6.2 Start at the Source: Earth Rotating Around the Sun

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 orbit around the sun.

Credit: Credit: The Seasons. National Oceanic and Atmospheric Administration/National Weather Service. (NOAA) (Public Domain)

The current tilt of Earth’s orbit. More solar radiation is absorbed by a square meter of Earth's surface near the equator than a square meter at high latitudes as a result of the curvature of the Earth.

Credit: National Park Service. (NPS)

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 22.1^o to 24.5^o during a 41,000-year cycle. The obliquity is currently 23.4^o 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 that occur over time (top). Eccentricity (non-circular shape) varies over 100,000 years (middle). Obliquity (tilt of Earth’s rotation axis) varies every 41,000 years (bottom). Precession (obliquity relative to position in Earth’s orbit) varies over a cycle with a period of approximately 20,000 years.

Credit: Credit: CO2CRC.

Variation of solar irradiance (i.e., forcing) observed as a result of the changes in eccentricity, obliquity, and precession. Links to glaciation are complex but the distribution of solar irradiance on Earth is important for climate.

Credit: Milankovitch Variations from Wikimedia Commons is licensed under CC BY-SA 3.0

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.