THE SUN AND SUNSPOTS

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

Sunspots

    Photograph from NASA  

   A typical star, the Sun has a diameter of approximately 865,000 miles (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., 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., while the umbra is about 6,300 degrees F.  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 the Earth.   Therefore during sunspot maximums, the Earth will see an increase in the Northern and Southern Lights and a disruption in power grids and radio transmissions.  The storms can even change polarity in satellites which can damage sophisticated electronics.  

But the jury is still out on how much sunspots can (or do) affect the Earth's climate.  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.  From the mid 1600s to early 1700s, a period of very low sunspot activity (known as the Maunder Minimum) coincided with a number of long winters and severe cold temperatures in Western Europe, called the Little Ice Age.  It is not known whether the two phenomena are linked or if it was just coincidence.  The reason it is hard to relate maximum and minimum solar activity (sunspots) to the Earth's climate, is due to the complexity of the Earth's climate itself.  For example, how does one sort out whether a long-term weather change was caused by sunspots, or maybe a coinciding El Nino or La Nina?  Increased volcanic eruptions can also affect the Earth's climate by cooling the planet.  And what about the burning of fossil fuels and clear cutting rain forests?  One thing is more certain, sunspot cycles have been correlated in the width of tree ring growth.  More study will be conducted in the future on relating sunspot activity and our Earth's climate.  

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.  A chart of the current cycle is available from the NOAA Space Weather Prediction Center

NASA/Marshall Space Flight Center 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.)  This chart from NASA/Marshall Space Flight Center shows the sunspot number prediction for solar cycle 24.

One interesting aspect of solar cycles is that the sun went through a period of sunspot inactivity 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 the question remains, do solar minimums help to create periods of cooler than normal weather, and do solar maximums help to cause drought over sections of Earth?  This question is not easily answered due to the immensely complex interaction between our atmosphere, land and oceans.  In addition, there is evidence that some of the major ice ages Earth has experienced were caused by Earth being deviated from its "average" 23.5 degrees tilt on its axis.  The Earth has tilted anywhere from near 22 degrees to 24.5 degrees on its axis.  The number of sunspots alone does not alter the overall solar emissions much at all.  However, the increased/decreased magnetic activity which accompanies sunspot maxima/minima directly influences the amount of ultraviolet  radiation which moves through the upper atmosphere.  


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