The connection between solar activity and the earth's climate is an area of ongoing and sometimes controversial research.
A sunspot is a relatively dark, sharply defined region on the solar disc – marked by an umbra (dark area), which is 2000 degrees Celsius cooler than the effective photosphere temperature.
The average diameter of a sunspot is 4000 km, but can exceed 200,000km.
The NASA Solar Physics website (and other websites, such as the Royal Observatory of Belgium), includes information on sunspot numbers, the Maunder Minimum, and sunspot cycle predictions.
The sunspot index is updated monthly and available from 1749. The last time the monthly sunspot number was above 100 for any significant period of time was September 2002 when the value was 109.6 and the last time the value was above 200 was in August 1990 when the value was 200.3.
Reading University's professor of Space Environmental Physics Mike Lockwood thinks the current period of solar activity is likely to become the first “grand solar minimum” for four centuries.
During a grand minimum, the normal 11-year solar cycle is suppressed and the sun has virtually no sunspots for several decades.
Lockwood thinks there is now a 25 per cent chance of a repetition of the last grand minimum, the late 17th Century Maunder Minimum, when there were no sunspots for 70 years.
But Mike says we should not expect a new grand minimum to bring on a new little ice age. There may, however, still be noticeable consequences. For instance, long-term cold winters in the UK are common when solar activity is low. And less solar activity can slow the jet stream, triggering a suite of interlinked extreme weather events.
There have been 24 grand solar minima in the last 10,000 years. Their history is reconstructed by looking for isotopes like carbon-14 that cosmic rays generate in the atmosphere.
The recent decline in solar activity may have helped cause the current ‘hiatus' in the pace of global warming.
The chart (from the Royal Observatory of Belgium) below shows the monthly (and 13-month smoothed number) values of the sunspot number since 1955.
For further information on climate matters see https://sites.google.com/site/climatediceandthebutterfly/