In physics, a black body is an object that absorbs all electromagnetic radiation that falls onto it. No radiation passes through it and none is reflected. It is this lack of both transmission and reflection to which the name refers. These properties make black bodies ideal sources of purely thermal radiation. That is, the amount and wavelength (color) of electromagnetic radiation they emit is directly related to their temperature. Black bodies below around 700 K (430 °C) produce very little radiation at visible wavelengths and appear black (hence the name). Black bodies above this temperature, however, begin to produce radiation at visible wavelengths starting at red, going through orange, yellow, and white before ending up at blue as the temperature increases.
Image: CIE Black Body Color Temperature Chart — “The Tongue” — click for full-sized chart
The color temperature of the Sun is near the yellow zone on the inset chart, while the color temperature of the Earth (obviously) is outside the visible radiation spectrum and in the far infrared (low temperature) corner of the chart. Given the (more or less) constant energy level emitted by the Sun, the Earth can only rise to a finite maximum temperature. An independent study predicts that we’re almost there!
John Ray (M.A.; Ph.D.), writing from Brisbane, Australia, passes along this article by Ian Schumacher, in which he offers six postulates supporting the maximum temperature argument: What is the maximum temperature of the Earth?
- The average temperature of a body in thermodynamic equilibrium with an external energy source can never exceed the temperature of a black body in the same environment.
- The maximum temperature of a body in thermodynamic equilibrium with an external energy source can never exceed the temperature of black body in the same environment.
- The greenhouse effect can never produce a temperature that is higher than the temperature of a black body in the same environment.
- The earth is operating very close to its maximum possible temperature.
- The transition from Ice Age to warm period and back to Ice Age is achieved through a runaway greenhouse effect and its opposite.
- The runaway greenhouse effect ends when the Earth has achieved a effective absorptivity as close to unity as it can get after which the earth becomes insensitive to further positive feedback changes.
Read the entire article to get a sense for the physics involved and narratives associated with these six postulates.
Hat tip to my colleague, Rick, for bringing this to my attention.