What causes climate change?
The Milankovich Cycles
- 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.
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.
References and further reading
- 2020: NASA Jet Propulsion Laboratory; Milankovitch (Orbital) Cycles and Their Role in Earth’s Climate
- 2020: NASA Jet Propulsion Laboratory; Why Milankovitch (Orbital) Cycles Can’t Explain Earth’s Current Warming
- Rose City Astronomers: The Milankovich Cycle and Celestial Mechanics
- The Nature Education Project: Variations and schematic diagrams of Milankovitch cycles
- Bloomberg graphics: What’s really warming the world?
- Open Source Systems, Science, Solutions: Milankovich Cycles
- Science Direct: The Law of Conservation of Energy
- 2013: IPPC Chapter 8: Anthropogenic and Natural Radiative Forcing in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change