What is Earth’s surface emissivity?

Emissivity is an important factor in all calculations involving Stefan-Boltzmann’s Law:

q = εσT⁴

where q is the flux in W/m²

Most of the time in climate science the emissivity (ε) is presumed to be 1.

Scientist Roy Spencer likes to use ε=0.98 (link):

… a broadband IR emissivity of 0.98 …”

Where does this number come from? It comes from a very narrow wavelength range, typically 8-14 microns. Not “broadband” at all.

When other scientists wish to specify an emissivity factor, they will choose a value between 0.95 and 1, also based on a narrow band range.

This is all wrong! We actually do have a very good broad measure of emissivity covering 5-200 microns (over 99% of Earth’s emission)! This data was collected by NASA’s ISCCP Project, and is available here:

https://isccp.giss.nasa.gov/pub/data/surface/

The relevant file is called D1AVGANN__BBEMIS. It consists of 6530 equal-area sections of the earth, thus we can just average all the numbers in the file.

Here is a one-liner to get the answer we’re looking for:

wget -qO- https://isccp.giss.nasa.gov/pub/data/surface/D1AVGANN__BBEMIS | od -An -tf4 -w4 --endian=big | awk '!/\*/{S+=$1;N++}END{print S/N}'

And the answer is: 0.93643

This data was collected from 1983 to 2004, so one would think most climate scientists would just use the right number. But of course we are assuming they are interested in science.

-Zoe

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