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Solar irradiance varies on 11-year (sunspot cycle) and longer timescales. Total solar irradiance variations are ~0.1%, corresponding to a forcing of ~0.2 W/m² at the 11-year peak, small compared to greenhouse gas forcing. However, solar variability may have contributed to the Maunder Minimum cooling (17th century) and continues to modulate climate on decadal timescales. Solar forcing is well-constrained from satellite observations and paleoclimate proxies.
You already know from studying Earth's energy balance that the Sun supplies virtually all of the energy driving the climate system, and from radiative forcing that any change in the energy input or output at the top of the atmosphere will push the climate toward a new equilibrium. Solar variability is the most obvious candidate for an external forcing mechanism — if the Sun's output changes, Earth's energy budget changes with it. The question is how much it actually varies and whether those variations are large enough to matter.
The Sun's luminosity is not perfectly constant. It fluctuates on an approximately 11-year sunspot cycle, during which the number of dark sunspots and bright faculae on the solar surface rises and falls. Counterintuitively, the Sun is slightly *brighter* at sunspot maximum because the bright faculae more than compensate for the dark spots. Satellite measurements since 1978 show that total solar irradiance (TSI) varies by about 0.1% over each cycle — roughly 1.4 W/m² out of a total of ~1361 W/m². After accounting for Earth's geometry and albedo, this translates to a radiative forcing of only about 0.2 W/m² at cycle peak, which is small compared to the ~2.7 W/m² forcing from anthropogenic greenhouse gases accumulated since pre-industrial times.
On longer timescales, solar output may have varied more substantially. The Maunder Minimum (roughly 1645–1715) was a period when sunspots nearly vanished, coinciding with some of the coldest decades of the Little Ice Age in Europe. Reconstructions of past solar activity use paleoclimate proxies — cosmogenic isotopes like beryllium-10 in ice cores and carbon-14 in tree rings, whose production rates increase when solar magnetic shielding weakens during low-activity periods. These proxies suggest that multi-decadal solar minima may have contributed a negative forcing of 0.1–0.3 W/m², enough to produce modest regional cooling when combined with volcanic forcing and internal climate variability, but far too small to explain the warming observed since the mid-20th century.
The practical significance of solar variability for modern climate science is twofold. First, it must be included in climate models as an external forcing to accurately reproduce observed temperature records — particularly the pre-industrial period and early 20th century when greenhouse gas concentrations were lower. Second, the small magnitude of solar forcing relative to anthropogenic forcing provides a critical constraint: solar variability can modulate climate on decadal timescales and contributed to past climate episodes, but it cannot account for the rapid warming trend of recent decades. The forcing numbers make this unambiguous — a 0.2 W/m² solar signal cannot drive the warming that a 2.7 W/m² greenhouse gas signal produces.