Total Solar Irradiance and Its Variability
The total solar irradiance, the total radiant energy coming from the Sun at the mean position of Earth, has been measured precisely from Earth-orbiting satellites for nearly 30 years, allowing the observation of nearly three solar cycles (Figure 4.2). To measure total solar irradiance precisely, it is important to remove the effect of the atmosphere’s absorption, which can be achieved by taking the measurements from Earth orbit. Also, satellite orbits can be chosen to be in constant sunlight, allowing continuous monitoring of solar irradiance changes. These measurements show that the variation in total solar irradiance associated with the Sun’s 11-year cycle is about 0.1 percent. Variations of 0.2 percent are associated with the Sun’s 27-day rotation at times of high solar activity (Hickey et al. 1980, Willson et al. 1981, Willson and Hudson 1988, Frohlich and Lean 2004). These changes are small compared to the effect of greenhouse gases on the energy balance of Earth. It is important to monitor the energy exchange between Earth and space so that observed changes in Earth’s climate can be attributed to and partitioned correctly among various causal mechanisms, including solar variability, atmospheric particles induced by volcanic eruptions, human-induced greenhouse gases, and aerosols.
FIGURE 4.2 Time history of total solar irradiance (TSI) observed from seven different orbiting TSI monitors, along with monthly sunspot number. The average change in TSI during the solar cycle is about 1.5 W/m2 or about 0.1 percent. SOURCE: Figure courtesy of Dr. Greg Kopp, University of Colorado, http://spot.colorado.edu/~koppg/TSI/.