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The publication of research into the solar-climate connection by Chinese astronomers, albeit in an obscure publication, has once again posed the question whether the recent warming observed on Earth has something to do with the sun’s behaviour. The usual answer is that solar changes are too small to account for terrestrial changes both in absolute and relative terms.

Changes in solar irradiance are small in relative terms, amounting to about 0.1% of the sun’s output – called the solar ‘constant.’ But that 0.1% in absolute terms is a huge number given the size of the solar constant – the amount of energy coming from the sun. In real terms it has an effect over the 11-year solar cycle about equal to the forcing caused by changes in greenhouse gasses over the same period.

This is often a surprise for those who automatically discount solar influences. There is however a qualitative difference between solar and greenhouse gas forcing. Solar is cyclic – up and down – whereas greenhouse gas forcing is monotonically increasing. So solar variations are short-term fluctuations on a long-term upward trend of higher temperatures on Earth.

Unless that is there is a long-term trend in the solar constant.

Estimating the value of the solar constant before the onset of space-borne instrumentation is difficult and subject to large uncertainties. We do know that the solar constant does vary and has increased to some degree since the Little Ice Age.

One way to estimate the output of the sun is to link it to sunspot numbers that go back to 1609. Sunspot numbers clearly show the 11-year solar cycle as well as the Maunder Minimum between about 1640 -1720 (coincident with the Little Ice Age) as well as the so-called Grand Solar Maximum of the 20th century when sunspot activity has been estimated to be the strongest in many thousands of years. Analysing sunspot numbers as an indicator of solar activity shows a peak in the 1950s and the 1980s.

One could simply correlate the increasing sunspot activity with rising global temperatures and say ah ha, there is a connection. But whether or not this is acceptable to many scientists – in the UK and the USA mainly – depends upon how it is done.

If, as these Chinese scientists have done, a correlation is found (between Total Solar Irradiance and global surface temperature, click on image to enlarge) then it is treated as dubious. But if one reaches the opposite conclusion the reception can be somewhat different.

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A case in point is a now infamous paper written a few years back. It was written just after there were hints that the sun’s activity was tailing off from the grand high of the 20th century. It looked at the rising temperature of the Earth and the rising and then falling solar activity and said because the sun’s activity was declining and the earth’s temperature was still rising there was no correlation. It was a paper that was definitely of its time being written just as the recent global surface temperature pause was being recognised. Today if a similar analysis was performed using up-to-date data it would have to reach the opposite conclusion.

But when it comes to the possibility of a link between changes in solar activity and climate change, correlations between changing solar parameters and global temperature can arouse suspicions but they only go so far. It is true that when faced with a series of observations one of the first questions a scientist will ask is, what does it correlate with? These Chinese scientists have shown us once again that there is a correlation between solar activity and global surface temperature over long periods. At present it is an inconvenient fact to some, and a fact that we cannot really explain.