouse effect. If the temperature didn’t change with height, there would be no greenhouse effect.

Greenhouse gases in the air trap heat. The air close to the ground is thick with these gases and cannot let much heat escape easily. Higher up, where the air is thinner, more heat can escape into space. This difference in how heat escapes at different heights helps explain why Earth’s surface stays warmer than it would without these gases.

Infrared absorbing constituents in the atmosphere

Greenhouse gases

Main article: Greenhouse gas

A greenhouse gas is a special kind of gas in the air that helps keep heat close to the Earth. These gases trap heat that would otherwise escape into space, making our planet warmer. Most of the greenhouse effect on Earth comes from these gases.

Some gases can trap heat because they can absorb and send out infrared radiation. Gases with molecules that have two different atoms, like carbon monoxide (CO), and all gases with three or more atoms, such as water (H₂O) and carbon dioxide (CO₂), can trap heat. Gases with just one atom, like argon, or two identical atoms, like nitrogen (N₂) and oxygen (O₂), do not trap heat very well because their molecules are balanced and do not change shape when they vibrate.

These greenhouse gases absorb heat and share it with the air around them. They also send out heat, which can either stay in the atmosphere or escape into space, depending on the temperature of the air. This process helps control how warm or cool the Earth is.

Basic formulas

The greenhouse effect is how certain gases in a planet's atmosphere trap heat, making the planet warmer. Imagine the atmosphere acting like a blanket that keeps the planet's surface temperature higher than it would be without these gases.

Scientists measure the greenhouse effect in different ways. One way is to look at how much heat the planet's surface gives off compared to how much heat escapes into space. Another way is to calculate the temperature difference — how much warmer the surface is because of these heat-trapping gases. For Earth, this effect helps keep our planet at a temperature that supports life.

Misconceptions

Sometimes people have wrong ideas about how the greenhouse effect works. One common mistake is thinking that more carbon dioxide (CO₂) can only warm the Earth by sending extra heat back to the ground. But this isn't the whole story. Even if the air near the ground already traps heat well, adding more CO₂ stops some of the heat from escaping into space. This creates an imbalance that makes the whole planet warmer.

Another misunderstanding is that greenhouse gases can send heat from the cooler air to the warmer ground, which seems impossible. But this isn't what happens. Heat naturally moves from the warm ground up to the cooler air and space. Greenhouse gases do send some heat back down, but this just slows down how fast the ground loses heat overall.

Simplified models

Further information: Idealized greenhouse model

Simplified models help us understand how the greenhouse effect works and how it changes the temperature on Earth’s surface. These models imagine the atmosphere as a single layer that exchanges heat with the ground and space. Some models add more layers or include movement of air to make the picture more detailed.

One way to simplify things is to think of all the heat leaving Earth as coming from a certain height in the sky where the temperature matches Earth’s average temperature. As there are more gases that trap heat, this height moves higher. This idea helps explain why more of these gases make Earth warmer. Scientists have noticed that this height has been rising, matching a slow warming of Earth’s surface over many years.

Related effects on Earth

Scientists have found that sometimes, in certain parts of Antarctica, the usual greenhouse effect can work in reverse. When the air above is warmer than the ground, greenhouse gases can actually help cool the area by letting more heat escape into space. This is called a negative greenhouse effect.

Another interesting idea is the runaway greenhouse effect. This is when greenhouse gases build up so much that they trap heat continuously, making the planet’s temperature rise quickly. This has happened on planets like Venus, where the surface is extremely hot.

Bodies other than Earth

In the Solar System, apart from the Earth, at least two other planets and a moon also have a greenhouse effect.

Venus

The greenhouse effect on Venus is very strong, making its surface extremely hot — as high as 735 K (462 °C; 863 °F). This is because Venus has a very thick atmosphere made up mostly of carbon dioxide.

Even though Venus is closer to the Sun than Earth, it doesn’t absorb as much sunlight because its thick clouds reflect most of the sunlight away. Without the greenhouse effect, Venus’s surface would be much cooler. Scientists think Venus once had a strong greenhouse effect that changed its atmosphere forever.

Mars

Mars has lots of carbon dioxide in its atmosphere, but its greenhouse effect is small — only about 6 K (11 °F). This is because Mars’s atmosphere is very thin and lacks water vapor, which helps trap heat.

Titan

Saturn’s moon Titan has both a greenhouse effect and something called an anti-greenhouse effect. Gases like nitrogen, methane, and hydrogen in Titan’s atmosphere help trap heat, making the surface warmer by about 21 K (38 °F). However, a high layer of haze blocks some sunlight, cooling the surface by about 9 K (16 °F). Overall, Titan’s surface is about 12 K (22 °F) warmer because of these effects.

Effect of pressure

The size of the greenhouse effect doesn’t just depend on how many heat-trapping gases there are. The pressure of the atmosphere also matters. Higher pressure lets gases trap more heat, while lower pressure makes them less effective. This is because, under high pressure, gas molecules bump into each other more often, which helps them trap heat over more wavelengths. On Venus, with very high pressure, this effect is strong. On Mars, with low pressure, it is weak.