A Morning on the Moon
Imagine two neighbours sharing one Sun. Earth and the Moon sit at almost exactly the same distance from that star, and each receives roughly the same flood of sunlight. Yet on the Moon, the ground at midday is hot enough to boil water, and at night it plunges far below the coldest Antarctic winter. Earth, wrapped in its thin skin of air, settles into a far gentler rhythm. The difference is not the Sun. It is the blanket.
In the last level you met the watt, a joule of energy arriving every second. Hold on to that idea, because this story is about a flow of watts that goes into a planet and back out again. At the top of Earth's atmosphere, sunlight delivers about 1,361 watts onto every square metre facing the Sun directly. That is a little more than a kettle's worth of power on a patch of ground the size of a small rug, arriving every second without a break.
Light Coming In
Follow a beam of sunlight down from space. Most of its energy arrives as visible light, the light your eyes evolved to see, along with some ultraviolet and near-infrared. The air is largely transparent to this light, much as a clean window is, so much of it passes straight through.
Not all of it gets in. Bright clouds, pale deserts, snowfields and sea ice act like mirrors, and taken together Earth bounces about 30 per cent of incoming sunlight straight back to space. Scientists call this reflectiveness albedo. That reflected light leaves without warming anything. The rest is absorbed, mostly by the oceans and the land, with some taken up by the air itself on the way down. The surface warms, the way a dark car bonnet warms on a summer afternoon.
The Glow Going Out
Here comes the turn in the story. Anything with a temperature glows. A stove ring glows red when it is very hot, but even a cool stone or your own skin is glowing all the time, just in infrared light that your eyes cannot see. Thermal cameras show this glow. The warmed Earth does the same, sending its energy back upward not as visible sunlight but as infrared.
If the air were as transparent to infrared as it is to sunlight, that glow would escape directly to space, and the planet would settle at a frigid average of about minus 18 degrees Celsius. Instead, Earth's average surface temperature is about 15 degrees. That gap of roughly 33 degrees is the work of the greenhouse effect.
The Gases That Catch the Glow
Certain molecules in the air absorb outgoing infrared. Having caught it, they warm and glow in turn, sending infrared out in every direction, some upward towards space and some back down to the surface. The ground therefore receives energy from two sources: sunlight by day, and this returned glow from the sky around the clock. That is why a cloudy night often feels milder than a clear one.
The molecules doing this work are the greenhouse gases. Water vapour is the biggest player, and with clouds it does most of the job. One widely cited NASA analysis credited water vapour with about half of today's greenhouse effect, clouds with about a quarter, and carbon dioxide with about a fifth, with methane, nitrous oxide and ozone making up the rest. But water vapour behaves differently from the others. It rains and snows out of the air within days, so how much the air holds depends mainly on temperature. Warm the air and it holds more water vapour; cool it and the water falls out. Carbon dioxide does not rain out, and it stays in the air for a very long time. So water vapour reinforces a warming that something else starts, and does not start it by itself.
What about nitrogen and oxygen, which together make up about 99 per cent of dry air? They are almost invisible to infrared. Each of their molecules is two identical atoms locked together, and when such a pair stretches and vibrates its electric charge stays evenly balanced. A molecule has to shift its charge as it vibrates to grab infrared, and carbon dioxide, water and methane, built from three or more atoms, can do so. The bulk of the air is a clear window, and a few trace gases are the panes of frosted glass.
The name, incidentally, is a little misleading. A glass greenhouse stays warm mainly because its walls stop warm air from drifting away. The atmosphere works by absorbing and re-emitting infrared, which is why a blanket is the better picture.
The Blanket Grows Thicker
In 1856, Eunice Foote filled glass cylinders with different gases, set them in the sun, and found that the one holding carbon dioxide grew hottest. A few years later John Tyndall measured how gases absorb infrared and showed that water vapour and carbon dioxide do so strongly. Their insight sets up the second half of our story.
The natural greenhouse effect is the old, life-giving blanket that kept oceans liquid long before people existed. The enhanced greenhouse effect is the extra thickness that human activity adds. Around 1750, carbon dioxide stood at about 278 parts per million of air. Burning coal, oil and gas and clearing forests have pushed it to an annual average of about 427 parts per million at Hawaii's Mauna Loa observatory in 2025. More catching gas means more of the outgoing glow is held back, and the surface warms until the planet finds a new balance.
So when a headline says greenhouse gases are dangerous, it helps to know which effect is meant. Without the natural effect we would freeze. The concern is the human-made addition on top of it.
Key takeaways
- Sunlight arrives mostly as visible light; about 30 per cent is reflected, and the rest warms the surface and the air.
- The warmed Earth glows back in infrared, and greenhouse gases absorb part of that glow and send some back down.
- Water vapour and clouds do most of the natural trapping, but water vapour follows temperature and amplifies changes instead of starting them.
- Nitrogen and oxygen, made of two identical atoms, barely interact with infrared.
- The natural effect lifts Earth from about minus 18 to about 15 degrees Celsius; human emissions add an enhanced effect on top.