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

The Earth's climate system is mainly driven by solar radiation. Therefore, the question naturally arises whether changes in solar radiation have contributed to the changes in the global climate observed in recent decades.

On the one hand, the solar radiation incident on Earth changes in the long term due to fluctuations in the orbit and the axial inclination and orientation of the Earth. These fluctuations are regarded as the main cause of the geological sequence of ice and warm periods. However, their time scales are far too long to provide plausible explanations for the global climate changes observed in recent decades.

On the other hand, the radiation of the sun itself changes on all time scales due to its astrophysical development. Here, it is mainly the intensity changes in connection with the sunspot cycles observed for many centuries that could be relevant for the observed climate changes of the last decades.

Satellite-based measurements available since 1978 show a fluctuation of solar irradiance of about 0.1% over the course of the 11-year sunspot cycles, which causes fluctuations in the global mean temperature of less than 0.1 °C. Reconstructions of the irradiance averaged over the sunspot cycles show an increase of about 0.5 W/m2 for the first half of the last century. The associated additional radiative forcing of about 0.1 W/m2 has contributed significantly to global warming at that time. Since then, however, the irradiance has decreased again by a similar amount, which counteracts the observed global warming trend of recent years.

Compared to the total anthropogenic radiative forcing of about 2.3 W/m2 to date, the influence of solar changes is therefore relatively small and is likely to have contributed less than 0.1 °C to the observed increase in global mean temperature since the end of the 19th century.

 

Sources:
IPCC 2013: Chapter 8.4 and FAQ 5.1

Kopp, G. et al. (2016) The Impact of the Revised Sunspot Record on Solar Irradiance Reconstructions. Solar Physics, Vol. 291, 2951–2965.


Authors:
Stephan Bakan, Hauke Schmidt, MPI-M