There are several proposed mechanisms1
through which the 11-year solar cycle (SC) could influence the Earth’s
climate, as summarised by Figure 1. These include: (a) the direct impact of solar irradiance variability on
temperatures at the Earth’s surface, characterised by variation in the total incoming solar irradiance (TSI); (b) the
indirect impact of variations through the absorption of Ultra-Violet (UV) radiation in the upper stratosphere
associated with the presence of ozone, with accompanying dynamical responses that extend the impact to the Earth’s
surface; (c) the indirect impact of variations in energetic particle fluxes into the thermosphere, mesosphere and
upper stratosphere at high geomagnetic latitudes; and (d) the impact of variations in the generation of ions by
galactic cosmic ray (GCR) penetration into the troposphere. Although different in their nature, these four pathways
may not work in isolation but their influence could be synergetic. For example, there is modelling evidence that the
influences of TSI at the surface in combination with the stratospheric response to the spectrally-resolved solar
irradiance (SSI) variability could reinforce solar influences on regional scales, such as the tropical Pacific2
.
Furthermore, the surface imprint of energetic particle precipitation may be similar to influences of SSI variability3
.
This paper provides a summary of evidence and our current understanding of the first two of these proposed
mechanisms, namely those involving solar irradiance variability, with a focus on surface impacts in the Pacific and
in the Atlantic / European sector.
One of the primary challenges in this research field is the difficulty in identifying the optimal indicator of solar
variability in the various wavelength bands. Direct satellite measurements of TSI, i.e. the total irradiance across all
spectral bands, have been available since the late 1970s, starting with the NIMBUS7 Earth Radiation Budget (ERB)
in 19784
, and continuing to this day with the currently flying Solar Radiation and Climate Experiment (SORCE)
Total Irradiance Monitor (TIM)5
and TSI Continuity Transfer Experiment (TCTE)/TIM. However, this period
encompasses less than four solar cycles, and is inadequate for the analysis of long-term data such as sea level
pressure and temperature where global datasets extend back to the mid 19th century and for European datasets that
extend even further back, to the 17th century. For these analyses a variety of ‘proxy’ data indices are employed to
represent past solar variability, for example sunspot number. The situation is even more challenging for the analysis
of responses that involve modulation of the amount of UV radiation, since direct observation of spectrally-resolved
solar irradiance (SSI) over a wide range of UV wavelengths has only been achieved since 19816
, and with
sufficiently good stability to capture accurate solar cycle changes above 250 nm since 19917
with the Upper
Atmosphere Research Satellite (UARS) Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)8
. There has also
been much uncertainty in recent spectral observations from SORCE, particularly in the size of the change in UV
radiation between solar minimum and solar maximum9,10.