Um, what is this denial 'case' you refer to anyway? I don't think the deniers have a case, they don't need one. Remember, its the alarmists who are making the claims of future catastrophe, it is up to the alarmists to support those claims. Extraordinary claims require extraordinary proof, and so far that hasn't been delivered. Its easy being a denier, all one needs to do is pick holes in the alarmists evidence. But deniers aren't really making any claims, they don't have a case that I am aware of, they don't require a case. So what is their case again?It is true that the normal rules of logical discourse require those making the claims to back those claims with proof. What this commenter is doing is moving the goalposts. Anthropogenic Global Warming (AGW) has been backed by reams of research into multiple lines of evidence, independent verification and has made many predictions borne out by later evidence. Climate Change is quite plausibly the single most deeply studied topic in the history of human scientific endeavour. The Theory meets all the usual criteria (testable predictions, etc) to be a sound and widely accepted scientific capital "t" Theory, which is precisely why it is nearly universally accepted in the relevant scientific communities. But deniers simply assert that the evidence isn't good enough, and that they aren't satisfied based on some never explained asymptotically unreachable standard of evidence.
To answer this person's last question, when someone makes a claim and then provides evidence for it, the onus is now on the sceptics to explain why the evidence is unacceptable. What is the denier case against the evidence presented? In attempting to make such a case, deniers present us a bunch of charlatans, kooks and crooks who make relentlessly mendacious, specious and fallacious arguments where they misinterpret the data, cherry pick and derive trends from short term local maxima. As the video shows, deniers put together a petition of 30,000 "scientists" who oppose the consensus, but it turns out very few of these people are actual scientists of any sort, and even fewer of them are scientists in a relevant discipline. They sometimes deny the planet is warming, and other times they agree it is warming but assert that humans are not responsible. They often claim the whole notion is a giant fraud, yet strangely publish absolutely no original climate research which could show this. After all, if the planet really isn't warming, or it is but there is some other better (and natural) explanation for it, some original research could blow the whole AGW "fraud" wide open. Al Gore and the IPCC don't have a monopoly on sending up weather balloons and drilling ice cores. But really, the Oregon petition fraud is par for the course for deniers. When Senator Inhofe (who has his own infamous fraudulent list) and Viscount Monckton are your leading lights, one really must ask why no more credible opponents to AGW have appeared?
"Extraordinary claims require extraordinary proof"
As a matter of pure logical reasoning, I'm not convinced this old dictum is actually correct. I'd say that extraordinary claims require ordinary proof, the way any other claim requires. You should assess the evidence in and of itself, not in relation to how "extraordinary" you find the basic claim. Your bias that something is unlikely to be true should not impact your view of the evidence. Sometimes men bite dogs.
The bigger problem here is that nothing about the basic case for AGW requires an "extraordinary" claim. There is no Quantum Mechanical weirdness. No magic or miracle. It doesn't hinge on proving the existence of the Higgs-Boson nor are scientists attributing this to Dark Matter or String Theory. Here's the basic claims required for AGW to be generally correct:
1) The composition of the atmosphere of a planet impacts its dominant climate conditions, most particularly its average temperature.
This is really quite intuitive. It's easily provable by just looking at Venus, which, despite being signficantly further from the Sun than Mercury, has a hotter surface temperature due to the Greenhouse effect of its carbon dioxide rich atmosphere.
2) Changing the composition of a planet's atmosphere will change its climate.
Again, intuitive; give Mercury a CO2 atmosphere and it would be much hotter than Venus. Give Mars an atmosphere as thick as Earth's and it might be warm enough for human habitation.
3) The composition of Earth's atmosphere has changed, with a significant increase in the concentration of certain gases, such as methane and carbon dioxide.
This is a simple matter of measurement, which has been done every way imaginable, and including drilling miles-thick ice cores from the Antarctic icesheet, we have hundreds of thousands of years of atmospheric composition records. In fact, even though this claim is not extraordinary, we have what I would call "extraordinary" proof of it.
4) Human activity is primarily responsible for the increase in measured concentrations of these gases.
