Leaving aside the quality of this claim as a statement of fact, isn’t this a logical fallacy when used as a counter-argument?
No. Referring to overwhelming scientific consensus or expert opinion is not in the least bit an unreasonable counter argument or argument. Virtually everyone agrees with this except when they have strong religious or political ideologies that conflict with this level of agreement. For example, if three neurologists told person A they had a brain tumor, and person B asked, "Why do you believe you have a brain tumor?" it wound not be fallacious for person A to respond "Because three neurologists told me that I did." Likewise, accepting the fact of evolution is not fallacious based on 97% of all earth scientists on the planet. We don't expect every person to go digging for fossils or study DNA. Nor we expect (or want) laymen thinking that their Google search is more likely to get them to the "truth" than the overwhelming consensus derived from tens of thousands of scientists from around the globe, working independently, with 7+ years of specialized education in a particular field and often a lifetime of lab and field work in the area. Logical fallacies are about errors with reasoning. When referring to issues where meta analyses have been done on thousands of independent studies such as evolution, climate change, GMO safety, age of the universe, vaccine safety and others, where overwhelming scientific consensus has been reached (often defined as 80% or more agreement among experts in related fields), it is NOT unreasonable to accept these conclusions provisionally.
Before I get to the other questions, let's be clear on something. "Environmental Activists" and the scientific consensus on climate change is NOT the same thing. Many activists grossly distort the truth and facts because of their ideology and passion. In case you already don't know this, here is the official statement
"Observations throughout the world make it clear that climate change is occurring, and rigorous scientific research demonstrates that the greenhouse gases emitted by human activities are the primary driver."
reference:
So let's keep this in mind when activists say things and spew "facts." What they say might be true, might not be, or might be exaggerations.
ADVOCATE: All you need to know is that 97% of scientists agree that global warming is real, caused by humans and will be catastrophic.
Perhaps avoiding the issue. The advocate probably did not have answers to those questions, and rather than say "good questions... NASA answers these is great detail at..." they avoided answering them.
ADVOCATE: Denying that truth is no different than denying the Holocaust.
Weak analogy. I would argue that there are more differences than similarities with the two. However, denying climate change is very much like denying evolution.
In this case, I have read that many of the scientists who comprise the “97% percent” have no expertise in the impact of greenhouse gases on global temperature
Assuming this is true? What does "many" mean? Are we talking 90%? 50%? 20% or 1%? Misinformation campaigns thrive on ambiguity to raise doubts and challenge authority. This is why reason requires statistical thinking and not emotional thinking. Let's pretend that 50% of the scientists and organizations have no clue about the impact of greenhouse gasses on global temperature. Let's do some math based on an estimated 5000 climatologists globally who weighed in on climate change:
5000 climatologists total
2500 experts in greenhouse gasses on global temp
50% experts say no effect (therefore, don't make up consensus)
1250 nay, 3750 yea (75% agreement)
but we have a 97% agreement, so let's see if we can get closer...
10% experts say no effect (therefore, don't make up consensus)
250 nay, 4750 yea (95% agreement)
So even if 50% of all climatologists have no clue what they are talking about when it comes to greenhouse gasses and their effect on global temperature, we still have an overwhelming consensus (90% of those who do know what they are talking about).
The bottom line of all this is, we still don't have a "false authority." At best we have a less authoritative figure than what is made out to be.