Introduction

At periods of higher solar activity the Earth is subject to enhanced solar irradiance, a greater incidence of solar energetic particles and fewer galactic cosmic rays. This review presents an assessment of the extent to which solar variability affects the climate of the Earth’s lower and middle atmosphere and considers how the apparent response is brought about.

A summary of the potential routes whereby solar variability might influence the climate of the lower atmosphere is presented in Table 1. The absorption of solar radiation determines the Earth’s mean temperature and radiation budget, while the latitudinal distribution of the absorbed radiation is the primary driver for atmospheric circulations. Through photochemical processes solar radiation and solar energetic particles also play an important part in determining middle atmospheric composition. Now the radiative heating rate at any point in the atmosphere is the net effect of solar heating and infrared cooling, the latter being intrinsically related to the atmospheric composition and temperature structure. Thus variations in solar output have the potential to affect the atmospheric temperature structure in a complex and non-linear fashion. Section 3 below discusses the energy balance of the Earth and assesses the potential role of the Sun in the radiative forcing of global climate change. Section 4 considers how variations in the total solar irradiance incident at Earth may affect the climate while in Section 5 the processes involving solar ultraviolet radiation, and how these are modulated by solar activity, are presented. Galactic cosmic rays are an important cause of atmospheric ionisation and, as their intensity is modulated by solar activity, there is also a solar signal in the concentration of atmospheric ions which are involved in chemical processes in the middle atmosphere. To what extent they might also provide a source of cloud condensation nuclei at lower altitudes is an area of current research and this is the subject of Section 7.

Table 1:

Summary of routes through which solar variability may influence the climate of the lower atmosphere.


Forcing factor	Generic mechanism

Total solar irradiance (variations due to orbital variations or to variable solar emission).	Radiative forcing of climate. Direct impact on sea surface temperatures and hydrological cycle.

Solar UV irradiance.	Heating the upper and middle atmosphere, dynamical coupling down to troposphere. Middle and lower atmosphere chemistry and composition; impacts temperature structure and radiative forcing.

Solar energetic particles.	Ionisation of upper and middle atmosphere; impact on composition and temperatures. Magnetosphere – ionosphere – thermosphere coupling.

Galactic cosmic rays	Ionisation of lower atmosphere; impact on electric field. Impact on condensation nuclei.

First, Section 2 considers how information on past climates is derived and look at some of the (mainly circumstantial) evidence that variations in solar activity have affected climate on a wide range of timescales.

1 Introduction