By Ed Caryl
Several researchers have noted that the 11-year solar cycle does not show up in temperature or precipitation data. Most recently, Dr. David Evans has introduced his “notch filter” answer to the problem. I think the answer is much simpler.
The solar influence on earth has several components. The Total Solar Insolation (TSI), varies over the 11-year solar cycle by about 0.1%. Solar UV varies by much more, up to 10%, but those wavelengths carry much less energy than TSI, and affect only the top of the atmosphere. The solar wind has little energy, but influences the cosmic ray influx. Other influx, such as Forbush Events, from Coronal Mass Ejections (CME’s), have short term effects. These last two, the solar wind and Forbush Events, do not follow the solar cycle very closely, so tend not to show up in a time power spectrum of climate, or even of cosmic rays.
So why does the TSI cycle not show up in climate data? Because annual and daily changes completely drown out the signal. First, the earth’s orbit around the sun is not circular, but elliptical. We are closer to the sun during the northern hemisphere winter by 3.4% compared to the distance in the summer. Because the TSI varies by the square of the distance change, the change in TSI is 6.8% from summer to winter. This alone would make the 11-year 1% signal difficult to detect, but there are other, much large variations.
The earth is tilted on it’s axis by 23.5°. This causes a further variation in insolation even at the equator. At the equator, the annual variation is almost 12%, with the maximum occurring at the spring and fall equinoxes, and the minimum occurring on June 21st.
Figure 1: is the insolation seen at three different latitudes. Source here
At 40° latitude, this variation grows to over 100% around the average, and at 80° degrees, to over 200% around the average, including days of 24 hours of sunlight and more days of 24 hours of darkness.
Besides the large annual variations in insolation, there is the diurnal variation, night and day, with another signal that is nearly a square wave. But this isn’t all. The variation in clouds causes albedo changes that are nearly random, and imposed on the above insolation curves. I say nearly random, because of solar effects on clouds due to changes in cosmic rays. But these effects have only small correlation with solar cycles.
So, a 0.1% TSI change on an 11-year cycle, is buried in the much larger annual orbital and axial tilt cycle, the diurnal cycle, as well as random albedo changes due to clouds. An analogy would be hearing the 50/60 Hertz power supply hum on your HiFi, while playing Tchaikovsky’s 1812 Overture, complete with cannons, at full volume.