Has the climate in recent years been affected by changes in solar radiation?

The possible effect of variations in solar radiation on our climate is a subject that has been discussed for a long time. The energy output of the sun is subject to fluctuations linked to sunspot activity – a high sunspot number means an increase in solar radiation, connected at the same time to a slight shift in the spectrum of solar output towards the shortwave (UV) end of the scale.

There are two recognisable activity cycles. There is the ‘Schwalbe’ cycle, with a period of 11 years and an amplitude of about 0.1%, and the ‘Gleissberg' cycle, with a period of 80 years and an amplitude of 0.2-0.3% of the total solar output. The energy supplied by the sun can thus fluctuate by about 0.6W/m2 - as a comparison, the current effect of the anthropogenic greenhouse effect, caused by human emissions of CO2, methane, CFCs and nitrous oxide, is estimated at 2.5W/m2 . In the last 100 years, the solar constants have, on average, risen and are now estimated to be about 0.25% higher than they were 100 years ago.

How much do these solar fluctuations influence the climate? To answer this question, simulations have been carried out with the same global climate models used to estimate the effect of the anthropogenic greenhouse. They suggest that a part of the warming observed over the last century can be explained by the rise in solar intensity, although only about a third of it (0.2ºC). It has thus been concluded that solar variability cannot solely be responsible for the observed global warming of the last 100 years.