Tautology is crucial in logical structure. For example, any logical link between words must already exist in one of those words, if it is to be true by tautology.
For example, saying "Cats are animals" is only true if "animals" by definition already include cats.
So if word "animals" by its own definition includes "cats, dogs, rabbits", then saying that cats are animals is true by tautology, and cannot be false.
Tautology from dictionary:
"a statement that is true by necessity or by virtue of its logical form.
"all logical propositions are reducible to either tautologies or contradictions""
Tautology in simple terms means saying same thing twice in different forms. It is similar to law of identity.
But there is also anti-tautology, or something which is false by tautology.
For example, the claim "Humans are animals".
If word "animals" is defined in such a way to exclude humans in its definition, then saying that humans are animals is false by tautology.
Tautology in causation also works in same way.
Causation is defined as "If one thing exists, then the other thing must exist as its result".
For example, the claim "CO2 causes global warming" can only be true by tautology if definition of CO2 or global warming already includes this causation. And it can include this causation in 3 ways:
1. If main definition includes it (which is then a truism),
2. If it indirectly includes it (in definition of other words used in main definition),
3. If later expanded agreed definition includes it, directly or indirectly.
Tautology depends on how well defined the word is. Not all definitions are equally good and not all definitions include all there is to include. But if, in debate, definitions are agreed upon, then claim is usually true by tautology or false by tautology. There is usually no 3rd option, because definitions either support the claim or they dont. But in case definition is incomplete, there is more space to debate, but this then requires expanding existing definitions. In case of CO2, its main definition would have to be expanded to include causation of global warming, usually done through observation evidence to connect causation to CO2, making them impossible to separate.
The anti tautology can also be applied to causation. For example, definition of CO2 doesnt limit CO2 to Earth, and CO2 which is far away from Earth doesnt cause any global warming on Earth by definition, thus not all CO2 causes global warming.
Sure, this could be considered as focusing too much on details in definitions, but it does help if there are no other ways to attack or defend the topic.