// I'm sure they have and then discounted [the suggestion that the world has had another temperature rise in its 4.5 billion (not million) years existence]... //
Even this is completely wrong. The early history of Climate Change science was precisely about understanding why the Earth's Climate had varied
in the past, not how it might vary in future. But, by around the end of the 19th Century, a link had been made by multiple scientists between varying levels of CO2 ("carbonic acid gas", as it was known then) and previous Ice Ages.
For example, this, from the 1850s by Eunice Newton Foote:
// ...the highest [warming] effect of the sun's rays I have found to be in carbonic acid gas. ... An atmosphere of that gas would give to our earth a high temperature; and if, as some suppose, at one period of its history, the air had mixed with it a larger proportion than at present, an increased temperature from its action...must have necessarily resulted. //
Or, as Svante Arrhenius in the 1890s noted, that a halving of the then-atmospheric levels of CO2 would result, or have resulted in, an Ice Age, and similarly that a doubling (again, compared to 1890 levels) could give a total warming of around 5 degrees C. (These calculations have doubtless been revised, but the point is that the link between CO2 and global temperature was already made well over 100 years ago.)
Then there's this, in 1912:
https://tinyurl.com/5fz8ssyc
Now, it's true that at the time these theories were not universally accepted, but then it's also true that other ideas were advanced, including but not limited to solar activity and variations in the Earth's orbit (Milankovitch cycles, as they are now known). But an important point about this is that the role of Solar Activity as a source of short-term climate change was firmly debunked as a result of the early 20th-century research, while the Milankovitch Cycles remain well-established but are relevant only on ten-thousand-year+ scales. And at the same time "greenhouse theory" was generally regarded with scepticism, for example because of the belief that oceans would absorb CO2 too quickly for human emissions to have an effect.
The overall point, then, is that the scientific consensus today, that greenhouse gases (GHGs) are primarily responsible for the present epoch of Climate Change, and that human activity is primarily responsible for the increase in GHGs, has arisen out of a lengthy period of extreme scepticism, where it has found an answer to all attacks and has survived all attempts at serious scrutiny. It's *not* because other ideas have been crushed underfoot by financial interests.