Radiation is the primary mechanism that warms Earth’s atmosphere. The sun delivers roughly 340 watts per square meter of energy to Earth, and about 240 of those watts are absorbed by land, ocean, and air after the planet reflects roughly 30% back to space. The absorbed energy is then re-radiated as infrared (heat) radiation, and greenhouse gases in the atmosphere intercept much of that outgoing heat, trapping it and raising temperatures. Without this process, Earth’s average surface temperature would sit around minus 18°C (0°F) instead of the roughly 15°C (59°F) we experience.
How Energy Arrives and Leaves
The sun emits shortwave radiation, mostly visible light and ultraviolet, which passes through the atmosphere relatively easily. Of every 100 units of solar energy that reach the top of the atmosphere, clouds reflect about 23 units and Earth’s surface reflects another 7, for a combined reflectivity (albedo) of about 0.30. The remaining 70 units are absorbed by the surface, clouds, and atmospheric gases, warming the planet.
Earth doesn’t keep that energy. The warmed surface radiates it back out as longwave infrared radiation, the same type of heat you feel radiating from hot pavement. Of the outgoing energy, about 49 units leave from the atmosphere itself, 9 from clouds, and only 12 directly from the surface through to space. That breakdown is critical: most of Earth’s heat doesn’t escape directly from the ground. It’s first absorbed by the atmosphere, then re-radiated upward in stages. This is where greenhouse gases do their work.
How Greenhouse Gases Trap Heat
Carbon dioxide, water vapor, methane, nitrous oxide, and ozone all share a molecular trait: they can absorb infrared photons. When an infrared photon strikes a CO2 molecule, the molecule begins to vibrate, absorbing that energy. In a textbook scenario, the molecule later emits another infrared photon in a random direction, some of which heads back toward Earth’s surface rather than escaping to space.
In the real atmosphere, though, something more direct usually happens first. Before the CO2 molecule can re-emit a photon, it typically collides with neighboring nitrogen or oxygen molecules and transfers its extra energy to them, speeding them up. Since temperature is simply a measure of how fast gas molecules are moving, those collisions raise the temperature of the surrounding air. This is the physical heart of atmospheric warming: greenhouse gases convert infrared radiation into molecular motion, which is heat.
Not all wavelengths of infrared radiation get trapped. The atmosphere has a transparency window between roughly 8 and 13 micrometers where very little absorption occurs. Heat radiated at those wavelengths passes straight through to space. This window is the reason Earth can shed enough energy to maintain a livable temperature, and it’s also the range that climate scientists watch closely, because any gas that absorbs within that window has an outsized warming effect.
The Greenhouse Effect by the Numbers
Since the start of the industrial era in 1750, human emissions have measurably shifted Earth’s energy balance. The Intergovernmental Panel on Climate Change estimates that CO2 alone has added about 2.16 watts per square meter of extra energy retention to the atmosphere. Methane contributes another 0.54 watts per square meter. These numbers may sound small against 240 watts of total absorbed solar energy, but applied across the entire surface of the planet and accumulated over decades, they drive significant warming.
Aerosols, tiny particles from pollution and volcanic eruptions, partially offset that warming by reflecting sunlight. Their cooling effect is estimated at roughly minus 1.1 watts per square meter. This means the net human-caused energy imbalance is smaller than greenhouse gases alone would suggest, but still firmly positive, pushing temperatures upward.
Clouds Play a Double Role
Clouds complicate the picture because they interact with radiation in two opposing ways. Their white tops reflect incoming sunlight back to space, a cooling effect of about minus 50 watts per square meter globally. But clouds also absorb and re-emit infrared radiation rising from below, trapping roughly 30 watts per square meter. The net result is a cooling effect of about minus 20 watts per square meter, meaning clouds currently cool the planet more than they warm it. The balance between these two effects depends on cloud height, thickness, and type, which is one reason climate projections carry uncertainty.
Water Vapor Amplifies the Warming
Water vapor is the most abundant greenhouse gas in the atmosphere, and it creates a powerful feedback loop. For every degree Celsius the atmosphere warms, it can hold about 7% more moisture. Satellite and weather balloon data confirm that atmospheric water vapor is increasing by 1 to 2% per decade as the climate warms. That extra moisture absorbs more infrared radiation, which raises temperatures further, which allows even more evaporation. Scientists estimate this feedback more than doubles the warming that would occur from rising CO2 alone.
Water vapor is considered a feedback rather than a forcing because it doesn’t initiate warming on its own. If CO2 levels dropped and temperatures fell, water vapor would condense out of the atmosphere and the effect would reverse. But as long as CO2 and methane keep rising, water vapor will keep amplifying their impact.
Why the Upper Atmosphere Cools
One of the clearest fingerprints of radiation-driven warming is what happens at different altitudes. The lower atmosphere (troposphere) warms as greenhouse gases trap more infrared radiation near the surface. But the upper atmosphere (stratosphere) actually cools. This pattern was predicted as early as 1967, when scientists modeled what would happen if CO2 levels doubled, and satellite observations have since confirmed it.
The reason is straightforward. More CO2 in the lower atmosphere means less infrared radiation reaches the stratosphere from below. At the same time, CO2 molecules in the stratosphere are efficient radiators: they emit infrared energy out to space. With less energy coming in from below and the same amount going out, the stratosphere loses net energy and cools. This simultaneous tropospheric warming and stratospheric cooling is a signature unique to greenhouse gas forcing, distinct from other possible causes of warming like increased solar output, which would warm both layers.
Earth’s Temperature Without an Atmosphere
A useful way to grasp how much radiation trapping matters is to calculate what Earth’s temperature would be without it. Using the physics of how objects radiate energy (the relationship between emitted energy and temperature to the fourth power), and accounting for Earth’s 0.30 albedo, the planet’s effective emitting temperature works out to about 255 K, or minus 18°C. The actual average surface temperature is about 33 degrees warmer than that. The entire difference is the greenhouse effect: infrared radiation being absorbed and recycled within the atmosphere before it can escape to space.