All weather on Earth, from the surface of the planet out into space, begins with the Sun.  Space weather and terrestrial weather (the weather we feel at the surface) are influenced by the small changes the Sun undergoes during its solar cycle.  

The most important impact the Sun has on Earth is from the brightness or irradiance of the Sun itself. The Sun produces energy in the form of photons of light. The variability of the Sun's output is wavelength dependent; different wavelengths have higher variability than others. Most of the energy from the Sun is emitted in the visible wavelengths (approximately 400 – 750 nanometers (nm)).  The output from the sun in these wavelengths is nearly constant and changes by only one part in a thousand (0.1%) over the course of the 11-year solar cycle.

At Ultraviolet or UV wavelengths (120 – 400 nm) the solar irradiance variability is larger over the course of the solar cycle, with changes up to 15%. This has a significant impact on the absorption of energy by ozone and in the stratosphere. At shorter wavelengths, like the Extreme Ultraviolet (EUV), the Sun changes by 30% - 300% over very short timescales (i.e. minutes). These wavelengths are absorbed in the upper atmosphere so they have minimal impact on the climate of Earth. At the other end of the light spectrum, at Infrared (IR) wavelengths (750 – 10,000 nm), the Sun is very stable and only changes by a percent or less over the solar cycle.

The total energy from sunlight, integrated over all wavelengths,  is referred to as the Total Solar Irradiance (TSI). It is measured from satellites to be about 1361 Watts/m² at solar minimum, increasing to 1362 Watts/m² at solar maximum. An increase of 0.1% in the TSI represents about 1.3 Watts/m² change in energy input at the top of the atmosphere.  When averaged over the entire Earth, this change is only about 0.2 Watts/m².   A typical residential solar panel has an area of about 1 square meter.  So, in short, the change in solar energy received from the Sun over the course of the 11-year solar cycle is not enough to power even one additional 15-Watt light bulb with a solar panel.

This change in TSI is too small to have a major impact on the Earth’s climate.   Furthermore, it is cyclic in nature: over the past few centuries, solar activity has regularly risen and fallen every 11 years (approximately).  This pattern bears little resemblance to the steady increase in global temperatures on Earth over the twentieth century (see Figure).   So it is no surprise that the Intergovernmental Panel on Climate Change (IPCC)  finds solar activity to be a minor contributor to climate change compared to anthropogenic factors such as the emission of greenhouse gasses.

However, though the effects of solar variability on the climate are not noticeable to most of us, they are detectable by dedicated scientists.   Careful measurements suggest that solar activity does in fact warm the Earth by about a tenth of a degree (0.1° C) during solar maximum relative to solar minimum.  Furthermore, solar radiation appears to have a subtle influence on some of the climate’s internal modes of variability, such as the North Atlantic Oscillation (NAO). 

This influence is more than might be expected from a global solar energy change of only 0.2 W/m² so it is believed that there are positive feedbacks that serve to amplify the climate response.   One such feedback scenario is called the “top-down” mechanism and is attributed to the heating of the stratosphere by the absorption of UV radiation.  As mentioned above, UV light from the Sun varies much more over the course of the solar cycle than TSI and may lead to changes in the circulation patterns between the stratosphere and troposphere.  Another “bottom-up” positive feedback mechanism has to do with the absorption of excess solar radiation by the ocean in cloud-free areas of the subtropics.  This can establish self-reinforcing circulations by enhancing evaporation,  amplifying the strength of the trade winds.

There are other ways that solar activity can affect the climate that are even more subtle and less understood.   Sunlight is not the only sort of radiation that reaches the Earth from space; energetic particles also rain down on our atmosphere, mostly in the form of high-speed protons.  High levels of solar magnetic activity can simultaneously enhance lower-energy particles that are produced  by solar storms and suppress higher-energy particles that originate from outside the solar system.  The latter are referred to as galactic cosmic rays and they are diverted by the solar magnetic field that permeates interplanetary space.   Changes in energetic particle fluxes can potentially induce subtle changes in the chemical composition of the atmosphere or seed cloud formation.  However, the impact of these changes on climate variability is still a matter of debate.

Whatever the mechanism, climate changes induced by solar activity can be expected to be enhanced during extended periods of low or high activity that may last multiple cycles. This includes the series of stronger-than-average cycles from 1933-1964 (see Figure) as well as so-called Grand Minima when magnetic activity is suppressed for decades. The most notable example of the latter is the well known Maunder Minimum of the seventeenth century, when sunspot activity largely ceased for nearly 70 years.  Given the historical rarity of such events, particularly in the last 200 years, their climate impacts are not well understood.