Dr Theodor Landscheidt

Schroeter Institute for Research in Cycles of Solar Activity
Nova Scotia, Canada

“Solar Constant” Variations in the 11-Year Sunspot Cycle and Climatic Effects

Atmospheric circulation, the cause of weather, is driven by the sun’s energy. Climate is the integral of weather over periods of more than a year. This integral also depends on the flux of solar energy. The same applies to variations in the energy flux caused by the sun’s varying activity. Satellite data show that the “solar constant” S is variable. The solar irradiance decreased from the sunspot maximum 1979 to the minimum 1986, increased again on the way to the next maximum in the 11-year sunspot cycle, and decreased anew in the descending phase. This came as a surprise as it is plausible that the dark sunspots with their strong magnetic fields impede the free flux of energy from the sun’s interior to the outside. Yet P. V. Foukal and J. Lean [22] have shown that bright faculae in the vicinity of sunspots increase even more than sunspots when the activity grows stronger, so that an irradiance surplus is established.

IPCC scientists hold that the corresponding variation in the solar constant (Delta S) is smaller than 0.1% and has no impact on climate that could count in comparison with the greenhouse effect [94]. Yet they fail to appreciate that quotes of 0.1% in the literature refer to the absolute amplitude of the sinusoidal variation in the solar constant, not the whole change from minimum to maximum, or from maximum to minimum [25, 32, 39]. Figure 1 after C. Fröhlich [25] shows this distinctly. The data at the top of the figure, designated by `HF’, represent NIMBUS-7 measurements. The smoothed curve shows the 81-day running average related to the interval of three solar rotations of 27days. The horizontal axis indicates the investigated period, above in years, below in days since the first day of 1980. The vertical axis measures the solar constant S in W/m2. The scale in the middle of Figure 1 indicates the range of 0.1%. When this scale is taken to measure the variation in the smoothed curve from the sunspot maximum 1979 to the minimum in 1986, the result is Delta S approximately equal to – 0.22%. IPCC scientists cannot object to this higher value on the grounds that it is not a common practice to assess the total variation in such a way. They proceed equally by relating the rise in global temperature to the minimum at the end of the 19th century and not to the long-term temperature mean.

According to satellite measurements, the mean value of the solar constant is S = 1367 W/m2. 0.22% of this amount of energy equals 3 W/m2 . This result may also be read from Figure 1. The maximum of the smoothed curve is at 1374.2 W/m2 and the minimum at 1371.2 W/m2 . The variation of 0.22% does not affect climate in its entirety. The solar constant defines the amount of energy which just reaches the outside of the earth’s atmosphere. 30% of this energy is not absorbed by the atmosphere, but reflected. Furthermore, it has to be taken into account that the irradiated sectional area of the earth constitutes only a quarter of the surface to which this thermal energy has to be distributed. So there is only 239 W/m2 available to heat the atmosphere. Consequently, the variation of 3 W/m2 has only a climate effect of 0.53 W/m2 . How this affects global temperature depends on the general circulation model used to assess the climate sensitivity. C. Fröhlich [25] proceeds from a value between 0.3° and 1.4° C / W/m2 . When we choose the mean value 0.85° C / W/m2 to avoid an overestimation, the climate effect of 0.53 W/m2 yields a temperature effect of 0.45° C. The chosen mean value lies within the range given in the literature [19, 31, 33, 82, 87, 89, 115]. Even if a four times longer smoothing interval is chosen as
in Figure 1, the variation of the solar constant reaches 2.2 W/m2 [74] with a temperature effect of 0.33° C.

Variations in global temperature of 0.45° or 0.33° C in the course of seven years cannot be considered negligible. This all the more so as the observed rise of temperature during the last hundred years amounts to merely 0.4° C. From the value 0.5° C, quoted in the literature, 0.1°C has to be subtracted because it is due to urban warming that causes a spurious rise in global temperature [39]. Observed climate data, which follow the rhythm of the 11-year sunspot cycle, indicate that the effect of irradiance variations on the atmosphere is enhanced by positive feed-back processes or stochastic resonance. This form of resonance involves the cooperative interplay of random and periodic stimuli. Noise can improve the response to small periodic or quasiperiodic signals so that the small input is able to entrain large scale fluctuations. This effect is strongest in nonlinear systems with a high level of noise.

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