What is the greenhouse effect? How Earth stays warm

The greenhouse effect is the warming of a planet's surface by gases in its air that let sunlight in but absorb the heat the ground gives off, then send part of that heat back down. Without it, Earth's average temperature would be about −18°C, well below freezing. With it, the average is about 15°C. The natural effect makes Earth livable. Adding more of these gases strengthens it.

In short

• Sunlight arrives mostly as visible light and leaves as invisible infrared heat; greenhouse gases are transparent to the first and partly opaque to the second.
• The natural greenhouse effect is good: it keeps Earth about 33°C warmer than an airless planet just as reflective at the same distance from the Sun.
• Burning fossil fuels has raised carbon dioxide from about 280 to about 426 parts per million, which thickens the blanket and warms the planet.

How does the greenhouse effect work?

Everything warm gives off energy as light. The Sun, at about 5,500°C on its surface, shines mostly in visible light. Earth, at about 15°C, glows too, but in infrared, which our eyes can't see. That difference in color is the whole trick.

Nitrogen and oxygen, 99% of dry air, let both kinds of light pass. A few gases don't. Water vapor, carbon dioxide (CO2), methane, nitrous oxide and ozone let most sunlight through but absorb infrared. Their molecules have three or more atoms that can bend and stretch at just the frequencies infrared carries. When one absorbs heat rising from the ground, it re-emits it in a random direction, so some goes up toward space and some comes back down. The surface gets warmed twice: by the Sun, and by the atmosphere returning part of its own heat.

What does Earth's energy budget look like, step by step?

Here is the arithmetic, with figures from NASA's Earth energy budget. It shows where the −18°C comes from.

1. Sunlight arriving. At Earth's distance, sunlight carries about 1,360 watts per square meter (W/m²) on a surface facing it. Earth is a sphere that turns, so its whole surface, four times the area of the disk that catches the light, gets an average of 1,360 ÷ 4 = 340 W/m².
2. Reflected. About 29% bounces straight back to space off clouds, ice and bright ground. That leaves about 340 × 0.71 ≈ 240 W/m² absorbed.
3. The balance. To stay at a steady temperature, Earth must send 240 W/m² back to space as infrared. Physics (the Stefan–Boltzmann law) says how hot an object must be to glow that brightly: about 255 kelvin, or −18°C.
4. The surface. But Earth's surface averages 15°C, and at that temperature it gives off about 390 W/m², far more than 240.
5. The gap. About 150 W/m² leaves the ground but doesn't make it out to space. It is absorbed by greenhouse gases and clouds, and much of it comes back down. That gap, about 33°C of warming, is the natural greenhouse effect.

The same calculation works for other planets, with temperatures from NASA's planetary fact sheets.

Planet	Air	Without a greenhouse effect	Actual average	Warming
Venus	Thick, mostly CO2, about 90 times Earth's pressure	about −46°C	464°C	about 510°C
Earth	Thin, a little water vapor and CO2	about −19°C	15°C	about 34°C
Mars	Very thin CO2, under 1% of Earth's pressure	about −63°C	about −59°C	about 4°C

Venus reflects so much sunlight off its clouds that, if its air trapped no heat, it would be colder than Earth. Instead it is the hottest planet, hotter than Mercury, which is closer to the Sun. The note on why Venus is so hot tells that story.

Why does more carbon dioxide make Earth warmer?

Adding CO2 makes the air more opaque to infrared, so the heat that finally escapes to space leaves from higher up, where the air is colder. Colder air glows less. For a while, Earth sends out less energy than it absorbs, and it warms until the balance returns.

CO2 matters most because it lasts. Water vapor is the largest contributor to the greenhouse effect, but the amount the air holds depends on temperature: extra vapor rains out within days. That makes water vapor a feedback, as NASA describes it: warming from CO2 lets the air hold more vapor, which adds more warming. CO2 stays in the air for centuries, so it sets the level the rest responds to.

Before the Industrial Revolution, CO2 was about 280 parts per million (ppm), according to NOAA. NOAA's global average was 425.6 ppm in 2025, about 52% higher, mostly from burning coal, oil and gas. The strengthened effect is what drives climate change, and it is why so much effort goes into renewable energy.

Is it really like a greenhouse?

Only partly. A real greenhouse stays warm mainly because its glass stops warm air from rising and blowing away, not because it traps infrared. The atmosphere has no lid. It works by absorbing and re-emitting radiation. The name stuck anyway: the Swedish meteorologist Nils Ekholm first applied the word "greenhouse" to it in 1901. "Blanket" is a closer picture. A blanket doesn't make heat; it slows how fast your body loses it, so you settle at a warmer temperature.

Who discovered the greenhouse effect?

The idea is two centuries old, and was settled physics long before climate change was a public question.

• 1824. Joseph Fourier, a French mathematician, argued that Earth is warmer than its distance from the Sun alone would make it, and that the atmosphere must hold heat in.
• 1856. Eunice Foote, an American scientist, set glass cylinders of different gases in sunlight and found that one filled with CO2 grew hottest. She wrote that more of it in the air would give Earth a higher temperature.
• 1859. John Tyndall, in Britain, measured how much infrared different gases absorb and showed that water vapor and CO2 do, while nitrogen and oxygen don't.
• 1896. Svante Arrhenius, a Swedish chemist, made the first estimate of how much warmer Earth would get if CO2 doubled.

These dates are summarized in the Wikipedia history of the greenhouse effect, which cites the original papers. Since then, the question has been how much warming, how fast, and with what effects, not whether the effect exists. Over longer spans, changes in Earth's orbit and in greenhouse gases together have paced the coming and going of each ice age.

Check yourself

• Why would Earth be about −18°C without greenhouse gases, and where does that number come from?
• Why do greenhouse gases block heat leaving Earth but not sunlight arriving?
• Why is water vapor called a feedback rather than the main cause of today's warming?

Sources

• What is the greenhouse effect? — NASA, 2024
• Climate and Earth's energy budget — NASA Earth Observatory
• Climate change: atmospheric carbon dioxide — NOAA Climate.gov
• Trends in atmospheric carbon dioxide, global — NOAA Global Monitoring Laboratory
• Venus fact sheet, with Earth comparison — NASA Space Science Data Coordinated Archive
• Greenhouse effect — Wikipedia