Without the greenhouse effect, Earth would be a frozen world with an average temperature far below freezing. With it, our planet has stayed warm enough for liquid oceans and life for billions of years. The greenhouse effect is not a theory invented recently; its basic physics was discovered in the nineteenth century and has been measured in laboratories and in the atmosphere ever since. Yet it is also at the center of one of the most important challenges of our time, because human activities are adding greenhouse gases to the atmosphere and strengthening the effect. This explainer explains how the greenhouse effect works, which gases are involved and how scientists know that today's changes are caused by people.
Earth's energy balance
The Sun constantly sends energy to Earth, mostly as visible light, along with some ultraviolet and near-infrared light. About 30 percent of this incoming sunlight is reflected straight back to space by clouds, ice, snow, deserts and other bright surfaces. The rest is absorbed by the land, oceans and atmosphere, warming them.
Any warm object gives off energy as radiation. The hotter it is, the more energy it radiates and the shorter the wavelengths. The Sun, with a surface temperature of about 10,000 °F (5,500 °C), radiates mostly visible light. Earth, much cooler, radiates energy mainly as infrared radiation, which we cannot see but can feel as heat.
For Earth's temperature to remain stable over time, the energy it radiates to space must balance the energy it absorbs from the Sun. If more energy comes in than goes out, Earth warms until a new balance is reached.
What the greenhouse effect does
If Earth had no atmosphere, the infrared radiation emitted by the surface would escape directly to space. Calculations show that in this case, the average surface temperature would be about 0 °F (−18 °C).
But Earth's atmosphere contains gases that absorb infrared radiation. These greenhouse gases absorb some of the heat radiated by the surface and then re-emit it in all directions, including back toward the ground. This additional downward radiation warms the surface and the lower atmosphere further. As a result, Earth's actual average surface temperature is about 59 °F (15 °C), roughly 33 °C (about 60 °F) warmer than it would be without the greenhouse effect.
The natural greenhouse effect is therefore essential for life. The concern today is not that it exists, but that it is getting stronger.
Why is it called the greenhouse effect?
The name comes from a comparison with greenhouses, where glass lets in sunlight and keeps the inside warm. The comparison is not perfect: real greenhouses stay warm mainly because the glass stops warm air from rising and mixing with cooler outside air. The atmosphere works differently, through absorption and emission of infrared radiation. A better image might be a blanket that slows down the loss of heat from your body.
Which gases trap heat, and why
Not all gases absorb infrared radiation. The main components of air, nitrogen (about 78 percent) and oxygen (about 21 percent), are made of two identical atoms and are almost transparent to infrared. Greenhouse gases have molecules that can vibrate and bend in ways that absorb and emit infrared radiation at specific wavelengths.
| Gas | Main sources | Notes |
|---|---|---|
| Water vapor (H₂O) | evaporation from oceans and land | the largest contributor to the natural greenhouse effect; its amount depends on temperature |
| Carbon dioxide (CO₂) | burning fossil fuels, deforestation, cement; natural exchanges with oceans and plants | long-lived; the main driver of human-caused warming |
| Methane (CH₄) | livestock, rice paddies, landfills, natural gas leaks, wetlands | traps much more heat per molecule than CO₂ but lasts about a decade |
| Nitrous oxide (N₂O) | fertilizers, industry, burning | long-lived and potent |
| Fluorinated gases | refrigerants, industrial processes | very potent; some also damage the ozone layer |
Water vapor is the most abundant greenhouse gas, but it acts mainly as a feedback rather than a driver. The amount of water vapor the air can hold depends on temperature. If something else warms the planet, such as more carbon dioxide, the air holds more water vapor, which amplifies the warming.
The history of the discovery
The greenhouse effect has been studied for nearly two centuries:
- In the 1820s, the French mathematician Joseph Fourier realized that Earth is warmer than it would be based on sunlight alone and suggested that the atmosphere acts like an insulating layer.
- In 1856, the American scientist Eunice Foote showed in experiments that carbon dioxide and water vapor warm up more in sunlight than ordinary air.
- In 1859, the Irish physicist John Tyndall measured precisely how much infrared radiation different gases absorb, showing that water vapor and carbon dioxide are strong absorbers.
- In 1896, the Swedish chemist Svante Arrhenius calculated that doubling the amount of carbon dioxide in the atmosphere would warm Earth by several degrees.
These scientists established the basic physics long before global warming became a public concern.
