What causes climate change?

What causes climate change?

Menu: The causes

 

The Milankovich Cycles

Summary

  • Changes to Earth’s climate over long periods are caused by the natural variations in intensity and distribution of solar radiation that reaches the planet.
  • These cycles are because Earth’s orbit around the Sun is eccentric (Fig. 1), it’s axis is tilted (Fig. 2), and it wobbles (Fig. 3), each over different time frames or cycles (Fig. 4).
  • The cycles are named after the geophysicist and astronomer Milutin Milanković, who developed James Croll’s theory that these cycles were the main climate change forcings (Video 1).
  • Today, when the Milankovich Cycles are taken into consideration, Earth should have been slowly cooling over the past 9,000 years. Instead, we’re warming (Fig. 5 & Video 2).
  • This gradual warming was because people developed agriculture.  On balance, the climate remained relatively stable until the Industrial Revolution when we started burning huge amounts of fossil fuels. This rapid anthropogenic forcing rapidly outweighed the gradual cooling effects from the Milankovich cycles.
Fig. 1: Earth’s eccentric orbit around the sun (100,000 year cycle). When Earth is further away from the sun it receives less solar energy and therefore less warmth.
Fig. 2: Earth is slightly tilted (40,000 year cycle). As most of the land mass is now in the Northern Hemisphere (see the link in the menu ‘How to start an ice age’), when less of that hemisphere is facing the sun in winter, it receives less warmth.
Fig. 3: Precession (26,000 year cycle). This wobble can either add to the cooling or warming depending on which phases of the other two cycles that Earth is in (Figures 1 and 2).
Fig. 4: Each cycle occurs over different time frames. When they coincide, the effects are multiplied. However, other factors such as plate tectonics (see the first video ‘How Ice Ages happen’) and the balance of the carbon cycle can exaggerate, reduce, or cancel the effects of the Milankovich cycles over thousands or even millions of year. (Image: The Nature Education Project)

Video 1

Video 2

Fig. 5: The day grey line shows the observed temperature changes for the past 125 years, while the pale blue line shows Earth’s orbital changes. The cooling influence of the Milankovich Cyles over the past 9,000 is hardly noticeable in this short time frame. (Credit: Bloomberg)

Explainer

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. Conversely, if less energy escapes than gets in, the planet warms. This is due to 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, including the Milankovich cycles the position of Earth’s continents, and volcanoes
  • 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).

References and further reading