What makes them different apart from the subject? People saying anthropogenic climate change isn't real are ignoring overwhelming historical evidence, as with Holocaust deniers.

First of all, Holocaust deniers deny that the Holocaust happened, when the Holocaust is an indisputable historical fact with thousands of survivors and millions of relatives of victims and denying that is offensive and malicious. So-called "climate deniers" deny neither the existence of climate, nor climate change, nor anthropogenic climate change (most of them anyway), they are simply not convinced that the magnitude of the effect of human carbon dioxide emissions is represented correctly in the alleged "consensus" (which it provably isn't) of climate scientists. Especially after climate scientists were caught in the act of tampering with data in order to hide past warming periods. So there is a difference regarding the malicious intent, the moral dimension and the legitimacy of claims. As anyone who tries to debate it with an open mind and honest arguments would know.

Well, I don't know what to tell you but CO2 emissions from fossil fuels accumulate in the atmosphere. There is this weird thing happening that since 1960 the CO2 content in the atmosphere has risen from 300ppm to 400ppm. It's up to you to believe whether that is a coincidence. Random googling tells me we emit 36 billion tonnes per co2 per year. Even more random googling tells me there are 3 trillion tonnes of CO2 in the atmosphere. It's not hard to put them together and conclude that humanity responsible for growing the CO2 in the atmosphere by 1% every year right now. Now lets say these numbers are complete garbage and inaccurate (which they certainly are) and say they are off by 70% (which they certainly are not) and the a highly conservative number is closer to 0.3%. After 30 years 9% of CO2 in the atmosphere is man made assuming we start off with 0% man made CO2 (obviously not true).

The point of this comment is not to prove that there is a certain percentage of man made CO2 in the atmosphere because I don't have accurate numbers. The point is that humanity is powerful enough to reach an arbitrarily high percentage of man made CO2 in the atmosphere and it is certainly possible for 50% of the CO2 in the atmosphere to be of human origin if we wait long enough and do nothing.

> After 30 years 9% of CO2 in the atmosphere is man made assuming we start off with 0% man made CO2 (obviously not true).

So why do you try to argue something based on assumptions that are by your own admissions obviously not true?

What is the "normal" amount of CO2 in the atmosphere? What amount is optimal? What difference does it make to the atmosphere whether the CO2 is according to us of human origin or not?

Presumably, nobody sane believes we're at historically high levels of CO2 right now, past levels at 1000-4000ppm are not really disputed in the scientific community.

It's the rate of change in CO2 levels that is unprecedented in human history and for a long time in earths history. The effects of that change include ocean acidification and habitat loss for many species. It stands to reason that atmospheric change on a timescale much smaller than what natural selection and species adaptation normally occur at could be very damaging to all sorts of ecosystems.

Ah, I generally don't consider doubts of that nature (merely the magnitude of our effect being questionable) to qualify someone as a "climate denier", at least personally. I reserve it for the ones who call it a hoax, or say that warming trends are completely natural, or one of the many other easily refutable claims.