The Sun is the primary external energy source driving Earth's climate. Variations in the Sun's energy output — known as solar variability — constitute a natural external climate forcing. Understanding how solar output changes over time is essential for distinguishing natural climate drivers from anthropogenic ones.
Total Solar Irradiance (TSI) is defined as the total solar power per unit area incident on a surface perpendicular to the Sun's rays at the top of Earth's atmosphere. Its average value is approximately:
This value is also called the Solar Constant, though it is not perfectly constant — it varies slightly with solar activity.
TSI is significant for climate because it determines the total energy input into the Earth system. Even small changes in TSI can influence global mean temperatures over long timescales.
Sunspots are cooler, darker regions on the Sun's photosphere caused by intense localised magnetic fields that inhibit convective heat transport. Despite being darker than the surrounding surface, sunspots are accompanied by bright regions called faculae.
Solar activity — including sunspot numbers, solar flares, and faculae — follows a periodic cycle of approximately 11 years, known as the Schwabe cycle or solar sunspot cycle:
The variation in TSI over one solar cycle is small (approximately ), but cumulative effects over many cycles can influence climate.
Historical records reveal periods of prolonged low solar activity. The most notable is the Maunder Minimum (approximately 1645–1715 CE), during which sunspot activity was almost entirely absent for several decades.
This period coincided with the Little Ice Age — a prolonged interval of cooler temperatures across the Northern Hemisphere, characterised by:
The correlation between the Maunder Minimum and the Little Ice Age provides evidence that reduced solar output acts as a negative external forcing, lowering global mean temperatures. However, it is important to note that the Little Ice Age was also influenced by increased volcanic activity during the same period.
In the framework of climate forcings (SLO P-12-F-55), solar variability is classified as a natural external forcing:
| Forcing Type | Example | Effect |
|---|---|---|
| Natural External | Solar variability (TSI changes) | Warming (solar max) or cooling (solar min) |
| Natural External | Volcanic eruptions | Short-term cooling (aerosols) |
| Anthropogenic External | CO₂ from fossil fuels | Long-term warming |
While solar variability is a real and measurable forcing, scientific consensus indicates that recent (post-1980) global warming cannot be explained by solar variability alone — TSI has shown no significant upward trend since the 1980s, while global temperatures have continued to rise, pointing to anthropogenic greenhouse gas emissions as the dominant current forcing.