Greenhouse effect

The greenhouse effect occurs when heat-trapping gases in a planet's atmosphere keep heat from escaping into space. This makes the planet's surface warmer.

On Earth, the Sun sends out shortwave radiation (sunlight) that passes through these gases and warms the ground. The Earth then sends out longwave radiation, which the greenhouse gases mostly absorb. This slows down the cooling process.

Without the greenhouse effect, Earth would be much colder. Human activities, like burning fossil fuels, have increased amounts of carbon dioxide and methane in the air. This has caused global warming.

All objects above absolute zero give off heat as radiation. The Sun emits shortwave radiation, while Earth emits longwave radiation. Greenhouse gases absorb the longwave radiation, trapping heat and warming the planet. Human actions have made this natural process stronger, changing our climate.

Definition

The greenhouse effect is how certain gases and particles in Earth’s atmosphere trap heat. These gases, like greenhouse gases, along with clouds and 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.

Terminology

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

History of discovery and investigation

See also: History of climate change science

Smart thinkers started learning about the greenhouse effect in the early 1800s. In 1824, Joseph Fourier thought that some gases in the air could trap heat. Later, Claude Pouillet and Eunice Newton Foote found more proof that gases like water vapor and carbon dioxide help keep Earth warmer.

John Tyndall tested how different gases trap heat. He learned that even a little water vapor and carbon dioxide can make a big difference. In 1896, Svante Arrhenius guessed how much warmer Earth would get if there was more carbon dioxide in the air. The idea was named the "greenhouse effect" by Nils Gustaf Ekholm in 1901.

Measurement

The greenhouse effect can be measured by how warmer Earth is because of it. Without greenhouse gases, Earth's average temperature would be about −18 °C. Because these gases trap heat, the average temperature is around 15 °C. This makes Earth 33 °C warmer.

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. 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 gets stronger when we add more gases to the air from things we do. This is called the enhanced greenhouse effect. Scientists see this by measuring how much heat stays on Earth. The main reason for this 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 we find in old ice records from the past 800,000 years. Changes in CO₂ levels have helped change Earth’s climate over time.

Energy balance and temperature

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

The Earth also sends heat back into space. Greenhouse gases in the atmosphere trap some of this heat, keeping the planet warmer. This makes Earth’s surface much warmer than it would be without these gases. The balance between energy coming in and going out 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. Other air moves down, gets squeezed, and warms up. This creates a pattern where temperatures change with height, which is 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 has lots of these gases and cannot let much heat escape. Higher up, where the air is thinner, more heat can escape into space. This difference 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 escape into space, making our planet warmer. Most of the greenhouse effect comes from these gases.

Some gases can trap heat because they absorb and send out infrared radiation. Gases with molecules that have two different atoms, like carbon monoxide (CO), and 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 well because their molecules are balanced.

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

Basic formulas

The greenhouse effect is how some gases in a planet's atmosphere trap heat, making the planet warmer. Think of the atmosphere like a blanket that keeps the planet's surface warmer than it would be without these gases.

Scientists study the greenhouse effect in different ways. One way is to see how much heat the planet's surface gives off compared to how much heat leaves into space. Another way is to find 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 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 learn about the greenhouse effect and how it changes Earth’s temperature. These models picture the atmosphere as one layer that shares heat with the ground and space. Some models add more layers or include moving air to give a clearer picture.

One simple way to think about this is to imagine all the heat leaving Earth from a certain height in the sky. This height changes as more heat-trapping gases build up, moving higher and making Earth warmer. Scientists have seen this height rise, which matches Earth’s slow warming over many years.

Related effects on Earth

Sometimes, in parts of Antarctica, the greenhouse effect can work differently. If the air above is warmer than the ground, greenhouse gases can help cool the area by letting more heat escape into space. This is called a negative greenhouse effect.

There is also something called the runaway greenhouse effect. This happens when greenhouse gases build up a lot and trap heat continuously. The planet's temperature can rise quickly. This has happened on planets like Venus, which has a very hot surface.

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. This is because Venus has a 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.

Mars

Mars has lots of carbon dioxide in its atmosphere, but its greenhouse effect is small. 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 in Titan’s atmosphere help trap heat, making the surface warmer. However, a high layer of haze blocks some sunlight, cooling the surface. Overall, Titan’s surface is 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. On Venus, with very high pressure, this effect is strong. On Mars, with low pressure, it is weak.