The Moon is the same distance from the Sun as Earth, yet its average surface temperature is far below freezing and swings by hundreds of degrees between day and night. Earth's average surface temperature is a comfortable 15 °C or so. The difference is our atmosphere, and in particular a handful of gases that make up only a tiny fraction of it. This lesson explains how those gases keep Earth warm, and why adding more of them makes it warmer still.
Earth's energy budget
Earth's temperature depends on a balance. Energy arrives from the Sun as shortwave radiation: mostly visible light, plus some ultraviolet and near-infrared. About 30% of it is reflected straight back to space by clouds, ice, snow, deserts and the atmosphere. That fraction, 0.30, is Earth's average albedo. The other 70% is absorbed, most of it by the land and oceans. A warm surface gives off energy too, but because Earth is much cooler than the Sun, it radiates longwave infrared radiation (heat radiation you cannot see). If Earth absorbs more energy than it radiates, it warms; as it warms it radiates more, until incoming and outgoing energy balance again.
The natural greenhouse effect
Most of the atmosphere (nitrogen and oxygen, about 99% of dry air) lets infrared radiation pass straight through. But greenhouse gases, mainly water vapour, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃), absorb infrared. The absorbed energy makes the molecules vibrate, and they then re-emit infrared in all directions, including back down toward the surface. The surface therefore receives energy from the Sun and extra energy from the atmosphere above it, and it settles at a higher temperature. This natural greenhouse effect raises Earth's average surface temperature from about −18 °C (what it would be with no greenhouse gases) to about 15 °C. Without it, oceans would freeze and life as we know it could not exist.
The anthropogenic (enhanced) greenhouse effect
Since the Industrial Revolution, people have added greenhouse gases to the air, mostly by burning coal, oil and natural gas and by clearing forests. CO₂ has risen from about 280 parts per million (ppm) before 1750 to over 420 ppm today, an increase of more than 50%, and methane has more than doubled. More greenhouse gas means more infrared is absorbed and sent back down before it escapes to space. For a while, Earth absorbs more energy than it loses, so it warms until a new, warmer balance is reached. This human-caused strengthening of a natural process is the anthropogenic or enhanced greenhouse effect. The Intergovernmental Panel on Climate Change (IPCC), which summarizes the work of thousands of scientists, concluded in 2021 that it is unequivocal that human influence has warmed the atmosphere, ocean and land.
- Caused by greenhouse gases that occur naturally in the air
- Keeps Earth's average temperature near 15 °C instead of about −18 °C
- Essential for liquid water and life
- Balanced over thousands of years by natural sources and sinks
- Caused by extra greenhouse gases released by human activity
- Has warmed Earth by more than 1 °C since the late 1800s
- Changes climate faster than many ecosystems and communities can adapt
- Can be slowed by cutting emissions and protecting carbon sinks
Albedo and the energy balance
| Surface | Approximate albedo | What it means |
|---|---|---|
| Fresh snow | 0.8–0.9 | Reflects most sunlight; absorbs little |
| Sea ice | 0.5–0.7 | Reflects about half or more |
| Desert sand | 0.3–0.4 | Reflects about a third |
| Forest | 0.1–0.2 | Dark canopy absorbs most sunlight |
| Open ocean | about 0.06 | Absorbs about 94% of the sunlight that reaches it |
| Earth as a whole | about 0.30 | About 30% of incoming sunlight is reflected to space |
Albedo changes the incoming side of the energy budget, while greenhouse gases change the outgoing side. Melting snow and ice lowers Earth's albedo, so more sunlight is absorbed: the same warming that melts ice makes Earth absorb even more energy. You will meet this feedback again in the lesson on oceans and climate.