A. What is a Black Body in Physics?
A black body in physics is a surface that absorbs all radiation with zero reflection while at thermal equilibrium. It is an ideal body that does not exist in nature. It is used only to judge how real world conditions compare to the perfect case. This definition excludes all conditions, such as colors, other than black, shapes, angles, movements, rotations, atmospheres, and all variables other than temperature, distance between the body and source, and the energy output of the source. Each temperature has its own black body criteria. A black body as it relates to a planet cannot rotate, because if it did the temperature would be constantly changing and would not be in equilibrium.
The ideal black body gives scientists and engineers a standard in judging how real world objects operate thermodynamically as compared to the perfect case. Having standards to measure things are important and common. Rockwell hardness test is used to measure hardness (ASTM E-18). For example, steel is high and wood is low. The black body standards are used in many practical applications, such as lighting, heating, thermal imaging, electric & gas fired furnaces, and various measurement and instrument criteria. The closer the light bulb compares to a black body the more efficient.
Black body radiation is used as a standard in thermal imaging. No albedo component is used in that analysis. The only variable is temperature, distance and source. Black body radiation is used in lighting. No albedo component is used in that analysis. Although emissivity is a variable in real world applications, it is always 0 or 1 with respect to black bodies.
There is one exception. Whenever the words “Greenhouse Effect” pops up, then an albedo element is inserted into the calculations. The IPCC avoid calling it a black body temperature, choosing to call it a “Surface Temperature in Absence of Greenhouse Effect.” There have been many name variants such as “effective temperature”, “emission temperature”, “equivalent black body temperature”, and probably others.
Colleges and Universities teach how to calculate the black body temperature of a planet such as the earth. The University of Victoria is representative and states that the effective black body temperature is equal to the temperature of the sun times the square root of the sun’s radiation as reduced by the planets reflectivity divided by the distance between the planet and the sun. It calculates the black body temperature for the Earth as 249K or -24°C. Almost all Universities use an albedo variable in the calculations.
Inserting reflectivity or albedo into a black body analysis is contrary to its original premise of being a perfect absorber. Complicating the problem is that albedo is a number that is determined empirically, i.e. by physical measurements. It is not something that can be calculated without supporting empirical data. By including reflectivity into the equation it violates the original premise of a perfect absorber where albedo is 0. If the calculation assumes albedo = 0 then the black body temperature of Earth would be 279°K or 5.85°C and not 255°K. This would indicate that the Earth is 9 degrees warmer than the black body temperature and not the customarily used 33°C.
A standard needs consistency. If different people or entities make changes to the standard while others do not, it diminishes its value. Originally, a planetary black body was defined like other black bodies, as a perfect absorber and a perfect radiator. Some have inserted limitations into the standard to support a particular goal oriented objective. For example, assume that Country X discriminates against a particular class of people and that country also produces a high quality grade of steel. Adding 5 mm of penetration into the Rockwell Hardness test for all steel produced by Country X would satisfy a “goal oriented objective” but degrades the value of the standard. Purchasers would believe that steel produced by Country X was too soft and buy from someone else.
It is uncertain why albedo was inserted into the black body analysis other than to promote a “goal oriented objective,” i.e. Greenhouse Effect. Albedo for the earth varies from day to day, season to season, and year to year. Albedo of land varies from .1 to .4, ocean is almost zero at .06, ice & snow varies from about .5 to .7, clouds vary the most from .1 to .9 depending on the thickness, water content, and angle. The following Table illustrates the variations in warming amounts used by different entities.
The changes in the warming temperature with an included albedo variable varied from 33°C to 39°C with an average of 35°C. The standard deviation was 2.5°C degrees.