Susan Fraser, of Wellington, asks :-

What is our current understanding of mechanisms for the sun’s influence on earth’s climate

David Wratt, NIWA's chief climate scientist, responded.

The Sun may influence our climate in complex ways, involving not only the direct effect of changing energy output, but also indirectly through changes in ozone, clouds and atmospheric circulation. The energy reaching the top of the Earth’s atmosphere from the sun only varies by about 0.1% over the 11-year solar sunspot cycle. Changes over longer time scales include so-called “grand solar minima” like the Maunder minimum, which occurred during the late 17th to early 18th Century, and could be slightly larger (up to 0.3%).

Global temperature varies slightly (by around 0.1°C) between the minimum and maximum of the 11-year solar cycle. Solar variability also probably contributed to cooler temperatures from 1550 to1850 AD, but it can’t explain the increase in global temperature since the mid-20th Century. Over this period the effect of solar variability (which on its own would have probably led to a slight decrease in temperature) was much less than the effect of increasing concentrations of greenhouse gases in the atmosphere.

One mechanism proposed by researchers to explain solar effects involves changes in ultraviolet (UV) radiation from the sun. UV varies more across the 11-year cycle than does total solar radiation. It is absorbed in the stratosphere (the region between about 12 and 50 km altitude) where it can cause heating and affect ozone. Stratospheric changes might in turn affect the lower atmosphere.

Some scientists have hypothesized that changes through the solar cycle in the number of galactic cosmic rays (GCRs) reaching Earth lead to changes in cloudiness. However such effects appear too weak to significantly affect global climate. GCR effects can’t explain the increasing global temperatures since the mid- 20th Century, since the overall GCR trend since the mid-20th Century would have led to a temperature decrease under this hypothesis.