National Weather Service United States Department of Commerce

Can an increase or decrease in sunspot activity
 affect the Earth's climate?



    Photograph from NASA  


A typical star, the Sun has a diameter of approximately 865,000 miles (1,392,083 kilometers) (nearly 10 times larger than the diameter of Jupiter) and is composed primarily of hydrogen.  The Sun's core is an astonishing 29,000,000 degrees F. (16,111,093 degrees C), while the pressure is about 100 billion times the atmospheric pressure here on Earth.  Under these conditions, hydrogen atoms come so close together that they fuse.  Right now, about half the amount of hydrogen in the core of the Sun has been fused into helium.  This took roughly 4.5 billion years to accomplish.  When the hydrogen is exhausted, the Sun's temperature at the surface will begin to cool and the outer layers will expand outward to near the orbit of Mars.  The Sun at this point will be a "red giant" and 10,000 times brighter than its present luminosity.  After the red giant phase, the Sun will shrink to a white dwarf star (about the size of the Earth) and slowly cool for several billion more years.

Sunspots:  One interesting aspect of the Sun is its sunspots.  Sunspots are areas where the magnetic field is about 2,500 times stronger than Earth's, much higher than anywhere else on the Sun.  Because of the strong magnetic field, the magnetic pressure increases while the surrounding atmospheric pressure decreases.  This in turn lowers the temperature relative to its surroundings because the concentrated magnetic field inhibits the flow of hot, new gas from the Sun's interior to the surface. 

Sunspots tend to occur in pairs that have magnetic fields pointing in opposite directions.  A typical spot consists of a dark region called the umbra, surrounded by a lighter region known as the penumbra.   The sunspots appear relatively dark because the surrounding surface of the Sun (the photosphere) is about 10,000 degrees F (5,538 degrees C), while the umbra is about 6,300 degrees F (3482 degrees C).  Sunspots are quite large as an average size is about the same size as the Earth.

Sunspots, Solar Flares, Coronal Mass Ejections and their influence on Earth:  Coronal Mass EjectionCoronal Mass Ejections (shown left) and solar flares are extremely large explosions on the photosphere.  In just a few minutes, the flares heat to several million degrees F. and release as much energy as a billion megatons of TNT.  They occur near sunspots, usually at the dividing line between areas of oppositely directed magnetic fields.  Hot matter called plasma interacts with the magnetic field sending a burst of plasma up and away from the Sun in the form of a flare.  Solar flares emit x-rays and magnetic fields which bombard the Earth as geomagnetic storms.  If sunspots are active, more solar flares will result creating an increase in geomagnetic storm activity for Earth.  Therefore during sunspot maximums, the Earth will see an increase in the Northern and Southern Lights and a possible disruption in radio transmissions and power grids.  The storms can even change polarity in satellites which can damage sophisticated electronics.  Therefore scientists will often times preposition satellites to a different orientation to protect them from increased solar radiation when a strong solar flare or coronal mass ejection has occurred.

The Solar Cycle:  Sunspots increase and decrease through an average cycle of 11 years.  Dating back to 1749, we have experienced 23 full solar cycles where the number of sunspots have gone from a minimum, to a maximum and back to the next minimum, through approximate 11 year cycles.  We are now well into the 24th cycle.  This chart from the NASA/Marshall Space Flight Center shows the sunspot number prediction for solar cycle 24.  The NASA/Marshall Space Flight Center also shows the monthly averaged sunspot numbers based on the International Sunspot Number of all solar cycles dating back to 1750.  (Daily observations of sunspots began in 1749 at the Zurich, Switzerland observatory.) 

One interesting aspect of solar cycles is that the sun went through a period of near zero sunspot activity from about 1645 to 1715This period of sunspot minima is called the Maunder Minimum.  The "Little Ice Age" occurred over parts of Earth during the Maunder Minimum.  So how much does the solar output affect Earth's climate?  There is debate within the scientific community how much solar activity can, or does affect Earth's climate.  There is research which shows evidence that Earth's climate is sensitive to very weak changes in the Sun's energy output over time frames of 10s and 100s of years.  Times of maximum sunspot activity are associated with a very slight increase in the energy output from the sun.  Ultraviolet radiation increases dramatically during high sunspot activity, which can have a large effect on the Earth's atmosphere.  The converse is true during minimum sunspot activity.  But trying to filter the influence of the Sun's energy output and its effect on our climate with the "noise" created by a complex interaction between our atmosphere, land and oceans can be difficult.  For example, there is research which shows that the Maunder Minimum not only occurred during a time with a decided lack of sunspot activity, but also coincided with a multi-decade episode of large volcanic eruptions.  Large volcanic eruptions are known to hinder incoming solar radiation.  Finally, there is also evidence that some of the major ice ages Earth has experienced were caused by Earth being deviated from its average 23.5 degree tilt on its axis.  Indeed Earth has tilted anywhere from near 22 degrees to 24.5 degrees on its axis.  But overall when examining Earth on a global scale, and over long periods of time, it is certain that the solar energy output does have an affect on Earth's climate.  However there will always be a question to the degree of affect due to terrestrial and oceanic interactions on Earth.