Before humans, climate changes were caused entirely by natural factors that affect how much solar energy reaches earth. More solar energy increases temperatures. Less solar energy decreases temperatures. Factors include:
- Variations in Earth’s orbit over the span of tens of hundreds of thousands of years. These changes probably caused past cycles of ice ages.
- Small ups and downs in the sun’s intensity occur in cycles.
- Aerosols ejected by volcanic eruptions and meteor strikes can block sunlight, causing cooling.
- Variations in the levels of greenhouse gases in the atmosphere.
Scientists have found no evidence that natural factors alone could have caused recent climate change.
Photo Credits: temperature graph: US-EPA, CO2 graph: NASA