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This chapter examines the factors that determine the amount of solar radiation that reaches the Earth, and which primarily controls its climate. On timescales of billion years, the Sun evolves as a main sequence star and there are variations in its luminosity and in the spectral distribution of this flux, especially in the ultraviolet wavelengths that affect atmospheric composition, including some greenhouse gases. Variations over hundreds of thousands of years are primarily due to cycles in the Earth's orbit, while on timescales of tens of years, much smaller changes occur in response to the Solar Cycle. The chapter quantifies these variations together with seasonal and daily variations over the globe, due to rotational and orbital effects. The global distribution of the incoming solar radiation for each season is given.

This chapter discusses how there are four general factors that contribute to the Sun's potential role in variations in the Earth's climate. First, the fusion processes in the solar core determine the solar luminosity and hence the base level of radiation impinging on the Earth. Second, the presence of the solar magnetic field leads to radiation at ultraviolet (UV), extreme ultraviolet (EUV), and X-ray wavelengths which can affect certain layers of the atmosphere. Third, the variability of the magnetic field over a 22-year cycle leads to significant changes in the radiative output at some wavelengths. Finally, the interplanetary manifestation of the outer solar atmosphere (the solar wind) interacts with the terrestrial magnetic field, leading to effects commonly called space weather.

In the last chapter we saw that sunspots, aurorae, and geomagnetic disturbances vary in an 11-year cycle. So do many other solar features, including faculae and plages, which are bright regions seen in visible and monochromatic light, respectively. If both bright faculae and dark sunspots follow 11-year cycles, does this mean the sun’s total light output varies? Or are these two contrasting features balanced so that the sun’s output of light remains constant? The light output of the sun is often discussed in two different ways: either as the solar luminosity, which is the sun’s omnidirectional radiant output, or as the solar constant, the output seen in the direction of the Earth. In this chapter, we explore the variable solar light output that has been the subject of vigorous discussions. The total solar irradiance or solar constant is defined as the total radiant power passing through a unit area at Earth’s mean orbital distance of 1 astronomical unit. Today the most common units of solar irradiance are watts per square meter (W/m²). Power is defined as energy per unit time, so the solar irradiance can also be expressed in calories per square centimeter per minute. Modern experiments indicate that the sun’s radiant output is about 1367 W/m², with an uncertainty of about 4 W/m². About 150 years of effort by many people have been required to establish the value to this accuracy. The sun’s radiant output is not an easy quantity to measure, and we will discuss some of the struggles required to measure it. In the late 1800s, many scientists considered the solar total irradiance or solar irradiance to be constant. Oceanographers Dove and Maury vigorously supported this viewpoint, so the solar irradiance was called the solar constant. For the next century, virtually every paper concerning the sun’s radiant output used the term solar constant. No physical justification for this nomenclature existed, only a philosophical bias. Yet by the 1950s this bias proved so strong and so prevalent that support for individuals who wished to measure variations in the solar constant became almost nonexistent.

This chapter shows that the carbonate-silicate geochemical cycle or Urey reaction, the long-term control on the steady-state atmospheric carbon dioxide level, is far from equilibrium on the present Earth, approaching this state only on a billion-year timescale in the future as solar luminosity and surface temperature climb. Moreover, the progressive increase in the biotic enhancement of chemical weathering in the last 4 billion years, culminating in the weathering regime of the forest and grassland ecosystems, has brought the steady-state atmospheric carbon dioxide level closer to the Urey reaction equilibrium state. In contrast, the abiotic steady state is always further from this equilibrium state than the biotic, except near the origin of life and at their future convergence. These are counterintuitive results from a classical Gaian view.