Agreed, CO2 and H2O are not toxic -- you'll gladly sit in a chamber consisting of a pure atmosphere of both, right? 
So in your mind drowning, asphyxiation and poisoning are all synonymous?
The dose makes the poison; so it is with biology, so too the atmosphere.
Tim
This is nonsensical. I'd have expected a more intelligent statement from you. Without CO2 & H20 plants die. Without water people die. Since we produce CO2 we are able to tolerate it to some degree, but our bodies are very sensitive to excessive levels.
The atmosphere has become highly toxic many times in the past in the vicinity of major volcanic events. Not that it matters much in the case of a caldera collapse as everything will already be dead from the white hot rock fragments raining down in the area affected by the release of H2S. At greater distances where the ash has had time to cool will kill everything out to a range of 1000 miles or more. The ash kills the plants by blocking the sunlight; the herbivores die for lack of food followed shortly after by the carnivores.
The problem is not just limited to the area of the ash fall. So much superfine ash is injected into the stratosphere that large expanses of the planet are covered with snow year around. But after a few years the ash eventually settles out and it warms up. The eruption of Mt. Tambora in 1815 resulted in what is referred to as "The Year Without a Summer" in 1816. Carl Sagan's nuclear winter was based on that. Mt. Tambora was big, but by no means the largest such event.
The releases of H2S accompanying the great basalt floods such as the Deccan traps would have been much larger than releases related to a caldera collapse. There is a lot of H2s associated with a million cubic kilometer volcanic eruption and it is quite toxic if you are downwind of it.
All of human history is the blink of an eye in geological terms. And the death toll in mass extinction events is truly astonishing, almost everything big enough to die does. Many years of effort have gone into trying to understand what happened. The earth is a *very* violent place. Organisms either adapt to the change or they die. The list of extinct organisms is *much* longer than the list of living organisms and vast numbers of organisms leave no fossil record other than some burrowing marks in the sediment.
The melting of the polar ice caps over the last 15,000 years raised sea level by about 600 ft. That is an *average* sea level rise of 1/2" every year for 15,000 years. We are now at about average historical sea level high stand. Sea level has been around 100 ft higher at times. So given that our reference is mean global high stand this rise might continue at about 1/2" per year for a while longer.
All of that water came from melting huge sheets of ice which had accumulated. That melting was the result of thousands of years of global warming which made it possible for the the small number of humans to move to higher latitudes. The Gulf stream brought a great deal of heat energy to Europe and making it especially hospitable.
There certainly is coastal flooding caused by humans. But it's been caused by pumping oil and water out of the ground. Venice is particularly affected by ground water pumping.
While sea level has been over 1000 ft below current MSL, it's unclear how much of that is the result of the addition of surface water from the release of H2O during volcanic eruptions and how much is the result of the continental plates moving around.
In all of recorded history there have only been a few large geological events and *no* major events. All of this was fairly well known long before "paleoclimate studies" got rebranded as "climatology".
All of this is well documented. The available data is freely accessible and various details hotly debated in the geological community in open discussions in which anyone who has an informed opinion may participate. It *is* science, so you *are* expected to know the facts and be able to justify your opinion with a logical argument which accounts for *all* the known facts. But there is no secret data and no one is going around proposing that those who disagree be put in prison.
So I suggest a bit of critical thinking might be in order. I have not been significantly involved in geology for 35 years, so this is just what I remember from school. I got into reflection seismology and got entranced by all the digital signal processing involved. Any good PhD level geologist, which I am not, my geology studies ended with my MS, can cite far more issues that Mann et al ignore or misrepresent.