The Sun’s Interaction with the Earth’s Thermosphere and Climate System

There are many processes associated with the Sun, the thermosphere and the Earth’s climate that are well understood, but the prediction of these systems is still far from accurate. There are several reasons for this. Some important physical, chemical or biological mechanisms may not be sufficiently well quantified, or indeed not yet determined. Complex interactions and feedback mechanisms operate over wide spatial and temporal scales, and make prediction of the emergent behaviour particularly challenging. The mean radiative forcing of the climate system is an example where there are still some important uncertainties (Fig. 1, from Intergovernmental Panel on Climate Change, 2001). This shows that two of the largest uncertainties are terms associated with the Sun and with aerosols. Whilst much is known about climate forcing by visible radiation, Marsh and Haigh (both this volume) give succinct summaries of several alternative mechanisms by which variations in the electromagnetic and charged particle output of the Sun can affect the climate system. There is ∼ 7% variation in the flux of ultraviolet radiation at wavelengths ∼200 nm through a solar cycle, and this causes marked changes in the stratosphere, and especially of the ozone there. Quantitative modelling (Haigh, this volume) demonstrates that the changes induced above the tropopause influence tropospheric climate, primarily through changes in the propagation and dissipation of gravity waves and planetary waves. This is an important finding as it provides a robust mechanism whereby changes occurring at high altitude can affect the climate at the Earth’s surface. Figure 1 shows that the effects of aerosols are the largest uncertainty in radiative forcing. Marsh (this volume) describes how the role of cosmic rays can be important in a chain of reactions whereby aerosols are formed and cloud cover may be altered. Changing cloud cover can either have a positive or a negative effect on 61