There are studies which point in the direction of a true climate catastrophe, possibly involving a runaway greenhouse effect. This would be due to positive feedbacks involving the release of methane currently sequestered in permafrost and clathrate in the continental shelves in Arctic areas. There is an enormous amount of methane present which has supposedly been associated with events like the Paleocene–Eocene Thermal Maximum
How seriously should we take this particular threat of true catastrophe, as opposed to climate change as we have come to understand it as mostly a question of CO2 emissions.
Are they not related? That is, it is the (so far) gradual raising of global temperature, for which CO2 emissions bear some responsibility, that is making it possible for the permafrost to melt and release all that methane, as I understand it. So that scenario is just that which comes after the serious if not catastrophic rise of sea levels due to the melting of glaciers and polar ice, if nothing is (or can be) done to stop it.
So the answer to your question is, very seriously, but not “as opposed to climate change” but “as a consequence of climate change.”
I am put in mind of the Reaganites who liked to ridicule climate change by claiming bovine flatulence contributed to methane in the atmosphere…blaming the cows for global warming.
Have you ever heard of SimEarth? You could use sliders and manipulate the chemical composition of the atmosphere. Highly oxygenated air detonates. Surprise! Forest fires on a global scale! Heavy carbon infusion in the upper atmosphere blocking sunlight. Ice age ensues! It was really a difficult thing to balance.
Methane trapped in vegetation trapped in ice released by rising mean temperatures. That’s always been present as a potential hazard. That we would artificially alter the mean temperature of the planet making its release imminent? When I was a kid, preposterous! Now? What’s the holdup?
As far as I recall it’s always been a part of the climate change discussion: that it could lead to catastrophic runaway effects.
However, that’s when we’re discussing the consequences of climate change.
If instead we are trying to educate and persuade people who are ignorant of the science, and do not accept anthropogenic climate change as a thing, it’s probably best not to bring up “heres an aspect where there is far less certainty”. Better to start with why we’re so sure about AGW.
Most people, even conservatives, can accept that humans are capable of radically and adversely transforming the environment on a local scale. One approach is to address the immediate, visible economic harm caused by human activity. Take commercial fishing, for example. Fishermen are business people who probably have a conservative outlook, but they are seeing firsthand how man-made activity is harming their way of life, and it goes beyond fertilizers and oil being dumped in the sea. Attempts to persuade skeptics ought to start here rather than predicting which glaciers are going to melt, and when.