Global warming is pushing Earth away from its natural glacial-interglacial rhythm because rapid anthropogenic warming overrides the slower influence of orbital changes. It does not stop orbital cycles; greenhouse gas emissions alter how the climate system responds to them.
This is HL-only content in IB ESS subtopic 6.1, Introduction to the atmosphere.
The Mechanism
Natural glacial and interglacial periods are paced partly by Milankovitch cycles: long-term variations in Earth's orbit and orientation. Changes in orbital eccentricity, axial tilt and precession alter the seasonal and geographical distribution of incoming solar radiation over tens to hundreds of thousands of years.
Small changes in solar radiation are amplified by feedback mechanisms. Cooling expands ice cover, increases Earth's albedo and reflects more solar radiation, causing further cooling. Warming melts ice, lowers albedo and increases absorption, reinforcing warming through positive feedback.
| Feature | Natural change | Current global warming |
|---|---|---|
| Main forcing | Changes in Earth's orbit and orientation | Anthropogenic greenhouse gas emissions |
| Timescale | Tens to hundreds of thousands of years | Decades to centuries |
| Energy mechanism | Redistribution of incoming solar radiation | Increased absorption and re-emission of outgoing infrared radiation |
| Likely effect | Alternation between glacial and interglacial conditions | Shift toward hotter climatic conditions |
Burning fossil fuels and deforestation increase atmospheric carbon dioxide, strengthening the enhanced greenhouse effect. This rapid forcing can delay orbitally driven cooling and move Earth toward a new, hotter equilibrium. The EPICA Dome C ice-core record demonstrates the historical relationship between greenhouse gas concentrations and glacial-interglacial temperatures.
The common misconception is that global warming has changed or stopped Milankovitch cycles. It has not: orbital changes continue, but anthropogenic forcing dominates their climatic influence.
Exam Technique
For an HL explain question, present the chain: greenhouse gas emissions, enhanced greenhouse effect, warming, ice melt, lower albedo and positive feedback. Distinguish orbital forcing from anthropogenic forcing.