Global Warming, Venus and Sunspots

Is the Greenhouse Effect the only Cause of Global Warming?

Mar 17, 2007

Most scientists think the greenhouse effect causes global warming as on Venus, but is there a contribution from sunspot cycles?

Venus, Greenhouse Effect, and Global Warming

At the dawn of the space age, probes to Venus discovered that beneath her cloudy veil the goddess of love and beauty is a hellish mistress. In the hour before she destroyed them, the probes learned that her surface melts lead. They also found that Venus' atmosphere is 95% carbon dioxide. Then young, Carl Sagan connected these two facts to correctly surmise that a runaway greenhouse effect dominates Venus. Carbon dioxide, a major greenhouse gas, traps the heat trying to radiate away from Venus causing a huge global temperature increase.

As shown by the success of Al Gore's An Inconvenient Truth, it has since become conventional wisdom that increased carbon dioxide in Earth's atmosphere causes similar but less extreme global warming. Burning fossil fuels, chopping down rain forests, and so on increase the carbon dioxide content of our atmosphere thereby raising Earth's temperature. In science it is always important to challenge the conventional wisdom. Is this hypothesis correct or complete?

Vikings in Greenland

In 982, Eric the Red was expelled from his native Norway for a few murders. Settling in Iceland did not break his bad habits. Banned for three years, he sailed west again. The land he discovered is now covered with a thick sheet of ice, but he named it Greenland. In 986, after returning from his banishment, Eric the Red led a fleet of 25 ships to found a colony on his newly discovered Greenland. The colony thrived for a few centuries, but was abandoned in the 14th century when the climate cooled. It appears that the name, Greenland, allowed Eric the Red to pull off the greatest real-estate scam in history. But, perhaps he really did discover a green land that later turned cold and icy.

Sunspots

With his newly invented telescope, Galileo discovered dark sunspots in 1610. After 400 years, they still baffle us. There is a well known 11 year cycle in the number of spots seen on the Sun. Are there longer cycles? In the August 1894 issue of Knowledge, E. Walter Maunder wrote that despite frequent observations by capable observers, very few spots were observed in the late 17th century. For example, the 1671 sunspot was the first in over a decade.

Carefully examining historic records and indirectly estimating sunspot activity before 1610, John A. Eddy in 1976 confirmed the Maunder minimum from 1645 to 1715, found another minimum from 1460 to 1550, and a prolonged medieval grand maximum from 1100 to 1250, when the Vikings were active on Greenland. During grand maxima the 11 year spot cycle occurs at a level much higher than normal. Eddy also speculated that we might be due for another grand maximum starting in the 22nd century. Bolstering this view, recent maxima are among the most active on record. Does increased sunspot activity help warm Earth's climate?

Satellite observations of the Sun's luminosity (total energy per second) over recent 11 year solar cycles show that the Sun is about 0.1% more luminous during sunspot maximum than minimum. What about longer cycles? Accurately measuring the Sun's luminosity centuries ago is clearly impossible, but there is indirect evidence. The Maunder minimum was apparently one of the coldest periods on record in Europe. The Viking colony that Flourished in Greenland during the medieval grand maximum was abandoned when the sunspot level decreased and Greenland simultaneously cooled. Studies of other solar types stars show a similar tendency for the luminosity to change with the spot cycle.

It certainly has not been proven, but there is evidence that the Sun's luminosity varies as part of long term spot activity cycles and that Earth's climate responds with long term climate cycles. If correct, then our current global warming might in small part result from solar induced climate cycles. There may of course be multiple causes, so this hypothesis does not in any way invalidate the greenhouse effect. It does however show that other factors are also at work. The problem is more complex than the average person realizes. It also seems that if the Sun causes additional warming beyond that caused by greenhouse gasses, we must strive even harder to reduce greenhouse gasses.