Solar forcing is the influence the Sun’s energy output has on Earth’s climate. It’s measured as a change in energy reaching the top of the atmosphere, expressed in watts per square meter (W/m²). While the Sun is the fundamental driver of Earth’s climate system, its output isn’t perfectly constant. It fluctuates on cycles of roughly 11 years and over longer timescales, and these fluctuations nudge global temperatures up or down.
How Solar Forcing Works
Earth’s climate is essentially a balancing act between energy coming in from the Sun and energy radiating back out to space. A climate forcing is anything that tips that balance, whether it’s a change in solar output, a volcanic eruption scattering sunlight, or a buildup of greenhouse gases trapping heat. Solar forcing specifically refers to the Sun’s side of that equation.
The total energy the Sun delivers to Earth, called total solar irradiance (TSI), averages about 1,361 watts per square meter. That number isn’t fixed. It rises and falls with the Sun’s activity level, primarily on an 11-year cycle tied to sunspot activity. During a solar maximum (peak activity), TSI increases by roughly 0.1% compared to the solar minimum (quietest phase). That translates to about 1.4 extra watts per square meter at the top of the atmosphere.
A 0.1% change sounds tiny, and in terms of direct heating, it is. But the climate system can amplify small nudges through feedback loops involving clouds, ice cover, and atmospheric chemistry.
Total vs. Spectral Irradiance
Not all of the Sun’s energy arrives in the same form. Sunlight spans a wide range of wavelengths: ultraviolet, visible light, and infrared. TSI captures the total energy across all wavelengths, while solar spectral irradiance (SSI) breaks that total down by wavelength. NASA’s TSIS-1 instrument package on the International Space Station tracks both, measuring SSI from 200 to 2,400 nanometers, which covers about 96% of the total energy output.
This distinction matters because different wavelengths affect the atmosphere differently. Visible light mostly passes through to warm the surface, while ultraviolet radiation is absorbed higher up, in the stratosphere. And crucially, the Sun’s UV output varies far more than its visible light output over the solar cycle, even though UV makes up a small fraction of total energy.
The UV Connection to the Stratosphere
Ultraviolet radiation plays a disproportionate role in solar forcing. When the Sun is more active, its UV-C emissions (the shortest, most energetic ultraviolet wavelengths) increase substantially. This extra UV energy drives photochemical reactions in the upper stratosphere, boosting ozone concentrations at altitudes of 30 to 60 kilometers. At the same time, ozone in the lower stratosphere (15 to 30 km) actually decreases slightly.
These ozone shifts alter how the stratosphere absorbs heat, which changes temperature gradients and wind patterns that can propagate downward to influence weather and climate at the surface. A 5% increase in solar UV output in the 200 to 370 nanometer range produces up to a 4.5% increase in upper-stratospheric ozone. This indirect pathway means the Sun can influence climate through atmospheric chemistry, not just raw heating.
The 11-Year Solar Cycle
The most well-documented pattern in solar forcing is the roughly 11-year sunspot cycle. Sunspots themselves are cooler, darker patches on the Sun’s surface, but they’re surrounded by bright regions called faculae. On timescales of a full solar cycle, the brightening from faculae outweighs the dimming from sunspots, so the Sun is actually about 0.1% brighter at solar maximum.
Satellite measurements since 1979 have tracked this cycle precisely. The peak-to-peak variation has ranged from about 0.063% to 0.096% across the four most recent cycles. The current cycle, Solar Cycle 25, is expected to reach its maximum around mid-2025, with a predicted peak sunspot number of about 115. NOAA’s Space Weather Prediction Center estimates the peak could fall anywhere between late 2024 and early 2026.
Solar Forcing Over Centuries
Beyond the 11-year cycle, the Sun goes through longer periods of higher or lower activity. The most famous low point is the Maunder Minimum, a stretch from roughly 1650 to 1710 when sunspots nearly vanished. During this period, temperatures across much of the Northern Hemisphere dropped noticeably. Climate models show the most pronounced cooling over eastern and central North America and northern Eurasia, contributing to what’s often called the Little Ice Age (though volcanic eruptions and ocean circulation changes also played roles).
Scientists reconstruct past solar activity using indirect markers: historical sunspot records, aurora observations, and isotopes like beryllium-10 and carbon-14 that accumulate in ice cores and tree rings when solar activity is low. These proxies extend the solar record back thousands of years, revealing that the Sun’s output has wandered enough to leave a measurable imprint on global temperatures over centuries.
Solar Forcing vs. Greenhouse Gas Forcing
The most common question about solar forcing is whether it explains recent global warming. The numbers make the answer clear. The IPCC’s most recent assessment puts the total solar forcing from 1750 to 2019 at just 0.01 W/m², with a likely range of -0.06 to +0.08 W/m². Over nearly 270 years, the Sun’s net contribution to the energy imbalance has been essentially zero.
By comparison, greenhouse gases have added 3.84 W/m² of forcing over the same period. Carbon dioxide alone accounts for 2.16 W/m², methane adds 0.54 W/m², and other gases contribute the rest. The total human-caused forcing, after subtracting the cooling effect of aerosol pollution, comes to 2.72 W/m². That’s roughly 270 times the solar contribution.
Satellite data reinforces this picture. Over the past four decades of direct measurement, there has been no upward trend in solar irradiance. The Sun’s output cycles up and down every 11 years, but it isn’t getting systematically brighter. Global temperatures, meanwhile, have risen sharply over the same period, tracking the increase in greenhouse gas concentrations rather than solar output.
How Scientists Measure Solar Forcing
Before satellites, estimates of solar forcing relied entirely on indirect evidence. Since 1979, space-based instruments have provided continuous, direct measurements. The current workhorse is NASA’s TSIS-1 (Total and Spectral Solar Irradiance Sensor), mounted on the International Space Station. It carries two instruments: the Total Irradiance Monitor, which tracks the Sun’s total energy output, and the Spectral Irradiance Monitor, which measures how that energy is distributed across wavelengths.
TSIS-1 continues a measurement record that now spans over 40 years, building on earlier instruments aboard the Solar Radiation and Climate Experiment (SORCE) satellite, which launched in 2003. Maintaining this unbroken record is critical because detecting long-term trends in solar output requires extremely precise, consistently calibrated measurements over many decades. Even small instrumental drifts could create false trends in a signal as subtle as 0.1%.