8.1 Solar variability and its effects on climate

The Sun's energy output isn't perfectly constant. It varies due to magnetic activity cycles and longer-term fluctuations. These changes in solar irradiance affect Earth's climate through direct heating and indirect effects on atmospheric composition and circulation patterns.

Solar variability has influenced historical climate events, but its impact is now overshadowed by human-caused greenhouse gas emissions. Understanding solar forcing helps you contextualize natural climate variations and see how they interact with anthropogenic factors.

Solar Variability and Its Impacts on Climate

Factors of solar activity variation

Solar magnetic activity is the main driver of variations in solar output. The Sun's magnetic field isn't static; it goes through regular cycles that change how much energy the Sun emits.

• Sunspot cycles occur on roughly 11-year intervals. Sunspots are dark, cooler patches on the Sun's surface caused by intense magnetic activity. Counterintuitively, more sunspots mean more total energy output, because the bright regions (faculae) that accompany sunspots more than compensate for the dark spots themselves.
• Magnetic polarity reversals happen every 22 years. The Sun's magnetic field completely flips direction, so it takes two 11-year sunspot cycles to return to its original polarity.

Long-term solar variability operates on timescales of centuries to millennia:

• Grand solar minima are extended periods of unusually low solar activity. The most famous example is the Maunder Minimum (1645–1715), when sunspots nearly vanished for decades.
• Grand solar maxima are periods of heightened solar activity, such as the Medieval Solar Maximum (roughly 1100–1250).

Solar irradiance variations affect the amount of energy reaching Earth, but the changes are small:

• Total Solar Irradiance (TSI) changes by only about 0.1% over the 11-year solar cycle. That translates to roughly $\pm 0.5 \text{ W/m}^2$ variation in radiative forcing at Earth's surface.
• Spectral Solar Irradiance (SSI) changes are much larger in the ultraviolet (UV) wavelengths than in visible light. UV output can vary by several percent, which matters because UV strongly interacts with ozone in the stratosphere.

Solar irradiance effects on climate

Direct radiative forcing is the most straightforward effect: when TSI increases, more energy enters Earth's atmosphere and surface, warming the planet slightly. When TSI decreases, less energy arrives.

But the indirect effects are where things get more complex. Solar variability influences atmospheric circulation and composition through several pathways:

• UV variations alter stratospheric ozone concentrations and temperature. More UV means more ozone production and a warmer stratosphere. This temperature change in the stratosphere can influence the strength and position of the polar vortex and jet streams, which in turn affect weather patterns at the surface.
• Changes in cosmic ray flux may play a role. When the Sun is more active, its stronger magnetic field deflects more cosmic rays away from Earth. Some researchers have proposed that fewer cosmic rays means fewer cloud condensation nuclei and therefore less cloud cover, but this mechanism remains debated and is not well supported by observations.

Amplification mechanisms can enhance the climate response to what would otherwise be a very small forcing:

• Ice-albedo feedback: A small solar-induced warming can melt some sea ice or snow, exposing darker ocean or land surfaces that absorb more heat, which causes further warming.
• Ocean heat storage and transport: Oceans absorb and redistribute heat through currents, which can amplify or delay the climate response to solar changes over years to decades.

Solar activity vs. historical climate

The Little Ice Age (roughly 1450–1850) coincided with the Maunder Minimum and other periods of low solar activity. During this time, global temperatures were modestly cooler, and regional effects were pronounced: expanded sea ice in the North Atlantic, harsher winters across Europe, and shorter growing seasons.

The Medieval Warm Period (roughly 950–1250) overlapped with the Medieval Solar Maximum. Some regions, particularly parts of the North Atlantic and Europe, experienced warmer temperatures. However, the warming was not globally synchronous, meaning different regions warmed at different times, unlike modern greenhouse-driven warming which is global in scope.

Establishing clear causality between solar activity and these climate events is difficult because of:

• Complex interactions with other climate factors, such as volcanic eruptions and ocean oscillations, which were also occurring during these periods
• Uncertainties in reconstructing past solar activity (from proxies like tree rings and ice cores) and paleoclimate conditions

Importance of solar variability in context

Solar variability is a real climate forcing, but it needs to be placed alongside other natural and human-caused factors.

• Anthropogenic greenhouse gases have become the dominant forcing factor in recent decades. The warming effect of increasing $CO_2$ and other greenhouse gases (roughly $+2.7 \text{ W/m}^2$ since pre-industrial times) far outpaces the $\pm 0.5 \text{ W/m}^2$ variation from solar cycles.
• Volcanic eruptions can cause short-term cooling (1–3 years) by injecting sulfate aerosols into the stratosphere, which reflect sunlight. These events can temporarily mask or complicate the climate response to solar forcing.
• Internal climate variability modes modulate regional climate responses to solar forcing. El Niño-Southern Oscillation (ENSO) affects global temperature and precipitation patterns on 2–7 year timescales. The Atlantic Multidecadal Oscillation (AMO) influences North Atlantic sea surface temperatures over decades.
• Milankovitch cycles are orbital variations that drive glacial-interglacial cycles on timescales of tens to hundreds of thousands of years. These involve changes in Earth's orbit shape (eccentricity), axis tilt (obliquity), and wobble of the axis (precession). They operate on far longer timescales than solar magnetic variability and are the primary pacemaker of ice ages.