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Causes Greenhouse gases (GHGs) & how they work

Solar flare image: NASA

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Summary

The sun warms the Earth through solar radiation. The Earth then radiates this warmth back up into the atmosphere where certain gases stop heat from escaping to space. These gases are amongst many other climate forcings. They’re called ‘greenhouse’ gases because, while the process is slightly different, the effect they have is similar to that of a greenhouse. That is, they act like a thermostat, helping to regulate the temperature of the atmosphere and with it, the planet as a whole. 
 
Greenhouse gases in the atmosphere are measured in parts per million (ppm). While the amounts seem small, without them, the average temperature on Earth would be -18°C; a frozen snowball (Video 1). But you can have too much of a good thing. Increasing them just a tiny amount ratchets up the thermostat; see the Stefan-Boltzmann Law.
 

What’s covered in this page:

Fig. 1: The Greenhouse Effect

The main GHGs

Other GHGs

*Not natural; created through industrial chemical processes

The global warming potential of each greenhouse gas

Before humans began burning large amounts of fossil fuels, the average temperature on Earth was around 14°C. In 2024 the average temperature reached 17.6°C. And we’re adding more of these gases every year (Table 1). 

Some GHGs have a much more powerful global warming potential than others, including entirely new man-made GHGs created through industrial processes (the red * in Table 2).

Table 1: Increase in GHGs since 1820

One million (1,000,000) molecules of air contains:
~780,000 molecules (parts per million or ppm) of nitrogen (N2)
~210,000 molecules (ppm) of oxygen (O2)
+ non-GHGs including helium, hydrogen and trace gases
+ the following greenhouse gases:

   Year: 1820 (ppm)

Water vapour              ~  3,900

Carbon dioxide          ~    284

Methane                    ~       0.774

Nitrous oxide              ~       0.27

     Year:  2025 (ppm)

~   4,170

~     429

~         1.946 

~         0.339

Main greenhouse gases 2025
Fig. 2: Increases in the three of the main greenhouse gases 1984 – 2025 (Image: Zach Labe, 06 Feb 2026).

Table 2: Comparative warming potential of different GHGs

Global warming potential

Carbon dioxide

Methane

Nitrous oxide

Sulphur hexafluoride*

CFCs -12*

HCFC -22*

Comparative value

1

25 x stronger

298 x stronger

22,200 x stronger

10,800 x stronger

1,760 x stronger

(* Man-made industrial chemicals)

The above tables are simplified summaries. The chemical interaction of GHGs, how long they survive in the atmosphere and where they’re located in the atmosphere all contribute to their global warming potential over hundreds of years. For full details see the IPCC Report here.

The radiative forcing (warming influence) of long-lived atmospheric greenhouse gases has accelerated, almost doubling in 40 years.
Figure 3: The heat that GHGs contribute to warming is measured in watts/ square metre. Carbon dioxide (red), methane (orange), nitrous oxide (green). Increasing amounts from the other man-made industrial chemicals GHGs (grey, blue, pink). While we’ve known for over a century that this is a problem for our climate, the radiative forcing (warming influence) of long-lived atmospheric greenhouse gases has accelerated, almost doubling in 40 years. (Image: rCraig009 based on data from NOAA).
Figure 4: GHGs (red line) are the largest contribution to rising temperatures or ‘climate forcing’. Some minor cooling occurs following volcanic eruptions (orange line), but otherwise temperatures (grey line) keep rising with rising GHGs. ‘Aerosols’ (dark blue line) are pollutants like ash and soot that prevent some sunlight from reaching Earth, resulting in slight cooling while they’re in the air. This highlights the complexity of climate change because when soot and ash eventually falls out of the atmosphere onto snow and ice, it increases warming by reducing the albedo effect (see below). (Image composite: Carbon Brief).


Why are garden greenhouses warmer when plants absorb so much CO2?

In both the atmosphere and in greenhouses of all sizes, solar energy is converted to heat energy. In the atmosphere, heating works by absorption and emission of electromagnetic radiation. Greenhouse gases prevent some of the absorbed heat from leaving the atmosphere through radiative transfer (RT).

In a greenhouses, the heat is absorbed by plants, the soil, and other objects. Concrete bricks, for example, absorb a lot more heat than wooden tables. The heat is circulated in this relatively small space through convection: warmer air goes up and cooler air flows down. However, the roof and walls, not greenhouse gases, prevents the heat from escaping. (The RT value in each greenhouse depends on the individual chemical properties of different greenhouse gases). 

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