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David Whitehouse: Is It The Sun Wot Done It?
Thursday, April 21st 2011, 4:58 PM EDT
Co2sceptic (Site Admin)
If you are not confused about the Sun’s (the star that is not the newspaper as alluded to by the headline) role in influencing the Earth’s climate then perhaps you haven’t been paying attention recently.

It is a common story in climate change science. Some researchers will offer definitive statements that the Sun couldn’t possibly be responsible to any significant degree in the recent warm period the Earth has experienced. Look behind such comments however and you will find that the science, as science often is, is more equivocal, as a few recent papers indicate. There is much food for thought.

The common view is that the Sun did have a majority influence up until about 1950 – 60, but since then, because of forcing from greenhouse gasses, its role has been miniscule.

Miniscule globally that is. Regionally it might be a very different matter. Solar scientists have been going on about this for years. The speculation is that the Sun’s influence could be significant regionally such as across, say, Europe in winter, but not significant globally. However, one does wonder how many local regions are needed to be present before such a solar effect might become more global?

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We all know of the way solar activity has been at a very high level in the past century. Solar activity peaked in the mid 1950’s and mid-1980’s and then declined (some researchers conveniently like to ignore the 1980’s peak and just focus on the 1950’s one). Solar activity was rising throughout the 20th Century at just the same time that the Earth’s annual average temperature increased, rising steadily with some standstills until about 2000 when it has remained static at the warmest level in the instrumental period (since 1860).

Add to this the observations that show when solar activity was low in the 17th century - when for decades there were hardly any sunspots to be seen - there was a coincident period called the Little Ice Age, and when solar activity was low for a decade or two at the start of the 19th century (the so-called Dalton Minimum) the Earth also chilled slightly has led some to speculate that there is a connection.

Low solar activity, cool earth: High solar activity, warm Earth? There must be a connection because all these correlations are straining explanation by chance. The problem is that no one has come up with a physical mechanism whereby the Sun and climate are linked in a convincing way. It doesn’t seem that solar modulation of Cosmic Rays affecting cloudiness can do it. So what is going on?

Total Solar Radiation

Recently the Sun and the Earth’s global temperature have been doing something very interesting. The last solar minimum was long and deep. We have to go back at least a century, and possibly back to the Dalton Minimum to find a parallel. The Sun probably reached an end of its 20th Century Grand Maxima phase in the last decade of the 20th century and now some speculate we are due for a very low sunspot cycle and a prolonged period of subdues activity. In the solar section meeting at the recent Royal Astronomical Society 2011 National Astronomy Meeting it was suggested that there is a one in twelve chance of the Sun entering another Maunder Minimum.

Some research has cast doubt on any connection between the Sun’s downturn in activity and the earth’s temperature standstill arguing that whilst the Sun’s activity declined the Earth’s temperature continued upward. However, as the Earth’s temperature stubbornly remains static for the past decade (or more) such a conclusion is now hardly justifiable.

Over the past three solar cycles (just over 30 years) we have instrumental data for the total radiation coming from the Sun and, as expected, it varies with the sunspot cycle, being higher at sunspot maxima and lower at sunspot minima by about 1 Watt per sq metre as measured at Earth. This is a remarkably small variation as the TSI for the past prolonged minimum is 1361 Watts per sq m – the solar ‘constant.’ The TSI doesn’t just vary on sunspot-cycle timescales. On the short term it can vary by 1 watt on a period of just days. For example on March 10th it was 1360.6 W and just five days later it was 1361.5. A week later it was 1360.8 and 11days later 1361.9.

It also varies on longer timescales and reconstructions of TSI (using its correlation to sunspot numbers) suggest that it was lower during the Little Ice Age (no sunspots) and has since been climbing out of this episode. It is hypothesised that variations in the Sun’s Total Solar Irradiance (TSI), modulated by aerosols emitted from volcanoes could be an explanation for the Little Ice Age (LIA), although in my opinion it’s a bit of a stretch for the climate models to reproduce it.

TSI was low between 1650 and 1720 and then started to rise until the Tamboora eruption which interrupted the rise, from which it recovered and then levelled off. It then started to rise about 1900 and continued. Over this period, 350 years, it is claimed that TSI varied from about 1364 – 1368 Watts per sq m.

New research casts doubt on this view. Karel Schrijver, of the Solar and Astrophysics Lab of the Lockheed Martin Advanced Technology Center, and colleagues suggest that TSI during the LIA that period may not have been as low as has been calculated from its correlation with sunspot numbers. They analysed direct measurements of solar activity during the recent 2008-2009 period of very low sunspot activity, which they suggest was similar to the activity level during the Maunder Minimum. They find that even when there were no sunspots, the Sun had a base level of magnetic activity that had not been accounted for in previous estimates of TSI during the Maunder Minimum, which were based solely on sunspot numbers. The researchers suggest that earlier estimates of the TSI during the Maunder Minimum were too low and that the Maunder Minimum probably had levels of magnetic activity and TSI similar to 2008-2009 values, which may or may not be true. It is possible that should the Sun enter another Maunder state it might settle down to a lower level of activity than that seen in 2008 – 2009.

Their conclusion is that factors other than low solar irradiance resulting from low sunspot activity must have contributed to the Little Ice Age. This, if true, throws ideas about what caused the LIA and its association to the Maunder Minimum into confusion.

That the Sun was different during the Maunder Minimum there is no doubt. One area of debate relates to the existence and meaning of a base value to the heliospheric magnetic field. From isotope abundances on earth, it has recently been deduced that the near-Earth Interplanetary Magnetic Field at the end of the Maunder minimum was 1.80 ± 0.59 nT which is considerably lower than estimates given by others of about 4 nT.

Other work suggests that the so-called “open solar flux” was of (0.48 ± 0.29) × 1014 Wb at the end of the Maunder Minimum.

The interesting thing about these estimates of the solar magnetic field is that the largest and smallest annual means recorded by instruments in space between 1965 and 2010 were 5.75 × 1014 Wb and 1.37 × 1014 Wb, respectively, set in 1982 and 2009, and the maximum of the 11 year running means was 4.38 × 1014 Wb in 1986.

What this means is that the average open solar flux during the Maunder Minimum was about 11% of its peak value during the recent grand solar maximum and less than half of its minimum value. Is this an indication that the recent prolonged solar minimum wasn’t the same as the Maunder Minimum?

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