What causes climate change?

What causes climate change?

Menu: The causes

The main GHGs

(Image: MIT)

Greenhouse gasses

Summary

  • The sun warms the Earth through solar radiation. The Earth then radiates this warmth back up into the atmosphere where certain gasses stop heat from escaping to space (Fig. 1 & Video 1).
  • These gasses 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’ gasses (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, mostly from burning fossil fuels but also from agriculture, so global temperatures have risen (Fig. 3).
  • The climate doesn’t instantly respond to adding excessive GHGs into the atmosphere. But once those gasses 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.

Other GHGs

A little has a huge impact

Greenhouse gasses in the atmosphere are measured in parts per million (ppm). While the amounts seem small (see Table 1), without GHFs, the average temperature on Earth would be -18°C; a frozen snowball. With just a few hundred ppm of greenhouse gasses in the atmosphere in the year 1850 (before humans began burning large amounts of fossil fuels), the average temperature on Earth was 13.7°C.

Fig. 1: (Image: Wikimedia commons)

Since 1850 (Fig. 2) we’ve added a lot more GHGs. And we’re continuing to add more every year. Fig. 3 shows much this is warming the planet.

Table 1

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-GHG gasses including helium, hydrogen and trace gasses
+ the following greenhouse gasses:

   Year: 1820 (ppm)

 

Water vapour              ~  3,900

Carbon dioxide          ~    284

Methane                     ~       0.774

Nitrous oxide               ~       0.27

     Year: 2017 (ppm)

 

~   4,100

~     407.9

~         1.85

~         0.33

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

Table 2

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.

Video 1: How greenhouse gasses work
Fig. 2: Greenhouse gasses (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 gasses’ include man-made HCFCs and HFCs. While we’ve known about the effect of CO2 for over a century that it’s a problem for our climate (scroll down to see the timeline), there has been a dramatic 43% increase in GHG emissions since 1990. (Image: NOAA).

We’ve known about greenhouse gasses and the impact of CO2 on the climate for a very long time.

See this short history of who knew what, when (this website).

“The effects will indeed be catastrophic (at least for a substantial fraction of the world’s population).”ExxonMobil

Explainers

Climate Forcing:

The term ‘climate forcing’ comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy reaching Earth’s atmosphere and the amount that escapes. If more solar energy escapes than arrives, the planet cools (negative RF). Conversely, if less energy escapes than gets in, the planet warms (positive RF). This is due to the The Law of Conservation of Energy, a basic law of thermodynamics, which states that: ‘Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.’

Different climate forcings each determine how much solar energy arrives and escapes.

  • Natural Forcings are those that happen through natural changes.
  • Anthropogenic Forcings are those due to human activities.

Click here to learn about the main forcings and how they work (links to page on this site).


Greenhouses:

Greenhouses work by convection, while atmospheric heating works by absorption and emission of electromagnetic radiation, preventing absorbed heat from leaving the atmosphere through radiative transfer (RT), which is affected by the individual chemical properties of different greenhouse gasses.

References and further reading