The Sun has a strong influence on the Earth in many respects (from magnetosphere to the climate) and on various time scales (from days to millennia). The Earth’s magnetos-phere is modulated by the solar wind (Sect. 2.2) in such a way that the near-Earth space (magnetosphere, ionosphere, thermosphere) is sensitive to the changes in the heliosphere and events originating on the Sun. The events produced by short-term variations of the Sun and their effects on the near-Earth environment are usually dealt by space weather.
For instance, shock waves created by coronal mass ejections (CMEs) are observed to cause geomagnetic storms. The radio bursts produced by solar flares could disrupt the radio communication system on the Earth. The solar energetic particles (SEPs) could also induce strong aurorae in the higher latitudes, sometimes even pose threats to satellites, electricity facilities, resulting in power shortages (e.g., the Carrington event in 1859).
Solar variations have also great influence on the Earth on longer time scales, particu-larly on the Earth’s climate, because the Sun is the main source of energy to the Earth.
By analysing the sunspot numbers records (Sect.2.1), the terrestrial cosmogenic isotopes data (Sect.2.4) and the climate-related quantities (such as global sea surface temperature (SST) and tree growth widths), the relationships between the Sun and the Earth’s climate have been observed (e.g.,Schneider and Mass 1975;Eddy 1976;Reid 1987;Bond et al.
2001). Friis-Christensen and Lassen(1991) has also found the link between the northern hemisphere temperature and the solar cycle length.
Recently, various surface climate records (e.g., corals, stalagmites, marine sediments) in the past have suggested that the Earth’s climate might be affected by the long-term variations of solar activity. One of the classical examples of the relationship between the
Mesosphere
Figure 1.1: Schematic indicating the potential mechanisms that might influence the Earth’s climate (details see text). Reproduced afterGray et al.(2010).
solar activity and the Earth climate is the Maunder minimum, a period between 1645 – 1715 AD. Sunspots seemed to have disappeared during the Maunder minimum, which was coincident with a long cold period in North America and Western Europe between mid-15th to mid-19th century (e.g.,Mann 2002), also known as the Little Ice Age (LIA).
LIA also covers another extreme of low solar activity between 1640 – 1550AD(Spörer minimum). Nevertheless,Jungclaus et al.(2010) has suggested that the LIA might have been caused by volcanism activities. The particles and aerosols injected along with the volcano eruptions into the stratosphere could cover the Earth’s surface and block a great percentage of the incoming solar irradiance, causing lower surface temperature.
There are also other potential astronomical mechanisms of influencing the Earth’s climate. For instance, the Earth’s orbit parameters change with time. The precession, the obliquity, and the eccentricity parameters vary with periodicities of≈23 000,≈41 000 and
≈100 000 years, respectively (Paillard 2001; Crucifix et al. 2006). With the change of these orbit parameters, the Sun-Earth distance and the incident angle of the sunlight also vary. The collective effect of these changing parameters on the Earth climate is known as theMilankovitch cycle, which is on time scales of a few tens of thousands of years and is
1.1 Sun-Earth connection the prime cause of the occurrence of the glacial/interglacial periods. Additionally, cosmic rays have also been proposed to affect the Earth global climate (e.g.,Ney 1959;Dickinson 1975; Svensmark and Friis-Christensen 1997; Marsh and Svensmark 2000a,b; Dorman 2012). The ions produced by the cosmic rays have been suggested to act as condensation nuclei and further cluster and become cloud condensation nuclei (CCN). The CCN could enhance the formation of the clouds (so-called cloud nucleation), increase the cloud cover area, and further cool the surface temperature. Although the correlation between the cloud cover and the cosmic ray intensity has been observed byMarsh and Svensmark(2000a), a solid physical connection between the cosmic rays and the cloud formation is lacking and the theory is still under debate. The Cosmics Leaving Outdoor Droplets (CLOUD) experiments (Kirkby et al. 2011) at CERN (European Organization for Nuclear Research) showed a very limited effect of cosmic ray strength on the CCN formation (Svensmark et al. 2016;Dunne et al. 2016;Gordon et al. 2017). Consequently, on the time scales we are concerned with in this thesis, i.e., from centennial to millennial, the solar radiative flux is still the prime solar source of influence on the global climate.
The two possible mechanisms by which the solar irradiance affects the Earth climate system: thetop-downand thebottom-upeffects, are described below and summarized in Fig. 1.1.
