What causes global warming? An energy imbalance. Greenhouse gases in the atmosphere are transparent to incoming sunlight but absorb outgoing infrared radiation, so heat that should escape to space is retained. Add more of those gases and the planet warms until it radiates enough to rebalance. That is the entire mechanism, and it has been understood since the 1800s.
How the Greenhouse Effect Works
Sunlight arrives mostly as visible and ultraviolet light, which passes through the atmosphere fairly easily and warms the surface. The warmed surface radiates that energy back out — but as infrared, at much longer wavelengths.
Certain molecules absorb strongly at exactly those infrared wavelengths: CO2, methane, water vapor, nitrous oxide, ozone. They absorb the outgoing radiation and re-emit it in all directions, including back down. The surface receives sunlight plus that returned infrared, so it ends up warmer than it would be with a transparent atmosphere.
Without any greenhouse effect, Earth’s average surface temperature would be about -18°C (0°F). With it, the average is about 15°C (59°F). The effect is not inherently bad — it is why the planet is habitable. The problem is the rate of change.
The Gases, Ranked by Impact
| Gas | Pre-industrial | Today | Warming potential (100 yr) | Main source |
|---|---|---|---|---|
| Carbon dioxide | ~280 ppm | ~425 ppm | 1 (baseline) | Fossil fuels, cement, land use |
| Methane | ~722 ppb | ~1,900 ppb | ~28-30x | Agriculture, gas leaks, wetlands |
| Nitrous oxide | ~270 ppb | ~335 ppb | ~265x | Fertilizer, combustion |
| Water vapor | Variable | Variable | — | Evaporation (a feedback, not a driver) |
Water vapor is the strongest greenhouse gas by total effect, but it is not the cause here. It does not accumulate independently — warmer air holds more moisture, so water vapor amplifies warming started by something else. It is a feedback, and a strong one.
How We Know It Is From Fossil Fuels
This is where the evidence is unusually direct. Three independent lines converge:
- Isotopic fingerprint — CO2 from burning ancient carbon has a distinct ratio of carbon-13 to carbon-12, depleted relative to modern biological carbon. Atmospheric CO2 shows exactly that shift, and it has grown as emissions grew. The carbon is tagged.
- Oxygen decline — combustion consumes O2. Atmospheric oxygen is measurably declining at the rate fossil fuel burning predicts.
- Stratospheric cooling — if warming came from the Sun, the whole atmosphere would warm. Instead the lower atmosphere warms while the stratosphere cools, which is precisely what greenhouse trapping predicts.
Add the observed energy imbalance measured directly by satellites, and the picture is not really ambiguous.
Solar Activity Is Not the Explanation
If not the Sun, then what causes global warming? The measured rise in heat-trapping gases.
Total solar irradiance has been measured by satellite since 1978, and reconstructed from sunspot records and cosmogenic isotopes for much longer. Solar output has shown no net increase over the past several decades — if anything, a slight decline since the 1980s, while surface temperatures rose fastest.
Volcanic activity contributes too, but in the cooling direction: sulfate aerosols from eruptions reflect sunlight. Large eruptions produce temporary global cooling of a few tenths of a degree lasting one to two years.
The Numbers So Far
- Global average surface temperature has risen roughly 1.3°C (2.3°F) above pre-industrial levels
- The last decade was the warmest in the instrumental record
- Sea level has risen about 20-25 cm since 1900, from thermal expansion plus melting ice
- Arctic sea ice extent has declined substantially in every month of the year
- Ocean heat content continues to rise, which is the least noisy indicator of the imbalance
What has not happened is worth stating too: the frequently repeated predictions from decades past — the ozone hole was fixed by policy; predictions of an ice-free Arctic by 2013 were wrong and came from outlier extrapolations, not the mainstream models.
Why It Is Hard to Act On
CO2 is not like a pollutant that degrades. It accumulates, and a significant fraction stays in the atmosphere for centuries to millennia. That means warming is roughly proportional to cumulative emissions, not annual ones — so emissions delayed are emissions avoided, but emissions already produced keep working for a very long time.
It also means the system has real inertia. Oceans hold over 90% of the excess heat and take centuries to equilibrate, so some further warming is already committed regardless of what happens next.
Keep Exploring
- Weather vs climate — why one cold snap proves nothing
- How do ocean currents work? — where the heat is going
- What is a volcano? — the natural forcing that cools instead
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