There is a number of physical factors that are thought to have an impact on the climate of the Earth including changes in the orbit of the Earth when revolving around the sun is perceived to result in ice ages; large volcanic eruptions that result in sudden cooling of the Earth that can last 2 - 3 years; internal variations in the climate of the Earth such as changes in ocean and atmospheric circulation; changes in concentration of greenhouse gases in the atmosphere; and fluctuations in solar activity, which is the focus of this paper (Benestad, 2006). Historical records indicate that the Sun is a crucial determinant of the climate of the Earth. For more than a century, reports have suggested an apparent relationship between the climate and solar activity. In 1801, William Herschel, a famous scientist, hypothesized that the number of sunspots had a direct effect on the price of wheat; this suggestion drew upon the observation that the rainfall was lower during the times of sunspots. Ever since, there have been numerous reports linking the climate of the Earth and solar activity (Rycroft, Israelsson, & Price, 2000).
Fluctuations in solar activity can be traced using history of atmospheric isotope levels, which were caused by galactic cosmic rays. These historical records indicate a relationship between cold climatic periods and low solar activity, as well as warm climatic periods and high solar activity in the course of the past 10,000 years (Rycroft, Israelsson, & Price, 2000). Specifically, the period 1000 to 1300 AD was characterized by warm climate, while at the same time, solar activity was high. After 1300 AD, there was a decrease in solar activity, which caused prolonged cooling of the climate. There are various ways through which solar activity affects the climate of the Earth including the link between temperature of the Earth and long-term fluctuations in solar activity; fluctuations in solar irradiance; and through galactic cosmic rays (GCR) (Haigh, 2003).
The Relationship between Cloud Cover of the Earth and Cosmic Rays
Satellite data has revealed a strong correlation between GCR and cloud cover of the Earth. Clouds have an effect on the atmospheric properties through heating by trapping outbound long-wave radiation and cooling by reflecting inbound shortwave radiation. The net radiative effect associated with a specific cloud depends significantly on its optical thickness as well as its height above the sea level (Dorman, 2012). Clouds having high optical thickness are likely to heat whereas clouds having low optical thickness tend to heat. Currently, it has been estimated that the net forcing of the Earth's cloud cover is 17-35 Wm-2 cooling, which implies that clouds play a crucial role in the radiation budget of the Earth (Dorman, 2012). As a result, any significant effect on the cloud properties of the Earth has a significant influence on the climate of the Earth. The figure 2 below shows satellite monitoring regarding the total cloud cover on the Earth. From the figure, the GCR variations are compared with cloud data and the 10.7 radio flux from the Sun. The comparison revealed differences between radio flux and GCR variations. During the period 1987 to 1995, the radio flux and GCR variations existed simultaneously. Nevertheless, there was a lag of about two years before 1987 (Haigh, 2003). Regarding this observation it is vital that the cloud cover of the Earth is followed by GCR fluctuations, which suggests that the ionization that takes place in the atmosphere as a result of GCR is vital in the link between the climate of the Earth and solar activity (Dorman, 2012).
GCR Influence on Earth's Temperature
After establishing that GCR fluctuations are likely to influence the climate of the Earth, using data spanning 1980-1995, it is crucial to look at fluctuations in solar activity based on data spanning longer periods. Nevertheless, it is imperative to note that no reliable data exists for cloud cover outside the period mentioned above (Dorman, 2012). An important assertion is that, if GCR fluctuations can result in a climatic effect, then the climatic effect ought to be manifested through fluctuations in the temperature of the Earth. This poses the need to explore long range data series relating to GCR fluctuations. Cosmic rays were first measured in 1935 using ionization chambers that primarily measure muon flux. The figure 2 below shows the cosmic ray flux for the period 1997 to 1994. The figure 2 below illustrates four distinct measures of solar activity during a longer period and the temperature of the Earth. The figure also shows the 11-year averages of marine and land temperatures in the northern hemisphere. It is evident from the figure that the temperature of the Earth during 1970-1990 increased by about 0.3 degrees Celsius (Rycroft, Israelsson, & Price, 2000). Therefore, simple assumptions allow to make a comparison of the cosmic ray flux fluctuations and changes in the temperature of the Earth. From numerical cloud modelling and cloud observations using satellites, it can be established that 1 percent change in the total cloud cover of the Earth will result in a change in the net radiative forcing by 0.5 W/m2. Rycroft, Israelsson, & Price (2000), point out that, during the period 1987 to 1990, global cloudiness showed a change of about 3.0 percent, which translates to 1.50 W/m2 (Rycroft, Israelsson, & Price, 2000). During the same time period, there was a 3.5 percent change in cosmic rays. The approximate radiative forcing can be computed by considering that mean 11 years average upsurge in cosmic rays during the period 1975 to 1989 is approximated to be between 0.6-1.2 percent, which translates to a change in cloud forcing of about 0.3-0.5 W/m2. Rycroft, Israelsson, & Price (2000), consider this to be a large cloud forcing, which is about 2-45 times the approximate fluctuation in solar irradiation. The basic inference from this calculation is that the volume of cloud in Earth depends significantly on solar activity; therefore, an increase in cloud cover leads to lower Earth temperatures. Thus, solar forcing can be used to explain the changes in Earth's temperature over the period 1935 to1994.
The Maunder minimum refers to a period 1645 to 1715 when extremely great number of sunspots was recorded on the Sun. Benestad (2006), suggested that, during the Maunder minimum period, there was a lower solar input, which can be used in explaining the extreme climatic conditions observed during the time. Ever since, there have been various efforts aimed at reconstructing the history of solar irradiance. The number of sunspots is considered to be one of the factors that can be used to measure solar activity. The number of sunspots has been recorded for the last four centuries. As a result, the relationship between the number of sunspots and solar irradiance fluctuations as observed by satellites during the past decades may help in reconstructing the historical changes in solar irradiance (Haigh, 2003).
Cosmic Rays as a Crucial Link between Climate Change and Solar Activity
A number of authors have outlined the effect of solar and galactic cosmic rays on the climate of the Earth. The primary source of air ionization at below 45-35 kilometers is cosmic radiation. It is only below 1 kilometer that the ionization of air is facilitated by radioactive gases found in the soil. According to Benestad (2006), fluctuations in ionization of the air might influence atmospheric optical transparency through a change in formation of aerosol or influencing the change of water state. The likely statistical relationship between solar activity and variations in the intensity of cosmic rays with climate change characteristics were considered by Rycroft, Israelsson, & Price (2000), who attempted to reconstruct variations in the intensity of cosmic rays during the last century based on the solar activity data and compared their findings with the radiocarbon data. Cosmic rays have also been found to play a crucial role in formation of lightning and thunderstorms. According to Haigh (2003), the atmospheric electrical occurrences suggest a likely relationship between the climate of the Earth and solar activity.
There is no doubt that the climate of the Earth has been undergoing fluctuations at different phases of history. Previously, these fluctuations were thought to be caused by internal causes only. For instance, the presence of volcanic dust within the stratosphere is likely to result in a cooling of 0.5 degree Celsius. Analyzing the data spanning for the 10 years given, it was found out that solar activity has a significant impact on fluctuations in the climate of the Earth. Researchers have found a striking correlation existing between fluctuations in the cloud cover of the Earth and cosmic flux. Owing to the fact that clouds play a crucial role the energy balance of the Earth, solar effect on the clouds is likely to be the primary cause of the documented correlations between the climate of the Earth and solar activity.