Rising carbon dioxide
Since 1958, scientists have measured carbon dioxide continuously at the Mauna Loa Observatory in Hawaii, building what is known as the Keeling Curve. Before the Industrial Revolution, carbon dioxide levels were about 280 parts per million (ppm). They passed 400 ppm in the 2010s and are now above 420 ppm, the highest in at least 800,000 years according to air bubbles trapped in Antarctic ice cores, and likely for millions of years.
How we know the extra carbon dioxide comes from people
Several independent lines of evidence show that the increase in carbon dioxide comes from human activities, especially burning fossil fuels:
- Quantities: Humans release more than 35 billion tons of carbon dioxide each year from fossil fuels. The atmosphere's increase is about half of this; the rest is absorbed by oceans and plants.
- Isotopes: Fossil fuels formed from ancient plants and contain carbon with a distinctive mix of isotopes, with less carbon-13 and no carbon-14. The atmosphere's carbon is shifting toward this fossil signature.
- Oxygen decline: Burning fossil fuels consumes oxygen. Measurements show a small, steady decline in atmospheric oxygen, matching the amount expected.
- Ocean acidification: The oceans are absorbing carbon dioxide and becoming more acidic, as expected.
How we know greenhouse gases are causing warming
Scientists also have multiple lines of evidence that increased greenhouse gases are warming the planet:
- Satellites measure less infrared radiation escaping to space at exactly the wavelengths absorbed by carbon dioxide and methane.
- Instruments on the ground measure more infrared radiation coming down from the atmosphere.
- The upper atmosphere is cooling while the lower atmosphere warms. If the Sun were causing warming, the whole atmosphere would warm. A cooling stratosphere is a fingerprint of a strengthened greenhouse effect.
- Nights are warming faster than days in many places, consistent with heat being trapped.
- Climate models, based on physics, can reproduce the observed warming only when human greenhouse gas emissions are included.
The Intergovernmental Panel on Climate Change has concluded that it is unequivocal that human influence has warmed the atmosphere, oceans and land.
Feedbacks: amplifying and dampening
The climate system responds to warming in ways that can amplify or reduce it:
- Water vapor feedback: Warmer air holds more water vapor, a greenhouse gas, amplifying warming.
- Ice-albedo feedback: Melting ice and snow expose darker ocean and land, which absorb more sunlight, causing more warming.
- Cloud feedbacks: Clouds can both cool, by reflecting sunlight, and warm, by trapping heat. Their overall effect is one of the main uncertainties in climate projections, though evidence suggests a net amplifying effect.
- Carbon cycle feedbacks: Warming can release carbon from thawing permafrost and reduce the ability of oceans and forests to absorb carbon dioxide.
- Radiative response: A warmer Earth radiates more energy to space, which acts as a stabilizing effect.
The greenhouse effect on other planets
- Venus has a thick atmosphere of about 96 percent carbon dioxide, producing a runaway greenhouse effect that heats its surface to about 870 °F (465 °C), hotter than Mercury, despite being farther from the Sun.
- Mars has a very thin carbon dioxide atmosphere and only a weak greenhouse effect, so it is cold.
These planets show that the greenhouse effect is real and powerful, depending on the amount and type of gases in an atmosphere.
Questions readers ask
Is the greenhouse effect bad? The natural greenhouse effect is essential; without it, Earth would be frozen. The problem is the extra warming from human-added greenhouse gases.
Why is carbon dioxide important if it is only a small part of the atmosphere? Even in small amounts, carbon dioxide strongly absorbs infrared radiation. Its concentration has increased by about 50 percent since preindustrial times.
Is water vapor more important than carbon dioxide? Water vapor contributes more to the natural greenhouse effect, but its amount depends on temperature. Carbon dioxide drives the change, and water vapor amplifies it.
How do we know the Sun is not causing the warming? Solar output has been measured by satellites and shows no upward trend in recent decades, and the cooling of the upper atmosphere points to greenhouse gases rather than the Sun.
Can the greenhouse effect be reduced? Reducing greenhouse gas emissions, protecting forests and developing ways to remove carbon dioxide from the atmosphere can slow and eventually stop the strengthening of the greenhouse effect.
In short
The greenhouse effect is a natural process in which gases like water vapor, carbon dioxide and methane absorb infrared heat radiated by Earth and send some of it back toward the surface, keeping the planet about 33 °C (about 60 °F) warmer than it would otherwise be. Its physics has been understood since the nineteenth century. Since the Industrial Revolution, human activities have increased carbon dioxide by about 50 percent, strengthening the effect. Isotopes, satellite measurements and the cooling of the upper atmosphere all show that this extra warming comes from human greenhouse gases. Understanding the greenhouse effect is the foundation for understanding today's changing climate.