Solar Power Output and Global Warming

Without the Sun we die! Yet how many of us give frequent thought to whether we can can continue to absorb its gift of energy? Or whether that gift will wax or wane? The true believers in Anthropogenic Global Warming (AGW) think we are keeping a lot more of that gift than is good for us, due to our increasing emissions of CO₂. However, even though they consider carbon dioxide to be the major cause of global warming, even they (in the guise of NASA) will grant that rising solar power output contributes about 25% of observable global warming. In our last post on global warming, we found evidence that decreasing cloud cover possibly accounts for all the global warming observed since 1971. Certainly the combination of decreasing cloud screening of incident solar radiation in combination with increasing solar output is more than enough to explain what we have experienced.

So what has the power received from the Sun been as a function of time? What we seek is called the solar irradiance, which is the power in watts in an area to which the radiation beam is perpendicular. Irradiance has units of watts/meter² [Wm^-2]. It is the area power density in the Sun’s outward-going radiation beam. Below is shown a graph of solar irradiance versus time in years from 1870 until about 1990. It is taken from a 1999 paper in Geophysical Research Letters. [M. Lockwood and R. Stamper, “Long-term drift of the coronal source magnetic flux and the total solar irradiance”, Geophysical Research Letters, 26, pp 2461-2464, 1999].  Click on the image for a better resolution view.

The solid line is the total solar irradiance. The peaks and dips you see in it are the solar maxima and minima of the well-known 11 year sunspot cycle. The dashed line running through the middle of the peaks is the running mean irradiance, I₁₁. The dash-dot line is the I₁₁ produced by a different research group. It clearly can be seen that solar power output has generally been increasing from about 1900 to around 1995. If you examine the graph closely, you can see an irradiance increase from 1900 to roughly 1940. From 1940 to around 1975, you can see that mean  irradiance was roughly constant to slightly decreasing with one large solar maximum during 1959. This corresponds to the slight cooling period observed during that time. The record then shows continued increase in mean solar power output until 1999. Unfortunately, this record does not extend beyond 1999 to show if there was a constant or slightly decreasing irradiance to correlate with the widely noted “pause” in global warming.

Highly correlated with solar power output is the number of observed sunspots, with sunspots increasing in number during solar maxima and decreasing to minima during solar minima. Sunspots are slightly cooler than the hotter regions surrounding them and therefore appear darker. They are great plasma tornados with concentrated magnetic field lines passing through them, and their number is a measure of the plasma turbulence in the Sun’s atmosphere. It would seem that as solar power out increases, the turbulence within the sun also increases. Below is a figure, taken from the Climate Data Information website, showing the great correlation between total solar irradiance (TSI) and sunspot number (SSN). The red curve is TSI and the blue is SSN.

The importance of this linkage is that we can use observed sunspot numbers as an indication of how much power the sun is pouring out. During the Maunder Minimum (1645-1715) sunspots were extremely rare, and the temperatures in the middle of the Little Ice Age were particularly cold.

So how many sunspots are currently being observed? In fact at the present time the Sun appears to have very few sunspots  (51 sunspots on August 14) and is very quiet. Supposedly we are in a solar maximum phase and should be seeing something like 160 sunspots or more. See here and here. There are even a few voices being raised  warning of a coming new “Maunder minimum”. That may or may not happen, but we may well have to worry more about global cooling than global warming.

There are a great many proposed mechanisms for the interaction between the Sun’s incident radiation and the atmosphere. Check out this list of 71 abstracts of papers published in refereed journals in 2013 to see some of the thoughts on how the Sun is the primary cause of global warming.

One of the more interesting and probable of these mechanisms is a connection between the sun, cosmic rays, and clouds. Recall that we determined in our last post that clouds are net coolers of the atmosphere. Therefore, anything that makes clouds more frequently and extensively formed will cool the Earth, and anything that discourages their formation will warm the Earth. The Sun-atmosphere interaction of interest here is an indirect one involving the Sun influencing the intensity of cosmic rays reaching our atmosphere. Ever since the 1970s physicists have been speculating that intense cosmic rays, solar radiation, and clouds had some kind of relationship. The first person to actually demonstrate a connection between cosmic ray level and cloud coverage was the Danish physicist Henrik Svensmark. Using satellite data, he along with a collaborator, Eigil Friis-Christensen, showed a close correlation between the three factors of solar activity, cosmic ray levels, and cloud cover.

These results inspired a CERN physicist, Jasper Kirkby, to see how cosmic rays could possibly affect cloud cover. He knew of evidence of 100 year cycles in pre-industrial climate, and that these variations correlated with activity of the solar wind. Correlations however are not proof; a mechanism was needed. Cosmic rays are composed primarily by high-energy protons and atomic nuclei from sources outside of the solar system. The natural speculation was that if the solar wind was strong, it would blow the cosmic rays away from the Earth. If it was weak, more cosmic rays would reach the Earth’s atmosphere, where it would ionize particulates that would would bind water molecules by electrostatic attraction. The more intense the cosmic rays reaching the atmosphere, the more condensation nuclei would be created and the more clouds would be formed. Therefore, the more active the Sun was, i.e. the greater the Sun’s power output, the stronger would be the solar wind sweeping cosmic rays away, and lessening the number of clouds. With fewer clouds and their cooling effect, the Earth would warm.

It was already known that strong solar winds would sweep cosmic rays away from the Earth (see here and here and here). What was needed was to show that cosmic radiation could create condensation nuclei in sufficient numbers to form enough clouds to cause global cooling. Kirkby decided to test the hypothesis by directing a CERN particle beam into a chamber simulating the atmosphere. After a great deal of time he was finally given CERN’s blessing to perform the experiment. The results verified Svensmark’s theory and were published in the journal Science.

The import of all this lies in the fact that the Sun was increasing in power output during the periods of global warming, and therefore allowed less cosmic radiation to help form clouds. This is an explanation of why cloud cover decreased during the warming periods that we noted in the post Clouds and Global Warming. We now have a complete, experimentally and observationally verified theory of the observed global warming that does not involve CO₂.

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