Saying global warming might lead to disaster is using trends to create a scenario or at least prediction. We measure the temperatures over many years and the UN makes its models.I actually disagree with this. I mean, obviously I agree with the consensus of climate scientists that carbon pollution is catastrophic and non-negligibly anthropogenic and that to the extent that our politics is not shaped by the end of creating a sustainable civilization and renewable infrastructure we are indulging in a form of genocidal, suicidal madness -- but I disagree with this analogy as a justification for the "trend" as a legitimate analytic object or useful methodological recourse. As I said before, there actually is no such thing as an historically agentic or otherwise forceful "trend." "Trends" are retroactive narrative constructions at their best, but usually their retroactivity is falsely projected as if from the vantage of a non-existing superior height (fashion trends announced from fashion authorities) or from the future (which is inhabited by no one at all) in which case they are always prescriptions masquerading as descriptions.
But I believe the climate model is a false analogy on which contemporary futurology especially depends to pretend it is a legitimate quasi-scientific methodology rather than a rather derivative clumsy kind of science fiction literature conjoined to hyperbolic marketing forms even more than usually prone to deception and self-deception. I should add that futurologically-inflected environmentalisms of the geo-engineering, "Bright" Green, technofixated elite Design sorts (despite some earnest adherents) mostly amount to reactionary greenwashing for apocalypse profiteers.
Anyway, given the complexity of ecosystems -- and their complex interactions from idiosyncratic local to planetary scales -- climate science provides a good fudge factor for futurologists to exploit in this way. It isn't accidental that climate change denialists are able to undermine scientific consensus in the field by displacing the debate onto a culture war terrain. Nor is it accidental that the scale of interventions futurologists pretend feasibly to propose in their geo-engineering yackety-yack would be less predictable in their actual effects -- apart from the obvious profits that would accrue to the polluting plutocrats for whom these proposals are actually made -- than the state of the weather already is.
Every legibly constituted discipline produces models of phenomena, every legibly constituted discipline has a foresight dimension. This is because knowing better how phenomena behave under various conditions facilitates more practically useful interactions with them, and leads us to form expectations and make plans accordingly. But "trends" are narratives more than models, strictly speaking, and it is not scientists but English lit majors and PR muckety-mucks who can explain how they operate: they solicit identification the better to peddle forms of consumption.
Futurological scenarios inevitably circumvent historically situated social, cultural, and political dynamisms while purporting to model these dynamisms in relation to physical phenomena. Scenario spinning superficially skims the objects of a host of disciplines without the least mastery or often even grasp of the specificities of those disciplines -- it is an anti-disciplinarian pretense of inter-disciplinarity (a very slippery but indispensable academic aim futurology isn't remotely fit for). I don't disagree that anything can be "trendified" and that you can "pick your trend" and then spit out talk that is legible for others who indulge this nonsense at Davos and TED and in Brockman's reductive bestseller salons, but that is far from saying that it makes the least sense to make this methodological move if one wants to actually understand the world or facilitate sustainable equitable outcomes rather than ride a lucrative gravy train as a guru wannabe.
And, no, scenario spinning doesn't become better or more useful (except of course as a sales pitch) if you entertain and invest in four fantastically impoverished alternative "futures" rather than one. This is the conjuration of a phony complexity, the circumscription of political possibility into a handful of choices on a menu provided by already established actors. Some of these difficulties beset all forms of social science, especially to the extent that they try to shoehorn their own scientificity into an image of engineering. But in futurology they constitute a kind of crisis, and a crisis akin to the crisis of neoliberal for which futurology is, after all, the quintessential discourse.
There are usually better ways of interesting people in a topic that actually affects them than scenario spinning, but I do agree with you that extended metaphors, little thought experiments, and anecdotal sketches have their place in mobilizing affect and concretizing abstractions. I teach rhetoric, after all. But I know better than to pretend rhetoric is a way to grasp the substance and stakes of research or policy outcomes -- rather than more effectively to communicate those stakes and change conduct to facilitate ends once they have been determined by other, better means.