Some deniers simply assert without foundation that the planet is simply too big, and the atmosphere too immense for humans to significantly affect it. They call it "arrogance." First, consider that scientists believe prehistoric bacteria were responsible for the ancient shift from a carbon dioxide rich atmosphere to an oxygen rich one (which made animal life possible). We know that algae today are responsible for a great percentage of the oxygen output in the atmosphere. If single celled organisms can impact the atmosphere, why can't 7 billion humans employing giant machines do so?
The most obvious reason for attributing this to humans is the obvious and direct correlation between measured CO2 levels and the rise of industrialization, and the massive increase in burning of carbon intensive fossil fuels. Just as a simple matter of diagnosing the cause of any phenomena, the first thing you look at is "what changed?" when comparing it to the situation before. There is no other known phenomena that could plausible explain the increase in greenhouse gas levels. We checked into the volcano angle, and it's a dead end. Beyond that, we can in fact tell the difference between CO2 that was released by burning fossil fuels, and "natural" CO2 from decaying plant matter or the like, by looking at the levels of certain atomic isotopes in the gas. We're not just guessing or deducing that the additional CO2 in the atmosphere is from us, we've measured. When it comes to methane, it is even easier, since methane doesn't stay "methane" for very long in nature as UV rays break the molecules, so whatever is up there has to have been released recently (and yes, methane released by cows is part of "human activity" since our demand for meat is what mostly controls the number of domestic grazing herd animals in the world).
5) The average global temperature is increasing.
Climate deniers are fond of saying "the climate has always changed, it will always change, nothing new here" as one of their favourite denialisms, which says that even they find the idea that the Earth's average temperature might change over time to be un-exceptional. Once again, this is a simple matter of measurement. Going back using human records, tree rings, select fossils and those ice cores, we have a very long record of Earth's average temperature. Even if one wants to rely solely on direct living human measurement of the past couple centuries, the trend is unmistakable.
6) The greenhouse effect is the primary cause of the observed warming.
So we have a phenomena observed, and it requires an explanation. The Earth is evidently getting warmer, why? There are, in the abstract, a number of possible causes to this. Everything from increased output from the Sun to cosmic rays to changes in Earth's orbit have been proposed, studied and discarded as explanations. The remaining sole plausible explanation is that the change to Earth's insulation system (the atmosphere) has altered the planet's energy absorbtion rate. What is extraordinary about this? Any child who has walked on dark pavement in bare feet understands that different substances get hotter or cooler in the sun. The idea that invisible gases in the air have a similar effect is probably beyond the intuition of most children (and most adults) but it remains real, and something we can empirically demostrate in labs. A transparent vessel filled with CO2 left in the sun will get hotter than one filled with normal air. The physics and chemistry of why this happens are more complex (the propensity of these gases to absorb the reflected solar energy in the form of infrared light the planet gives off which otherwise would get vented into space), but the reality of the phenomenon can be repeatedly demonstrated.
In fact, given that we can observe the increase in CO2 levels in the atmosphere, and understand this "greenhouse effect" thing, the question could never be "Are greenhouse gases warming the planet?" but only "how much are they doing so?" This idea is so obvious to scientists, that it was proposed back in 1896. Even then people who understood the relevant physics could look at our already prodigious carbon output, and ask what the eventual impact would be on the global temperature.
In summary, when you accept that a planet's climate is strongly correlated to the composition of its atmosphere, that Earth's atmosphere has been rapidly increasing in CO2 and MH4 levels, that Earth's average surface temperature has increased, that the rise in CO2 and MH4 levels are the only plausible explanation for the increased temperature, and that humans are the only plausible source of the increased levels of these gasses, then you have AGW. We can still debate particulars about how much warming how fast from a given amount of CO2, the impact on polar ice and glaciers, the effects on hurricanes, droughts and floods, but the basic premise is there, it flows logically from a series of very provable and reasonable claims and which hold up well under a very intensive search for evidence about them.
It is fair to demand people making factual claims provide suitable evidence, but it ceases to be fair when you set impossible standards of evidence and hide behind ill defined and shifting objections that are not internally consistent to justify refusing to accept the proposition. This is what deniers do, and it's why I call them "deniers" and not "sceptics."