The Sun & Earth's Climate
The Sun is the fundamental energy source for Earth's climate system, so it is natural to ask whether changes in solar output drive changes in climate. The answer is nuanced: solar variability does influence climate, but its role in recent warming is small compared to greenhouse gas emissions. This page summarises the current state of the science.
Total Solar Irradiance (TSI)
Total Solar Irradiance is the amount of solar energy arriving at the top of Earth's atmosphere per unit area. It has been measured continuously by satellites since 1978 and averages about 1,361 W/m².
TSI varies over the roughly 11-year solar cycle by about 0.1% — approximately 1.3 W/m² between solar minimum and solar maximum. At solar maximum, the Sun is slightly brighter overall despite having more dark sunspots, because the bright faculae that surround active regions more than compensate for the spots.
A 0.1% variation in TSI translates to roughly 0.2–0.3 W/m² of radiative forcing when averaged over Earth's surface and accounting for albedo (reflectivity). This is a real but modest forcing compared to other drivers.
Historical Context: The Maunder Minimum
Between roughly 1645 and 1715, the Sun went through a period of exceptionally low sunspot activity known as the Maunder Minimum. During several decades within this period, virtually no sunspots were observed. This coincided with the coldest phase of the Little Ice Age in Europe, when the Thames froze regularly and alpine glaciers advanced.
The temporal coincidence has long suggested a solar influence on climate. Reconstructions of past TSI suggest that the Sun may have been 0.1–0.3% dimmer during the Maunder Minimum, equivalent to a reduction in forcing of perhaps 0.2–0.5 W/m². This is enough to contribute to regional cooling, especially when combined with volcanic aerosol forcing and internal climate variability, but it is unlikely to have been the sole cause of the Little Ice Age.
Other grand solar minima (Spörer, Dalton) also appear in the proxy record and are correlated with cooler periods, reinforcing the idea that multi-decadal solar variability can modulate climate on regional scales.
Modern Warming: How Much Is the Sun?
Since the mid-20th century, the Sun's output has shown no net upward trend — if anything, recent solar cycles have been slightly weaker than those of the late 20th century. Meanwhile, global surface temperatures have risen sharply. This divergence is one of many lines of evidence that solar variability cannot explain the observed warming.
Quantitatively, the IPCC Sixth Assessment Report (AR6, 2021) estimates that the total radiative forcing from solar irradiance changes since the pre-industrial era (1750) is approximately +0.01 to +0.10 W/m². Compare this to the forcing from increased carbon dioxide alone, which is about +2.16 W/m², and the total anthropogenic forcing of roughly +2.7 W/m² (after accounting for aerosol cooling). Solar forcing is therefore a minor contributor — roughly 1–4% of the total change since 1750.
Climate models that include solar variability alongside greenhouse gases, volcanic aerosols, and other forcings reproduce the observed temperature record well. Models that include only solar and volcanic forcing cannot reproduce the warming of the last 50 years.
The Cosmic Ray–Cloud Hypothesis
In the late 1990s, Henrik Svensmark and colleagues proposed an indirect mechanism by which the Sun might influence climate more strongly than TSI changes alone would suggest. The idea is that galactic cosmic rays (GCRs) help ionise molecules in the lower atmosphere, promoting the formation of cloud condensation nuclei and thereby increasing low-level cloud cover. Since clouds reflect sunlight, more clouds would cool the planet. During solar maximum, the Sun's stronger magnetic field deflects more GCRs away from Earth, reducing cloud cover and warming the planet — and vice versa.
The hypothesis is physically plausible at the level of ion-induced nucleation. The CLOUD experiment at CERN confirmed that cosmic rays can enhance aerosol nucleation rates under controlled conditions. However, subsequent research has shown that the effect is too small to significantly alter global cloud properties. Multiple independent studies have failed to find a robust correlation between cosmic ray flux and global cloud cover in observational data. The IPCC AR6 assessed this mechanism as having very low confidence as a driver of climate change.
Putting It in Perspective
Research on solar influences on climate is legitimate and important. It helps scientists understand natural variability, which is essential for attributing observed climate changes to their correct causes. Solar variability can modulate climate on decadal to centennial time scales, and a future grand minimum could slightly offset a fraction of greenhouse-gas-driven warming for a few decades (studies suggest perhaps 0.1–0.3°C of offset, compared to projected warming of 1.5–4°C by 2100 under various emission scenarios).
However, the weight of evidence from multiple independent lines of research — direct TSI measurements, paleoclimate reconstructions, climate modelling, and atmospheric physics — consistently shows that solar variability is a minor contributor to the warming observed since the mid-20th century. The dominant driver of modern climate change is the increase in atmospheric greenhouse gas concentrations from human activities.
Acknowledging the Sun's modest but real influence does not diminish the urgency of addressing greenhouse gas emissions. The two findings are complementary, not contradictory.