Do Sunspots cause Global Warming?

September 25, 2009
In our last article on sunspots, we mentioned the strange period from 1645-1715 when sunspots seemed to disappear.  Called the Maunder Minimum, this period coincided with the "Little Ice Age", when Northern Europe and other parts of the world were plunged into a long period of cool summers and long winters when crop yields fell and rivers, harbors, and canals froze.

This quiet period of solar activity was closely followed by another, called the Dalton Minimum, from 1795-1825.  It also matches up well with a period of cooler climate, though the eruption of the Tambora volcano in 1816 made some contribution as well.

So you may wonder... do periods of little sunspot activity lead to cooler climate on Earth?  And do periods of increased sunspot activity, such as occurred from 1900-1950 account for periods of higher temperatures on Earth?  Can sunspots explain the rise in temperature during the 20th century, perhaps, rather than greenhouse gases produced by human activity?

In fact, the sun does get hotter when there are more sunspots.  Because although the spots are cooler, they're accompanied by hotter, brighter patches called faculae that cause the overall brightness of the sun to increase by 0.1% at visible wavelengths, and more at ultraviolet wavelengths.

Such increases in solar brightness are included in climate models.  It seems the 11-year sunspot cycle as well as the increase in solar activity earlier in the 20th century lead to an increase in average global temperature of 0.1 to 0.2 Celsius... which is only about 20% of the observed increase of 0.5 to1.0 degree.

So... case closed, right?  It is greenhouse gases, and not solar activity, that are the main cause of climate changes this past century? 

Well, not so fast.  Because when sunspot numbers rise and fall, there's more going on than simply changes in solar brightness.  Periods of reduced sunspot activity correspond to periods of reduced magnetic activity on the sun, and reduced outflows of charges particles from the sun (the so-called solar wind).  The solar wind whizzes past the Earth and deflects cosmic rays from deep space from hitting our atmosphere.  

A recent proposal from Danish scientists suggest that when cosmic rays strike our atmosphere, they create tiny aerosol particles that lead to increased cloud formation and less sunlight hitting the Earth.  So it's a double whammy... fewer sunspots mean a dimmer sun, which also means more cosmic rays into the atmosphere and more cloud cover which further cools the Earth.  And vice-versa when there is more solar activity.

Another recent theory suggests increased UV light from the sign drives energy flow from the upper to lower atmosphere by disrupting a layer of ozone high in the atmosphere.  How this affects climate is unclear.

As it turns out (as far as we know), computer models of the climate do not take these indirect effects of solar activity into account when calculating the change in global climate.  And while human activity counts for only 5% of carbon dioxide emitted into the atmosphere each year, the sun accounts for ALL the energy striking the Earth and driving its dynamic and enormously complex ocean currents and atmosphere.

So you see, despite what you hear in the media, there is still much uncertainty about how the Earth's climate really operates and changes over time, and how changes in solar activity drives climate change.  Healthy and open skepticism, as always, is appropriate.

And remember... the Earth is so complex that even the best computer model in the world can't tell you with any certainty whatsoever whether you'll need an umbrella when you head out the door to go the office a week from today.

Clear Skies,

Brian Ventrudo
Publisher, One-Minute Astronomer