The Greenhouse Effect: What It Is and How It Works

A blanket around the planet

Sunlight crosses the atmosphere almost undisturbed and warms Earth’s surface. Once warmed, the planet gives that energy back as infrared radiation: invisible heat climbing toward space. This is where the greenhouse gases come in, molecules able to absorb that outgoing heat and send part of it back down.

The result is what a blanket does on a cold night. The blanket makes no heat of its own: it slows the escape of yours. In the same way, greenhouse gases do not warm Earth by themselves, but they keep near the surface heat that would otherwise slip away.

Practical example: in deserts, the temperature plunges as soon as night falls. Dry air holds very little water vapor, which means very little “local” greenhouse gas, so the heat stored during the day escapes toward space almost unhindered. On a humid or cloudy night, instead, the heat stays trapped and the temperature drops far more slowly.

Meet the greenhouse gases

The atmosphere is made mostly of nitrogen and oxygen, which are “transparent” to infrared heat: they let it through. Greenhouse gases are a small minority of the air, yet they are enough to decide the climate of the whole planet.

Water vapor deserves a note. It is the most abundant greenhouse gas, but we do not emit it directly in significant amounts: it increases as a consequence of warming, because warmer air holds more moisture, and so it amplifies it. Scientists call this loop a feedback.

Natural, in fact vital

The greenhouse effect is not a recent invention nor, in itself, a problem: it has existed for billions of years and it is the reason Earth is livable. The planet’s average temperature is about 15°C; without the natural greenhouse effect it would be roughly 33°C lower, around -18°C: a planet of frozen oceans, where life as we know it would never have taken hold.

Nature, moreover, has kept the system in balance all along: photosynthesis constantly pulls CO₂ out of the air to turn it into sugars and wood, while the oceans absorb another share.

From the right blanket to a double one

The trouble began when we started adding layers to the blanket. With the Industrial Revolution, humanity began burning coal, then oil and gas, releasing into the atmosphere carbon that had stayed underground for millions of years. Since then, the growth of the world economy, the one we now measure with GDP, has moved hand in hand with emissions.

The numbers tell the rest. Before the industrial era, atmospheric CO₂ stood at about 280 parts per million (ppm); in July 2026 the Mauna Loa observatory in Hawaii measured a monthly average of 429 ppm. An increase of roughly 50%. And CO₂ alone, according to NOAA, is responsible for about 80% of the extra heating influence added by human-produced greenhouse gases since 1990. Methane is racing too: in 2024 it reached 1,897 parts per billion, its highest level in 800,000 years.

Practical example: since 1958, a station on the Mauna Loa volcano has measured the CO₂ in the air day after day: it is the longest record of direct measurements in the world. The resulting graph, the Keeling curve, climbs like a staircase, one step every year, with a small seasonal wiggle caused by Northern Hemisphere plants absorbing more CO₂ in summer.

The thickened blanket produces what we call global warming. According to the European climate service Copernicus, 2024 was the warmest year on record: an average temperature of 15.10°C, that is 1.6°C above the pre-industrial level (1850-1900), and the first calendar year past the 1.5°C threshold.

A mechanism to understand, not to fear

Telling the natural greenhouse effect apart from the enhanced one is the key to this whole topic, at school as much as in public debate. The mechanism itself has worked for billions of years and keeps us alive; it is the roughly 150 ppm of CO₂ added in a little over two and a half centuries that has upset the balance. Once that is clear, much of the news about climate suddenly makes more sense.