• Top-down mechanism: The chemical processes in the terrestrial atmosphere are highly wavelength and altitude-dependent. Particularly the stratosphere is sensitive to wavelengths shorter than 350 nm (viz., UV band). Oxygen molecules absorb the UV radiation in the Herzberg continuum (200 – 242 nm) and produce oxygen atoms and ozone. Thisoxygen photolysisprocess is responsible for the formation of ozone and heating up the stratopause region. Moreover, ozone is destroyed after absorbing the UV radiation in the Hartley-Huggins band (200 – 315 nm). This process is calledphotodissociation of ozone, which provides a strong radiative heating in the lower mesosphere and the upper stratosphere. Simulations show that the changes in the UV irradiance with the solar cycle affect the stratosphere patterns (Haigh 1996, 1999; Shindell et al. 1999; Larkin et al. 2000; Matthes et al. 2006; Haigh 2007). Perturbations in the stratosphere also influence the troposphere (Gillett and Thompson 2003;Scaife et al. 2005), where the main climate system performs. This stratospheric-induced heating propagates downwards to the troposphere and further affects the climate system. It is, therefore, called the “top-down UV effect” (Kodera and Kuroda 2002).
• Bottom-up mechanism: After the UV is absorbed by the Earth’s ozone layer, the remaining solar irradiance (mainly visible and IR radiation) is mostly absorbed in the cloud-free subtropical regions. The oceans in these regions are heated and va-porized, which enhances the humidity in the air and strengthens the Hadley and the Walker circulations. The Hadley (cell) circulation is a global tropical circula-tion driven by the uprising air in the tropical region and the converging air in the subtropical region. A strong Hadley circulation results in a larger latitudinal ex-tent and stronger trade winds (in the northern hemisphere, the warm air falls down to the surface at around latitude 30◦ and moves south-eastward due to the Corio-lis force). The Walker (cell) circulation is driven by the unevenly distributed heat in the eastern and western Pacific ocean. A strong Walker circulation results in a
higher pressure in the eastern Pacific ocean and lower pressure in the west, leading to stronger monsoon seasons in the western Pacific countries and cooler SST in the eastern Pacific ocean caused by the enhancing upwelling current from the bottom of the ocean. This phenomenon is the so-calledLa Nina. To the contrary, a weaker˜ Walker circulation results in warmer SST in the eastern Pacific ocean, causing flood in Peru and Ecuador and drought in south-east Asia and Australia (El Ni˜no condi-tion). This collective climate response to the absorption of TSI at or around the surface region is therefore called the “bottom-up TSI effect” (Cubasch et al. 1997;
van Loon et al. 2007;Meehl et al. 2009).
It is important to note that the response of Earth’s climate system to the solar radiative flux involves complex atmospheric circulations and feedback mechanisms. Therefore, both effects might only provide an initial trigger for an overall complexity in the climate system.
The concept ofradiative forcing (RF) is widely used to estimate and analyse the re-sponse of the Earth surface temperature to the perturbations in the energy budget. RF is defined as the energy change of the perturbing factors, which influence the energy balance between the incoming solar irradiance and the emission by the Earth’s atmosphere. These perturbing factors are called theRF agents, such as the greenhouse gases (GHGs, mainly methane and carbon dioxide), aerosols and clouds. The RF has units of [W/m2]. A posi-tive RF, such as an increasing amount of GHGs which absorb IR radiation and re-emit it back to the Earth’s surface, results in a global temperature increase. A negative RF, such as an increase amount of aerosol particles which reflect the incoming solar irradiance, re-sults in a global cooling effect. The change in the RF is found to have a linear relationship to the change in the surface temperature (Ts):
∆Ts= λs∆RF, (1.1)
whereλs is the climate sensitivity parameter with a typical value ranging from 0.3 – 1.0 K/(W/m2) (Haigh 2007) with a best estimate between 0.6 – 0.8 K/(W/m2) (Solomon et al.
2007;Le Treut 2012). Note that since the Earth reflects about 30% of the incoming solar irradiance and re-distributes the absorbed irradiance over the global spherical surface, the change in the RF is not one to one correlated with the change in the TSI. For instance, a 1.0 W/m2 increase in the TSI only results in a 0.175 W/m2 increase in the RF, which implies a 0.1 K increase in the surface temperature (by takingλs= 0.6 into account).
It has now been generally agreed within the scientific community that the recent glo-bal warming is mostly caused by the release of GHGs, which are produced by the large amount of fossil fuel burning (Solomon et al. 2007). Nevertheless, to better estimate the level of anthropogenic factors on the Earth’s climate, a good understanding of the natural cause (e.g., solar irradiation) needs to be taken into account. Since solar irradiance is an important energy forcing inputs in many climate models (Hansen 2000;Haigh 2001, 2003, 2007;Gray et al. 2010;Jungclaus et al. 2010;Schmidt et al. 2011;Jungclaus et al.
2016;Matthes et al. 2017), reliable reconstructions of both TSI and SSI further back into the past are needed.