Cataclysmic event? An incoming asteroid or the Yellowstone super volcano erupting - both cataclysmic I suppose with a finite chance of occurring. Both of these would probably have a chilling effect over the short term (couple of hundred thousand years).
Runaway warming following some massive release (from somewhere) of a greenhouse gas - possible one supposes but it does not look as if anything like that has happened during the last few million years.
The most likely medium term scenario based upon the rather limited historical climate data we have is that we will slowly move into another ice age. Now that will be cataclysmic for folks living in the Northern temperate zones where most of the current world economic activity takes place.
Cataclysmic is pretty unlikely. Do you think there is 1%+ chance of some major coastal cities becoming uninhabitable or requiring significant engineering to hold back the rising sea?
Interesting question and I suspect difficult to get a sensible answer to. I am assuming that you expect the 20th Century warming period to continue to raise Northern hemisphere temperatures to the levels experienced around 1,000 years ago. Certainly we know that Greenland had a lot less ice then - although we do not know where that water was. I am not sure if we have any information from Northern Canada or Asia. Regretfully I have not heard about any useful data from the South. Was the sea level higher a thousand years ago? Even extrapolating from coastal site evidence might not help as many land masses were still (and are still) rising gently following the last ice age.
There is not a lot of evidence that sea levels are rising substantially at the moment - you had best ask an oceanographer - I would stick with the science rather than ask a "climatologist".
What percent chance do you think there is of cataclysmic global warming, human caused or otherwise? I can't pick an exact number, but it is certainly above 0+ and it seems like a bad enough scenario that it is worth doing something to try and stop it if we can. Note that I don't think this type of analysis is very meaningful quantitatively, but it can reveal why people think the way they do. I really don't understand the contrarian perspective on global warming and I would like to hear how you justify your probabilities.
Well, one of the issues is magnitude: do we get, naturally or unnaturally, temperatures back to the level of the Medieval Warm Period or do we get a "cataclysm" (or do we get the next Ice Age, we are overdue one, and the same general group of people and some of the same people were warning of that before the '80s).
"Worth doing something" is nice and fine, but that "something" really needs to be connected to the probability of the risk. The extreme warmists want us to essentially spend all the world's disposable wealth on something that has at best a very shaky scientific foundation (cataclysm). It would be a real pity if we did that and it turned out we were entering the next Ice Age instead, or needed the money for some other greater threat instead (e.g. one of the ones mentioned in this discussion, like a megacaldera cutting lose). A maxim of warfare is that the side that tends to win in a battle is the one that is the last to commit its reserves. We really need to keep some reserves.
In the HN discussion in which you posted a pointer to this discussion I referred to Bjørn Lomborg, "The Skeptical Environmentalist". Back before the release of the CRUtape Letters he believed moderate AGW was happening, but he advocates moderate local expenditures to deal with that (moderate at least when compared to the "take over the world's economy" proposals of the extreme warmists), and that we spend some serious money on targeted interventions that will clearly help a lot of people (e.g. as I vaguely recall one involves addressing a common vitamin deficiency).
Unless a threat is truly cataclysmic it's a mistake to focus on it to the exclusion of all else, and the extreme warmists are just one example of an advocacy group that thinks they've found the one problem we're facing.
In Global Catastrophic Risks [1], many of the risks considered, including unfriendly AI and nuclear war, are argued to have a higher probability of causing catastrophe than climate change.
Runaway warming following some massive release (from somewhere) of a greenhouse gas - possible one supposes but it does not look as if anything like that has happened during the last few million years.
The most likely medium term scenario based upon the rather limited historical climate data we have is that we will slowly move into another ice age. Now that will be cataclysmic for folks living in the Northern temperate zones where most of the current world economic activity takes place.