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Causes: Greenhouse gases

Causes: greenhouse gases

Image: MIT

Greenhouse gases and how they work

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 (Fig. 1 & Video 1).
  • These gases are called climate ‘forcings’ because they act like a thermostat, helping to regulate the temperature of the atmosphere and with it, the planet.
  • Each gas has a different global warming potential (GWP), that is, how much it can potentially contribute to climate change.
  • They’re called ‘greenhouse’ gases (GHGs) because, while the process is slightly different, the effect is similar to that of a greenhouse.
  • The more GHGs there are in the atmosphere, the warmer the global climate (Fig. 2).
  • Anthropogenic (man-made) activities have added a lot more GHGs to the atmosphere, from burning fossil fuels and, destroying biodiversity for large scale agriculture, so global temperatures have risen (Fig. 3).
  • The climate doesn’t instantly respond to adding excessive GHGs into the atmosphere. But once those gases are up there, the planet will continue to warm for thousands of years. The more we add, the more we ‘lock in’ the effects for ourselves and future generations.

Image: MIT

Greenhouse gases and how they work

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 (Fig. 1 & Video 1).
  • These gases are called climate ‘forcings’ because they act like a thermostat, helping to regulate the temperature of the atmosphere and with it, the planet.
  • Each gas has a different global warming potential (GWP), that is, how much it can potentially contribute to climate change.
  • They’re called ‘greenhouse’ gases (GHGs) because, while the process is slightly different, the effect is similar to that of a greenhouse.
  • The more GHGs there are in the atmosphere, the warmer the global climate (Fig. 2).
  • Anthropogenic (man-made) activities have added a lot more GHGs to the atmosphere, from burning fossil fuels and, destroying biodiversity for large scale agriculture, so global temperatures have risen (Fig. 3).
  • The climate doesn’t instantly respond to adding excessive GHGs into the atmosphere. But once those gases are up there, the planet will continue to warm for thousands of years. The more we add, the more we ‘lock in’ the effects for ourselves and future generations.

A little has a huge impact

Video 1: How greenhouse gases work

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). With just a few ppm of greenhouse gases in the atmosphere before humans began burning large amounts of fossil fuels, the average temperature on Earth was around 14°C. Today, the average temperature is closer to 16°C. And we’re adding more of these gases every year (Table 1). Figure 3 shows how much this is warming the planet just over the last four decades.

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: 2017 (ppm)

~   4,100

~     407.9 (419 in 2021)

~         1.85

~         0.33

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

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.

Fig. 2: 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 ‘Feedback effects and tipping points’ in the menu). (Image composite: Carbon Brief).
Fig. 3: The heat that GHGs contribute to warming is measured in watts/ square metre. This graph shows the main contributors are CO2 (black) and CH4 (grey), with increasing amounts from the other GHGs (coloured). ‘Minor gases’ include man-made HCFCs and HFCs. While we’ve known for over a century that this is a problem for our climate, there has been a dramatic 43% increase in GHG emissions since 1990. (Image: NOAA).

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