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 Solar Output
People have always assumed that the sun's output is constant. In fact, we call the measurement of the sun's output the solar constant. But is the sun really constant?

We know that sunspots have an 11-year cycle. Over the past centuries there have been periods with many sunspots and times when sunspots have been few. But are sunspots related to solar output? In the past it was impossible to measure the solar constant with any degree of accuracy because of the atmosphere and clouds. Satellites with modern instrumentation have changed that. Since 1978, at least one satellite, usually more, has been measuring the total solar irradiance to see if it really is constant.

The figure below shows data from several of these experiments. There are two things to be learned from this graph. One, the various instruments do not give the same value for the total solar irradiance. Data from the most recent experiments are much closer, because scientists have agreed on how to measure solar irradiance so everyone now is doing the experiment the same way. Two, all of the data show cyclic variations which coincide with the sunspot cycle. Increased sunspot activity equals higher solar irradiance.

Accepted values of the solar constant or, more appropriately, the total solar irradiance, range from 1364 to 1368 W/m2 . Over the 11-year cycle the solar irradiance varies by 2 to 4 W/m2. For comparison, it is estimated that the increase in greenhouse gases has caused a radiactive climate forcing of 1 to 2.5 W/m2.

Image of a graph which shows the Total Solar Irradiance. This image links to a more detailed image.
Source: NGDC/NOAA

It is clear that solar output varies on a monthly, if not daily, basis as well as the long-term 11-year cycle. These fluctuations could cause short-term climate variability. If these fluctuations are truly cyclic and maintain their maximums and minimums over the long-term, that is, centuries or thousands of years, then it is unlikely that they could cause long-term changes in Earth's climate. Data is available for less than 20 years, so long-term predictions about the sun's output cannot be made at this time.

However, since sunspots seem to be linked to solar output, it is possible to look at historical records on sunspots and try to match these with historic climate data. The period from 1645 to 1715 was a period of prolonged sunspot absence. This time correlates well with one of the coldest periods of the "Little Ice Age." So, solar variability may play a role in climate over periods of 50 to 100 years.

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