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1.The Sun-Ultimate source of energy of all the planets including Earth. Earth is the only place that has all the right conditions for life to exist. The main reason for this is the right spot of Earth to receive the Sun's abundant energy that is needed for proper chemical reaction. The Sun is vital to life on Earth. Its heat influences the environment of all the planets, Moon andComets in our Solar System. Variations in the intensity of solar radiation hitting the Earth may produce changes in global and regional climate which are both different and additional to those from man-made climate change.

Since 90’s scientific research on climate change has included multiple disciplines and expanded. FromCharless Grelly Abott’s theory(1930) that persistently connected the sunspot as a main cause of climate change to Milutin Milankovitch’s (1950)improved theory of James Crall with tedious calculations of varying distances and angles of the Sun’s radiations convinced that sunspot variations were a main cause of climate change. Early 1970’s also brought claims that far slower variations in the Earth’s magnetic field correlated with climate, however it failed to convince. In 1973 Jack Eddy drew attention to a spell of low Carbon-14 and thus high solar activity. By the early 21^st century all the trends in the Sun that could have an influence on the earth’s climate has been in the opposite direction to that required to explain the observed rise in global mean temperature. Recent reviews have been presented by Haigh(2007), Lockwood (2012), Olayinka S. Ohunakin (2015), Jesse L.Reynlds (2019),Seleshi G. Yalew (2020).

Fig. (1) Earth and Sun

The Solar energy to the Earth is significant even after passing through millions of kilometers of the Earth’s atmosphere.  It is generally considered to produce a constant amount of power in spite of small variations. Average radiation that hits the edge of the Earth's atmosphere is known as the Solar Constant, that varies 7.1% according to the distance of the Earth from the Sun.There are two main sources of variation in solar radiation. The first one is the Inter stellar process that affect the radiant energy emitted by the Sun and the second is the Earth's orbit that directly affect the amount of energy hitting the Earth and its distribution around the Globe.

2.Effect of Earth’s Orbit-The solar energy flux i.e.,total amount of solar energy reaching the Earth in a given time period depends upon the distance of Earth to the Sun.Seasons exist because of the elliptical orbit of Earth that varies during the year. As Earth’s axis is tilted from the direction perpendicular to the plane of its orbits, so when one pole is tilted towards the Sun at a particular point in the orbit it is summer in the associated hemisphere and winter in the other hemisphere. Another parameter that measures the wobble of the Earth’s axis affecting the timing of the seasons relative to the Earth’s position is the elliptical orbit.On time scale of many millennia the amount of radiation received at the Earth is affected by variations in these orbital parameters. The Earth -Sun geometry play the major role in the variation of magnetic flux.The total solar energy that hits the Earth is basically depends upon its distance from the Sun and Its elliptical path. But the distribution radiation over the globe depends on the tilt & precession.

Fig. (2)The different parameters responsible for solar energy flux.

The amount of energy arriving in summer at high latitudes determines whether the winter growth of the ice cap will recede or the climate will be precipitated into an ice age. Changes to the seasonal irradiance, when amplified by other mechanism such as greenhouse gases released by the initial warming can lead to much longer- term shifts in climate regime. Data showing a connection between solar variations and climate are often dismissed as mere correlations since there is no generally accepted theoretical basis to explain these correlations. This is hardly an acceptable position given that the strongest arguments in approval of carbon dioxide in the atmosphere, mostly due to human activities. In the case of carbon dioxide, there is a generallyaccepted mechanism for linking changes in climate to variations in the concentration of this gas since it is an absorber of long wavelength infrared radiation and simple dimensional radiative equilibrium model illustrates the connection. It is defined as the change in net downward radiative flux at the troposphere resulting from any process that acts to perturb the climate system and measured in w/m².

It is convenient to characterize the response of the Earth’s surface and troposphere to a radiative perturbation after the stratosphere has come to thermal equilibrium considering the case of carbon dioxide, labeled as “the most important” greenhouse gas.

Fig. (3) Daily average solar radiation (Wm-2) entering the top of the atmosphere as a function of time, year and latitude.

If we increase the carbon dioxide by a factor of two it does not double the amount of infrared radiation absorbed by this. The reason for this is the position of carbon dioxide absorption bands relative to the earth’s emission spectrum CO₂ has three absorption bands at wavelength of 4.25, 7.52 and 14.99 microns. The Earth’s emission spectrum peaks at b/w 15 and 20 microns and falls off rapidly with decreasing wavelength.As a result, the CO₂ absorption band at 4.26 and 7.52 microns absorb negligible amounts of thermal radiation compound to the band at 14.99 microns.Accordingly adding more carbon dioxide to the atmosphere would contribute nothing to greenhouse gas effect.However, 14.99-micron band is essentially saturated so additional carbon dioxide may have some disturbance at the edge of the band. Because of this marginal effect, the change in forcing due to a change in carbon dioxide concentration is proportional to the natural logarithm of the fractional change in concentration of this gas.

ΔF= α/n(c/co)

Where c is the concentration of carbon dioxide at the time c₀ is the concentration at a given reference time and is the sensitivity of the climate to changes in carbon dioxide concentration.The approximation is valid in the range of practical interest therefore the earth’s temperature is relatively insensitive to changes in carbon dioxide concentrations.

3.Solar Influence on Surface Climate -

Climate variability and climate change depends crucially on the existence and accuracy of records of meteorological parameters. That would consist of long time series of measurements made by well calibrated instruments located with high density across the globe. But measurements with global coverage have only been made since the start of the satellite era just about thirty years ago.

Fig (4)- The most recent activity of solar cycle.

A key concern of contemporary climate science is to attribute causes, including the contribution of solar variability to the observed variations in temperature. Many approaches have been used but the simplest is to calculate the correlation of the time series of the temperature with that of the factor of interest, another approach is that of multiple linear regression analysis which seeks to drive simultaneously the magnitudes of signals due to a number of pre-determined factors. If the known uncertainties in both the data and the forcing factors are taken into account the result can be improved.

(a)   Millennial Time Scale-On long time scales changes in the earth’s orbit as well as variations in solar activity must be considered. Fig. (4) shows how interglacial periods tend to be associated with higher irradiance in summer high latitudes and also how the recovery from a cold period proceeds much faster than the rate at which it originally developed. This is probably due to a feedback effect whereby an initial warming is amplified by the release of the natural greenhouse gases. In this case the initial warming is assumed due to an increase in solar irradiance.

Fig-(5)Temperature deduced from18O records in air bubbles trapped in the Vostok ice core.

(b)   Centennial Time Scale-On these timescales, long term changes in the Earth’s orbit may be disregarded. Fig (5) presents reconstructions of the Northern Hemisphere surface temperature record over the past centuries. In long term variability, it is clear that current temperature is higher than the past two millennia climate change records suggests that the climate has been changing over the past century include the retreat of mountain glaciers, sea lever rise, thinner Arctic ice sheets and an increased frequency of extreme precipitation events.

Fig (6) Temperature change with the passing of years.

Different factors in the past century affecting the climate are represented in fig (2).

This suggests that the sun may have introducean overall global warming approximately 0.07⁰ C before 1960 but had little effect, since than studies using other solar indices have produced a larger signal of temperature increase before mid-century and a better match between observations and regressions model during that period.

Confirmation of hypotheses and validation of models can only be obtained by the continuingacquisition of long term, well-calibrated measurements of atmospheric properties including temperature, cloud properties and ozone concentration with sufficient spatial and temporal resolution. These need to be acquired as along side properly calibrated solar spectral data.

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