Both. I do believe that "communism will win" as an inevitability (with one big caveat, see below). Capitalism obviously is unsustainable and rife with internal contradictions that can only lead to its eventual demise. The obvious and broad example being that it requires infinite growth on a finite planet. But I think it can get very bad before it gets better, and expect it will further devolve into fascism (much more so than it already has) for most if not all of the western world, and the entire world will suffer as a result. Socialism, then communism will eventually emerge (since fascism is just as doomed by its contradictions as capitalism is), but before we get there, I expect there is going to be some truly unimaginably dark and horrible times on the way there. So in that sense, I am ultimately optimistic about the future of the world, but extremely pessimistic about its more immediate future.
But now for the caveat. I think that most people, even leftists, don't fully appreciate how much climate change is going to reshape the world. There is a real chance that it will get bad enough that civilization may not survive, that humanity as a species will be among the many that don't make it through the mass extinction we've only just entered. Even people fully on board with knowing climate change is bad and must be curtailed as much as possible as soon as possible still mostly don't realize how much a genuine existential threat it is on a planetary scale, on a scale of centuries and longer. It is by no means a certainty, but given the feedback loops we don't fully understand and definitely don't know how to interrupt, there is a possibility of Earth even going the way of Venus. Obviously I hope that's not the case, but it would be a mistake not to recognize the extreme potential of climate change. If we are able to mitigate it in time, I am like I said, ultimately optimistic. But I am beyond afraid that we won't be able to mitigate it in time.
In other words, it's not just "socialism or barbarism," it's socialism or annihilation.
Well... Earth becoming more like Venus is an inevitability in the high hundreds of millions of years (and for scale, multicellular life has been around for roughly 5-600M years, with the more than 3 billion before that just being simple single-celled prokaryotic life), but that is completely independent of anthropogenic climate change, it is because of the expansion of the sun towards it's red giant phase. In terms of being habitable to life, Earth is easily past the half-way mark already, no matter what. However, that is far enough out that it doesn't bear worrying about and isn't something we can have any sort of impact on.
That said, the climate change that we as a species are causing right now could lead to a runaway greenhouse effect on much shorter time scales. The fact is, there have been times in Earth's history where it has been so hot that complex life could mostly only survive at the poles (with the equator being a death zone to all but simple, single-cellular extremophiles) and there have been times where Earth was encased almost entirely in ice except perhaps at the equators - not just our usual conception of an ice age, but "snowball earth," and this was likely caused by certain forms of simple life, fascinatingly enough. The feedback loops we are triggering right now have a potential to drastically change the composition of the atmosphere on a far shorter timescale, one in which we are talking about an end to most complex life (obviously ourselves included). It was almost certainly volcanism that caused Venus to go from a mostly habitable planet to the completely, utterly inhospitable world it is. Volcanism has also been responsible for extreme heat and mass extinctions on Earth, but obviously it never tipped over into Venus-like territory. The thing is, right now we're changing the atmosphere at a rate far faster than volcanism has in the past! And rate of change matters a lot with this kind of thing. I'm repeating myself, but again, it is not a certainty but it is a possibility that anthropogenic climate change could hit tipping points that Venus-ifies Earth on a much shorter, nearer term than anything relating to the expansion of the sun, on time scales that are worth worrying about (if we value humanity as a whole), and is the sort of thing we can have an impact on.
Then you need to do more reading because I did work in this field and have read the science on it as well. First all, you have to take into account what time scales are being discussed. What you're reading is, I'm all but certain, just talking about the coming few decades, in which yes, millions at least will likely die. And even then the science that tends to reach the public is toned down, pacified, and doesn't represent the whole truth. You should be familiar with this as a communist trying to get an understanding of what's really going on with the world via popular journalism. Is what you're reading about "catastrophic weather events" also discussing what will be happening 1000 years from now? 10,000? Despite the longer scale, what we are doing right now and in the coming decades will have an effect on those longer scales. Climate change is so much more than simply an intensification of weather events. It is literally a rapid change to the composition of our atmosphere. An atmosphere which has, by the way, been completely altered by life in one of the most chemically fundamental ways possible, from a reducing atmosphere to an oxidizing one. This is what I mean when I say even many leftists just do not understand how extreme the risks are here. A runaway greenhouse wouldn't just kill a billion, it could well end our species and most other species of "higher lifeforms."