As a detailed example of uncertainty in climate studies, I thought it might be useful to provide a simple guide to problems in predicting solar-cycle intensity. Solar-cycle intensity is measured by the maximum number of sunspots. These are dark blotches on the Sun that mark areas of heightened magnetic activity. The more sunspots, the more likely it is that major solar storms will occur. Here is a list of previous solar cycles from 1755, numbered 1 - 23 (continuous monthly averages of sunspot activity are only really available from c.1749).

The next 11-year cycle of solar storms (known as Solar Cycle 24) was predicted to begin last autumn, 2006, but it appears to have been delayed, and it is now expected to take off in March, 2008, and to peak in late-2011 or mid-2012, up to a year later than thought. This delayed onset has somewhat confused the official Solar Cycle 24 Prediction Panel, leaving them evenly split on whether a weak or a strong period of solar storms lies ahead.

This difference of opinion can be seen clearly in the graph reproduced above. The official NOAA, NASA, and ISES Solar Cycle 24 prediction and graph were first released by the Panel on April 25, 2007. The graph presented above is the November 16 update of this. The Blue Line records the smoothed monthly number of sunspots recorded until October, 2007; the Dots are the recorded monthly values; and the two Red Lines represent the alternative projections for the smoothed predicted sunspot numbers put forward by the Panel. Half of the Panel predicts a moderately strong cycle of 140 sunspots, plus or minus 20, expected to peak in October, 2011. The other half predicts a moderately weak cycle of 90 sunspots, plus or minus 10, peaking in August, 2012. An average solar cycle ranges from 75 to 155 sunspots. The late decline of Cycle 23 moved the Panel away from its earlier inclination towards a strong Cycle 24. When the Panel finally converges on an agreed prediction, a single-line graph will be published. As yet, this has not proved possible.

During an active solar period, violent eruptions occur more often on the Sun. Solar flares and vast explosions, called coronal mass ejections, shoot energetic photons and highly-charged matter towards Earth, affecting the planet’s ionosphere and geomagnetic field, potentially disrupting power grids, critical communications, satellites, Global Positioning Systems (GPS), and even threatening astronauts with harmful radiation. These storms also illuminate night skies with brilliant sheets of red and green, called auroras or aurorae - the Northern Lights (aurora borealis) and the Southern Lights (aurora australis).

This was a period in the Little Ice Age when sunspots became very rare, as noted by observers of the era. During one 30-year period within the Maunder Minimum, astronomers recorded only about 50 sunspots. Although a simplistic correlation is rightly contested, the Maunder Minimum thus appears to coincide with the middle - and the severest part - of the Little Ice Age.

This may cause a shiver to run down your spine, because, as you may have already noticed from the Solar Cycle 24 graph, we are today likewise at a point with virtually no sunspots. We must surely hope that the current Solar Cycle 24 predictions are largely correct, and that sunspot activity starts up again soon, or else ‘global warming’ could change to ‘global cooling’ pretty sharply. Brrr.

And that really would be unpleasant!

So much, then, for “settled” climate science. Hot coffee all round?

[See also: ‘Place your bets for SC24’, Solar Science, November 20]