If the emissions (of CO2, etc) have an impact on global climate doesn't that mean a measurable effect on emissions has an impact on global climate? If you have doubts about emissions impacting climate say that, but that's another discussion.
There are many things not directly measurable which have an impact. Example: when learning a new programming language reading one paragraph of a tutorial does not have a measurable effect on you knowing the language - for most paragraphs. But if you read zero paragraphs you might not know anything.
There are many things not directly measurable which have an impact. Example: when learning a new programming language reading one paragraph of a tutorial does not have a measurable effect on you knowing the language - for most paragraphs. But if you read zero paragraphs you might not know anything.