>Anthropogenic climate change is not a “scientific fact”.

It is absolutely a scientific fact.

>and (to the best of my knowledge) an untestable one at that

You have no knowledge, so little it's hard to believe that you've in good faith ever looked it up. Foundation: the Earth is an open system, with the Sun providing the primary energy input and radiation into space being the thermal sink. The average energy of the atmosphere depends on the balance point between those. If all/most of the energy was trapped, temperature would skyrocket towards Venus levels. With little to none trapped (or all reflected) the atmosphere would freeze out. In between is a dynamic equilibrium. "Climate" and "weather" happens because thermal input is not even, which results an enormous heat engine that redistributes energy around the globe. The equilibrium point can be shifted by retaining more energy, or not.

The anthropogenic climate change overview comes down to three basic questions:

1. Is CO2 a greenhouse gas? The answer to this is yes. Most of the energy input from the Sun is in the visible part of the spectrum. Some is reflected outright, some used by vegetation for work, and around half is absorbed and then re-radiated in the IR part of the spectrum. CO2 absorbs light in the 2.7, 4.3, and 15 micrometer bands which makes it a strong IR absorber. You can verify this with an infrared spectrophotometer, or inversely by looking at the emission spectrum. This isn't cutting edge tech, spectral data has been a heavily used tool in a range of science for a long time now.

2. Is CO2 concentration increasing? Also yes. Like, we can just measure this directly and have directly for decades. We don't need to go back any further for the purposes of establishing global warming, because we already know increasing the greenhouse effect will warm things up. Though measures like air trapped in bubbles in ice at the poles can give us concentrations farther back too.

3. Is the increase in CO2 due to humans? A final and also unambiguous yes. Carbon, like other elements, has multiple isotopes. Only C-12 and C-13 are stable, with the ratio between them fixed overall on Earth. However, plants preferentially use the lighter isotope, in turn being consumed by other creatures, and as fossil fuels come from organic life they have a different C13/C12 ratio than inorganic carbon sources. Isotope geo and atmospheric chemistry has let us analyze and track the ratio over significant time, both directly in modern times and going back a significant period via tree rings. The C13/C12 ratio dropped dramatically starting in about 1850, along the industrial revolution, and has fallen ever since. The extra carbon in the atmosphere is therefor coming overwhelmingly from organic sources, primarily fossil fuels [1].

And that's really all there is to it at a basic level. CO2 traps heat. There is more of it. And it's due to us. Now, high precision predictions for exactly what the effects of energizing the atmosphere with far more heat will be for any given area and time are very hard. But the overall basis for anthropogenic global warming is not.

>I know how unpopular this comment will be in some circles

It's unpopular because it's fucking stupid. Willfully stupid. You are not an oppressed brave rebel speaking truth to The Man. You are just mindless repeating trivially disproved crap.

Edited to add:

1: Incidentally, primary source is fossil fuels because it's both a huge net change so mere regular biological cycles isn't enough, and because the C-14 ratio dropped too. C14 is an unstable isotope with a half-life of about 5730 years, generated constantly by cosmic ray bombardment in the atmosphere. The process to produce fossil fuels takes so long that only infinitesimal amounts of C14 remains, so burning them lowers the C14 ratio as well. This is the same principle for what it's worth behind radiocarbon dating.

Though it doesn't really matter in this question because even if it was due to mass deforestation caused by humans say, or toxic waste killing marine life or whatever, that'd still be anthropogenic. It's been well studied, like every other aspect of this. If you have any basic questions as a layman about this field, it is highly probable they already have been answered.