Climate alarmism of global warming claims to be supported by measurement of the energy balance of Earth+atmosphere by instruments like pyrgeometers, bolometers and radiometers with an accuracy of at best 1-2 Watts/m2 compared to a total of around 240 W/m2 and a projected total imbalance of 4 W/m2 as "radiative forcing" from doubling of atmospheric CO2 corresponding to a warming of 1 K.
The case for global warming may seem weak from these measurements, but nevertheless they serve to foster alarmism.
To properly evaluate the measurements it is necessary to understand how these instruments are designed and how they operate. For a pyrgeometer or bolometer using a thermocouple as sensor, there are two fundamentally different views:
- A thermocouple essentially measures incoming radiance from a source as a process variable.
- A thermocouple essentially measures a source temperature as a state variable.
It is natural to make a comparison in terms of a bank account:
- Difference between deposits and withdrawals as process variable.
- Total savings as state variable.
We understand that total savings may be fairly stable, while deposits minus withdrawals can fluctuate quite a bit. The same for temperature vs radiance imbalance.
What does then a thermocouple as sensor in fact measure? Radiance or temperature?
1. There is a widely spread view that a thermocouple essentially measures radiance and so can be used to reliably measure both incoming and outgoing radiance for Earth+atmosphere and so determine imbalance, even if the accuracy is not better than 1-2 Watts/m2, and so detect global warming. Radiance is then measured through a calibration process confronting the sensor with sources of known temperature $T$ with radiance according to an assumed Planck-Stefan-Boltzmann Law of the form $\sigma T^4$.
2. There is also a different view that a thermocouple essentially measures source temperature by essentially allowing the sensor to take on the source temperature by radiative equilibrium established optically at distance. In practice the radiative equilibrium source-sensor is only partially established by sensor cooling, but the principle of radiative equilibrium with equal temperature remains.
Case 2 builds on a clear physical principle of radiative equilibrium in stable measurement of a state variable.
Case 1 is based on instrument calibration vs sources/blackbodies of known temperature $T$ assumed to give radiance input of $\sigma T^4$, while the true input is PSB in the form $\sigma (T^4-T_i^4)$, where $T_i$ is instrument base temperature which is not 0 in general. Case 1 is thus based on a calibration process using an incorrect PSB law inflating input radiance. Moreover the measurement concerns a process variable prone to instability. There are cryogenic sensors with very small $T_i$ and better precision. A proof of the correct PSB Law in classical terms without statistics is presented here and in this talk.
Case 1 is consensus and is used to support alarmism from measured radiance imbalance of Earth+atmosphere as if this is a fact. But the measurement precision barely can capture any imbalance from doubled CO2. Unfortunately many climate skeptics embrace the idea that a pyrgeometer measures massive incoming radiance (Downwelling/Upwelling/Outgoing Longwave Radiation) and so go along with a basic alarmist argument: The measured energy imbalance is the result of more CO2.
A careful study shows that a thermocouple in fact measures source temperature as stable output, while derived radiance can be misleading because the calibration uses an incorrect PBS Law and is prone to instability. This means that measured energy imbalance can be questioned along with alarmism.
But the discussion is pretty much closed on 1 as the truth. Hopefully a new discussion can take place around the question: What does a thermocouple primarily measure and on what physical grounds? How can a thermometer acting at distance be constructed? Is an IR-camera such a thing?
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