FIGURE 2-5 U.S. Geological Survey land-cover data in Florida for (left) pre-1900 and (right) 1993. SOURCE: Marshall et al. (2004b).

SOLAR FORCING

Irradiance

The Sun’s electromagnetic radiation powers the Earth’s climate. The blackbody temperature of the solar surface is about 5800 K, and as a result, the spectrum of solar radiation peaks at visible wavelengths. Solar irradiance is the electromagnetic radiation from the Sun incident at the top of the Earth’s atmosphere at a distance of one astronomical unit (1 AU = 1.5 × 1011 m), corresponding to the mean Earth-Sun distance. Total solar irradiance (TSI), the integral of spectral irradiance, has a mean value of 1365 ± 1 W m−2 according to Active Cavity Radiometer Irradiance Monitor (ACRIM), Earth Radiation Budget Satellite (ERBS), and Solar and Heliospheric Observatory (SOHO) space-based radiometry. The recently launched Solar Radiation and Climate Experiment (SORCE) radiometer suggests a lower absolute value of 1358 W m-2. Approximately equal amounts of energy are radiated above and below 742 nm.

Solar irradiance forcing of climate arises from the change in electromagnetic radiation at 1 AU produced by variations in the Sun’s activity. This forcing is the result of changes generated within the Sun, not changes in the Earth’s orbit around the Sun. Calibrated radiometers on various



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