Solar Influences

In the early 1990’s a paper by Christensen was published that showed a striking correlation between the length of the sun’s sunspot cycle and the global average annual temperature, Fig 1. The shorter the cycle (short cycles are more intense) the higher was the earth’s annual temperature. It seemed to indicate that the sun was the dominant influence on the earth’s temperature variations. Click on image to enlarge. It is certainly striking that since the later part of the 19^th century and throughout the 20^th century there has been a general increase in the Earth’s global average temperature at the same time that the strength of the solar cycle was increasing in intensity as measured by the number of sunspots. In the last half of the 20^th century four out of the five most intense solar cycles occurred (the second largest cycle was around 1780) including the strongest ever which was in the 1950’s.Christensen linked these two together in what appeared to be a pleasing way. However, a few years after the work was published others found flaws in the way the final four (out of 24) data points were plotted. In Christensen’s paper the length of the solar cycle decreased between 1950 – 1990 with the last data point showing that the cycle length shortened at the same time that the recent global warming period started (post 1980). When this was corrected the concordance between the solar cycle length and the earth’s rising temperature broke down as it became apparent that the length of the solar cycle showed no trend as the earth’s temperature rose post-1980. Fig 2. Click on image to enlarge.It was heralded as proof of the hypothesis that the recent, post-1980 warming spell could not be due to the sun. Whereas many argued that the sun was the dominant factor prior to this period, the rate of warming was recently too great to be accounted for without a human influence. See here, here and here.It is now clear that in the past decade or so our sun has been behaving differently  from the way it did during the 20^th century. The current sunspot minimum has gone on for far longer than was expected. It did begin to show signs of an upturn in activity earlier this year, but has since faltered again. Some have suggested that this is a sign of the start of a new Dalton-like minimum – a period of low solar activity that occurred about 1800.Since the invention of the telescope and with it the ability to monitor the frequency of sunspots there has been two periods when sunspots were lacking. They are the Dalton Minima, which lasted about 20 years and the much longer Maunder Minimum of approx 1640 -1710. Both periods are coincident with cooler conditions on earth though we do not have a satisfactory explanation for how this occurs. Some believe that the Maunder Minimum can be explained by a combination of a reduction in solar radiation combined with volcanic effects, other are not so sure.It is interesting to see that as time goes by more and more solar scientists are expressing the possibility that it might be the start of a period similar to the Maunder Minimum. Only time will tell.The important point is that in previous periods of relatively low solar activity the solar cycle behaved differently than it did when solar activity was stronger.In broad agreement with the Christensen relationship, during the Dalton Minimum the period of the solar cycle increased. The longest cycle ever recorded was during the Dalton period. Also as sunspot numbers declined at the start of the period the solar cycle became more symmetrical and cycle rise and fall times converged at about 6 years.The last complete solar cycle, number 23 lasted 12.6 years (May 1996 – Dec 2008). Only four solar cycles have lasted longer than 12 years. Two of them, cycle 4 (1784 – 1798: 13.7 years) and cycle 5 (1798 – 1810: 12.6 years) occurred just before and during the Dalton Minimum.What’s more cycle 22 was unusually short (Sept 86 – May 1996: 9.7 years). There have been only two other solar cycles that have been shorter, Cycle 2 and cycle 3 which occurred immediately prior to the lengthy cycles of the Dalton Minimum.Scrutinising the gradient of sunspot cycle rise and falls it is obvious that usually the cycle rise is more rapid than the fall. During the Dalton Minima however they became equal. Then they resumed their normal relationship only to move once more to equality around 1910 when there were a few relatively weaker sunspot cycles. It is possible that the same thing is happening again. If so this would imply an approx 100 year periodicity, though this is only am impression, as we only have reliable sunspot records since about 1750.It is possible that changes in solar cycle lengths take place before periods of low solar activity, like the one we may be entering at present, and that might influence the later part of figure 1. If this is the case then the breakdown in the Christensen relationship post 1980 cannot be said with certainty to be due to the rise of human influences on the climate above solar ones.Feedback: david.whitehouse@netzerowatch.com