6.2 Start at the Source: Earth Rotating Around the Sun
Solar radiation drives the Earth system and makes life possible. Solar radiation is absorbed and then put to use to increase the surface temperature, to change the phase of water, and to fuel atmospheric chemistry. The uneven distribution of solar radiation on Earth’s surface drives atmospheric dynamics.
The total amount of solar energy per unit time and unit area, also called the solar irradiance, is 1361 W m–2 at the top of the atmosphere (Stephens et al., 2012, Nature Geoscience 5, p. 691). It is distributed unevenly over Earth’s surface. That distribution changes over the course of the seasons (see next two figures). The seasons result primarily from the Earth's rotation axis not being perpendicular to the plane of the Earth's orbit around the Sun.
Earth’s spin and its orbit around the sun are not constant, but instead, change with time, like a spinning top. The orbit’s eccentricity (i.e., how different it is from circular) varies with a 100,000-year period. The tilt of Earth’s rotation axis with respect to a line perpendicular to the plane of Earth's orbit, which is called the obliquity, varies from 22.1o to 24.5o during a 41,000-year cycle. The obliquity is currently 23.4o and decreasing. Finally, the precession of Earth’s orbit, which is the orientation of the rotation axis with respect to Earth’s orbital position, also varies with a period of about 26,000 years, although the orbit itself is also rotating around the sun, so that the effective period of precession is about 21,000 years. These motions, when taken together, slowly and periodically change the distribution of solar irradiance on Earth’s surface and are described by the Milankovitch Theory (see next two figures). The changes in solar radiation ultimately lead to very large changes in climate and greenhouse gas concentrations, particularly the ice ages, which occur about every 100,000 years. The greenhouse gas changes (methane and carbon dioxide, in particular) were presented in Lesson 4.
Changes in Earth’s orbit and spin are not the only ways that solar irradiance changes—the Sun’s energy output also changes. It has been increasing slightly (0.05–0.10%) over the past 300 years and varies by another ~0.1% over the course of the 11-year solar cycle. The ultraviolet (200–300 nm) irradiance has increased by about 3% in the past 300 years and varies by ~1.5% between solar maximum and solar minimum. This increased UV leads to greater stratospheric ozone production, which increases stratospheric heating, leading to the poleward displacement of the stratospheric meridional wind.