Introduction
In the realm of climatology and thermodynamics, the study of thermal spectra provides insights into the complex interactions between Earth's surface and its atmosphere. In this article, we revisit 1970 NIMBUS 4 experiment spectra [1, 2] of Antarctica and Sahara, two vastly different regions on our planet.
Comparative Analysis: Sahara vs. Antarctica
The Sahara Desert, characterized by scorching surface temperatures, stands in stark contrast to the frigid expanse of Antarctica. By comparing their thermal spectra, we gain insights into the influence of extreme temperatures on radiation patterns. This sheds light on the characteristics of these regions, emphasizing how thermal signals are shaped by the interplay between ground temperature (thermal irradiation), greenhouse gas specific absorption and atmospheric thermal radiation.
As correctly observed by W. Happer
Radiative forcing is negative over wintertime Antarctica since the relatively warm greenhouse gases in the troposphere, mostly CO2, O3 and H2O, radiate more to space than the cold ice surface, at a temperature of T = 190 K, could radiate through a transparent atmosphere.
Discussion / Background
The unique Antarctica spectrum is shaped by its frigid ground. The low thermal signal (extreme cold ground) gets overshadowed by the atmosphere's signal. The explanation is the well known Stefan-Boltzmann law which states that the total energy radiated by a black body per unit surface area is directly proportional to the fourth power of the black body's temperature (in Kelvin).
Remember: We delved into similar territory when discussing the misleading Venus article from WEF.
The equation is given by:
With T1=190K for Antarctica and T2=320K for Sahara, the total irradiation at 190K is approximately 90% less than the irradiation at 320K.
However, the graphs are showing the spectral emission intensity in the frequency domain. Using Wien’s law, the maximum is following a power 3 law.
With T1=190K for Antarctica and T2=320K for Sahara, the peak emission maximum at 190K is approximately 80% less than at 320K.
The peak maximum at 190K is shifted so far left that it’s cut of from the plot range. A rough visual estimate of ~50/200 = 0.25 is in line with the above calculation.
For amusement, Chat GPT got it wrong of course. It’s not able to do deterministic math. It’s a language model which guesses words. It’s neither intelligent nor understanding physics.
This is how it sees itself. It “excels”. At least it wasn’t a GPT user policy violation.
Antarctica Vertical Temperature Inversion
One of the fascinating observations from the 1970 NIMBUS 4 experiment is the atmospheric vertical temperature inversion.
The Stefan-Boltzmann law elucidates the resulting spectra, but the modulation of the black body theoretical curve by specific greenhouse gas emissions introduces additional bumps. This modulation can lead to a net negative forcing in polar regions, although the significance of these low levels is yet to be precisely calculated. The quantitative effect for this case wasn't included in the mentioned work.
Radiative forcing depends strongly on latitude, as shown in Figs. 7 and 8. Near the wintertime poles, with very little water vapor in the atmosphere, CO2 dominates the radiative forcing. The radiation to space from H2O, CO2 and O3 in the relatively warm upper atmosphere can exceed the radiation from the cold surface of the ice sheet and the TOA forcing can be negative
Appendix
The following graph illustrates the latitude dependence in the same way.
Related reading: https://arc.aiaa.org/doi/pdfplus/10.2514/6.2014-1905