Solar activity does influence Earth’s climate, but its contribution to the warming observed since 1980 is negligible. The sun’s brightness fluctuates on an roughly 11-year cycle, and during strong cycles, that variation affects global average temperature by 0.1 degrees Celsius or less. Meanwhile, Earth’s average temperature has risen by well over 1 degree Celsius since preindustrial times, driven overwhelmingly by greenhouse gas emissions.
The relationship between the sun and climate is real, but it’s far smaller than many people assume. Here’s how it actually works, what role it played in the past, and why it can’t explain what’s happening now.
How Solar Output Varies
The sun doesn’t shine at a perfectly constant brightness. Its output rises and falls in roughly 11-year cycles, driven by changes in magnetic activity that produce sunspots, solar flares, and shifts in ultraviolet radiation. At the peak of a strong cycle, the sun puts out about 1 extra watt per square meter compared to the cycle’s low point. That sounds modest, and it is. Spread across the entire planet, the effect on global temperature tops out around 0.1°C.
Solar Cycle 24 (which peaked around 2014) had one of the lowest maximums in 70 years. Solar Cycle 25, the current one, is expected to be comparable. Neither represents an unusual surge in solar energy reaching Earth.
The Sun Tracked Climate for a Century, Then Stopped
For roughly the first 120 years of the modern temperature record, solar brightness and Earth’s temperature moved in loose parallel. This is one reason the sun-climate connection gained traction: when researchers at the Max Planck Institute for Solar System Research reconstructed 150 years of the sun’s total radiation, ultraviolet output, and magnetic field strength, they found that solar variations ran parallel to climate changes for most of that period.
But starting around 1980, the two lines diverge sharply. Solar brightness, ultraviolet output, and cosmic ray intensity all continue their 11-year oscillation, but show no upward trend. Earth’s temperature, on the other hand, climbs steeply. The sun simply cannot account for the warming of the last four and a half decades.
Satellite measurements confirm this. Between 2000 and 2022, the amount of energy Earth absorbs from incoming sunlight increased by about 0.9 watts per square meter. But the change in incoming solar energy itself was just 0.02 watts per square meter, essentially zero. The extra absorbed energy comes from changes in Earth’s own reflectivity and atmospheric composition, not from a brighter sun.
Three Ways the Sun Can Influence Climate
Solar activity doesn’t just mean “more heat.” It affects Earth’s atmosphere through at least three distinct pathways, each operating at a different scale.
Direct heating. More total solar radiation means slightly more energy reaching Earth’s surface. This is the most intuitive mechanism, and the easiest to measure. It’s also small: the total variation across a solar cycle amounts to roughly 0.1% of the sun’s output.
Ultraviolet effects on the upper atmosphere. The sun’s ultraviolet output varies much more dramatically than its visible light, sometimes by several percent across a cycle. Extra UV radiation is absorbed in the stratosphere, where it boosts ozone production and warms that layer of the atmosphere. These temperature and wind changes can ripple downward, altering large-scale weather patterns like the North Atlantic Oscillation. Research published in Geophysical Research Letters found that while the global temperature effect of this mechanism is negligible, regional effects on sea-level pressure patterns are likely significant. In other words, solar cycles may nudge winter weather patterns in places like northern Europe without meaningfully changing the global average.
Cosmic rays and cloud formation. This is the most debated pathway. The idea: when the sun is less active, its weaker magnetic field allows more cosmic rays to reach Earth’s atmosphere. Those cosmic rays ionize air molecules, potentially seeding the formation of tiny aerosol particles that could grow into cloud droplets. More low-level clouds would reflect sunlight and cool the planet. It’s an elegant hypothesis, and CERN built an entire experiment (called CLOUD) to test it.
What CERN’s CLOUD Experiment Found
The CLOUD experiment confirmed that ions from cosmic rays can indeed boost the rate at which tiny aerosol particles form. But that’s only the first step in a long chain. Those particles need to grow from about 1 nanometer to 50 or 100 nanometers before they can serve as seeds for cloud droplets, and most of them get absorbed by existing particles before reaching that size.
When researchers modeled the full chain from cosmic rays to cloud condensation nuclei, the response was “strongly sublinear,” meaning each increase in cosmic rays produced a diminishing return. Removing ions entirely (simulating zero cosmic rays) reduced cloud condensation nuclei by 10 to 20 percent at the altitude of low clouds. But the actual variation in cosmic rays across a solar cycle is far smaller than turning them off completely. The team’s conclusion: cloud condensation nuclei respond too weakly to changes in cosmic rays to yield a significant influence on clouds and climate over the span of a solar cycle.
The Sun and the Little Ice Age
The most famous example of solar activity affecting climate is the Maunder Minimum, a period from 1645 to 1715 when sunspots virtually disappeared. It overlapped with part of the Little Ice Age, a centuries-long cool period stretching from roughly the 13th to the mid-19th century. For a long time, the Maunder Minimum was treated as the Little Ice Age’s primary cause.
That narrative has shifted. The Little Ice Age began well before the Maunder Minimum, which makes it difficult to pin the cooling on solar inactivity alone. Current scientific understanding points to volcanic aerosols as the leading driver, with natural fluctuations in ocean circulation, changes in land use, and reduced solar output all playing supporting roles. The sun contributed, but it wasn’t the star of the show.
What a Future Solar Minimum Would Mean
Some have speculated that the sun could enter another prolonged quiet phase, a “grand solar minimum,” and that this might counteract greenhouse gas warming. Researchers have modeled exactly this scenario, simulating a Maunder Minimum-scale drop in solar output during the second half of the 21st century. The result: global temperatures would decrease by about 0.1°C.
For context, under moderate emissions scenarios, greenhouse gas warming over the same period is projected at several degrees Celsius. A grand solar minimum would shave off a tiny fraction of that warming, roughly equivalent to delaying the projected temperature rise by a few years. It would not reverse, pause, or meaningfully slow the overall trend.
Putting the Numbers Side by Side
The clearest way to understand the sun’s role is to compare the energy numbers directly. Earth is currently gaining about 1.0 watt per square meter more energy than it radiates back to space, a figure that has doubled since the early 2000s. Of that increase, the change in incoming solar energy accounts for 0.02 watts per square meter. That’s 2 percent of the imbalance. The remaining 98 percent comes from changes in how Earth’s atmosphere traps and reflects energy, dominated by rising greenhouse gas concentrations.
Solar activity is a real climate factor. It shaped temperature patterns for centuries, it influences regional weather through stratospheric pathways, and it adds a small oscillation to global temperatures every 11 years. But it is not driving the warming trend that defines modern climate change, and even an unusually quiet sun would do almost nothing to offset it.