Greenhouse effect

The air keeps our world warm.

Sunlight comes down to Earth. It passes through the air to heat the ground. The ground then sends heat back up toward space.

Some gases in the air trap this heat. They act like a blanket. This stops the heat from leaving too fast.

Without these gases, Earth would be very cold. It would be much colder than it is now.

People burning fuel adds more of these gases. This makes the air trap even more heat. This makes our world warmer.

The air helps keep our planet warm. This is called the greenhouse effect.

It all starts with the Sun. The Sun sends out shortwave radiation. This is a type of light energy. This light passes through our air to heat the Earth. The Earth then sends heat back up toward space. We call this longwave radiation.

Some gases in the air trap this heat. These are called greenhouse gases. They absorb the longwave radiation. This stops the heat from escaping into space. Because of this, the Earth stays much warmer. Without these gases, Earth would be very cold. It would be about -18 degrees Celsius.

Humans are changing this system. We burn fossil fuels like coal and oil. This lets out more carbon dioxide and methane. These are greenhouse gases. More gases trap even more heat. This is called the enhanced greenhouse effect.

Since the Industrial Revolution, the Earth has warmed up. The average temperature has gone up by about 1.1 degrees Celsius. This warming is happening faster now than in the past.

The greenhouse effect is a way the Earth stays warm enough for life. It happens when certain gases in the air trap heat.

Without this effect, our planet would be much colder. The average temperature would be about -18 degrees Celsius. Instead, the greenhouse effect keeps the average temperature around 15 degrees Celsius. This difference of about 33 degrees makes a huge impact on our world. It allows the Earth to be a place where living things can thrive.

To understand how it works, we can look at energy flows. The Sun sends shortwave radiation, which is sunlight, toward the Earth.

The atmosphere only absorbs about 23% of this incoming sunlight. Most of it passes through to heat the Earth's surface. The warm surface then sends energy back up as longwave radiation. This is also called thermal radiation.

Greenhouse gases absorb 90% of this longwave radiation. This prevents the heat from escaping easily into space.

Scientists have studied this for a long time. Joseph Fourier first proposed the idea in 1824.

Later, Claude Pouillet added more evidence in 1827 and 1838. In 1856, Eunice Newton Foote showed that air with water vapour or carbon dioxide holds more heat than dry air. John Tyndall later measured how different gases absorb this heat. He found that even small amounts of carbon dioxide have a big effect. Finally, Nils Gustaf Ekholm used the term "greenhouse" in 1901.

We can measure these changes with modern tools.

At the Mauna Loa Observatory, scientists see that carbon dioxide levels are rising. In 1960, the level was about 313 parts per million. By 2013, it passed 400 parts per million. This is much higher than the 270 parts per million seen before the Industrial Revolution. Since that time, the Earth has warmed by about 1.1 degrees Celsius. This warming is happening at a rate of about 0.18 degrees Celsius every decade since 1981.

The name comes from how a real greenhouse works.

A glass greenhouse keeps heat in by blocking the movement of air. The greenhouse effect works differently by restricting how heat moves through radiation. When humans burn fossil fuels, we add more gases like methane and carbon dioxide to the air. This is called the enhanced greenhouse effect. These extra gases trap more heat than before. This extra energy stays in our atmosphere and changes our climate.

The greenhouse effect is a critical process that regulates Earth's temperature. It occurs when specific gases in the atmosphere prevent a planet from losing heat to space. This process raises the surface temperature of the planet. On Earth, this effect is essential for life. Without it, the average surface temperature would be roughly -18 degrees Celsius. Instead, the greenhouse effect maintains an average temperature of about 15 degrees Celsius. This represents a temperature difference of approximately 33 degrees.

To understand the mechanism, we must look at how energy moves. The Sun emits shortwave radiation, which includes visible light and near-infrared wavelengths. Because the Sun is very hot, its radiation has short wavelengths. This sunlight passes through the atmosphere to heat the Earth's surface. The atmosphere and clouds reflect or absorb about 23% of this incoming radiation. The Earth's surface absorbs most of it and then emits longwave radiation, also called thermal radiation.

Unlike the Sun, the Earth is much cooler. Therefore, it emits energy at longer, mid- and far-infrared wavelengths.

Greenhouse gases are the specific components that drive this warming. A gas is classified as a greenhouse gas if it absorbs longwave radiation. While the atmosphere only absorbs 23% of incoming shortwave radiation, it absorbs 90% of the longwave radiation emitted by the surface. This absorption causes energy to accumulate in the atmosphere. Each layer of the atmosphere containing these gases absorbs radiation from below. These gases then emit radiation in all directions, including back toward the surface. This reduces the rate at which the Earth can cool off.

Scientists have worked for centuries to understand this phenomenon. Joseph Fourier proposed the existence of this effect as early as 1824. Claude Pouillet later strengthened this argument with more evidence in 1827 and 1838. In 1856, Eunice Newton Foote conducted experiments showing that air with water vapour or carbon dioxide holds more heat than dry air. She concluded that an atmosphere of such gas would create a high temperature on Earth.

Later, John Tyndall measured the infrared absorption of various gases. He proved that a small proportion of the atmosphere, like water vapour and carbon dioxide, causes the effect. Finally, Nils Gustaf Ekholm applied the term "greenhouse" to the process in 1901.

We can measure the greenhouse effect using different scientific metrics. One way is by measuring the change in temperature. Another way is to measure the energy flux, which is the rate of energy flow per unit area. Currently, longwave radiation leaves the Earth's surface at an average rate of 398 W/m². However, only 239 W/m² actually reaches space. This means the greenhouse effect accounts for an energy flow change of 159 W/m². This represents about 40% of the longwave radiation that leaves the surface but is trapped.

Human activities are now creating an "enhanced greenhouse effect." This happens when the concentration of greenhouse gases increases due to human action. Burning fossil fuels increases the amounts of carbon dioxide and methane in the atmosphere. Since the Industrial Revolution, global warming of about 1.1 degrees Celsius has occurred. Since 1981, the global average surface temperature has increased at a rate of about 0.18 degrees Celsius per decade. Data from the Mauna Loa Observatory shows that carbon dioxide levels have risen significantly. In 1960, levels were about 313 parts per million (ppm). By 2013, they passed the 400 ppm milestone. This exceeds the natural historical maximum of approximately 300 ppm found in ice core data.

The term "greenhouse" is an analogy, but the physics are different. A real glass greenhouse retains heat by blocking convection, which is the movement of air. In contrast, the greenhouse effect retains heat by restricting radiative transfer. It limits the rate at which thermal radiation is emitted into space. By increasing these gases, humans are changing the Earth's energy balance. This shift affects how much energy stays in our climate system.