First of all, do you believe that "But as with the problem of global warming and its known solutions, what we lack is the will to change things" is incorrect?

I am not very good at estimating probabilities, but I would guess: 99% there is a global warming; 95% the human contribution is very significant; 95% in a rational world we could reduce the human contribution, though not necessarily to zero.

Parallel universes requires a long meta explanation before people can even grasp your point, and, more damningly, they are rejected by experts in the field.

Climate change also requires some investigation. As an example, I have never studied anything remotely similar to climatology, and I have no idea who the experts in the field are. (I could do this, but I have limited time and different priorities.) People are giving me all kinds of data, many of them falsified, and I don't have a background knowledge to tell the difference. So basicly in my situation, all I have is hearsay, and it's just my decision whom to trust. (Unless I want to ignore my other priorities and invest a lot of time in this topic, which has no practical relevance to my everyday life.)

Despite all this, during years I have done some intuitive version of probabilistic reasoning; I have unconsciously noticed that some things correlate with other things (for example: people who are wrong when discussing one topic have somewhat higher probability to be wrong when discussing other topic, some styles of discussion are somewhat more probably used by people who are wrong, etc.), so gradually my model of the world started strongly suggesting that "there is a global warming" is a true statement. Yet, it is all very indirect reasoning on my part -- so I can understand how a person, just as ignorant about this topic as me, could with some probability come to a different conclusion.

But if someone rejects the global warming consensus, then they are being irrational, and this should be proclaimed, again and again.

No one is perfectly rational, right? People make all kinds of transgressions against rationality, and "rejecting the global warming consensus" seems to me like a minor one, compared with alternatives. Such person could still be in the top 1 percentile of rationality, mostly because humans generally are not very rational.

Anyway, the choice (at least as I see it) is not between "speak about global warming" or "not speak about global warming", but between "speak about global warming in a separate article, with arguments and references" and "drop the mention in unrelated places, as applause lights". Some people consider this approach bad even when it is about theism, which in my opinion is a hundred times larger transgression against rationality.

Writing about global warming is a good thing to do, and it belongs on LW, and avoiding it would be bad. It just should be done in a way that emphasises that we speak about rational conclusions, and not only promote our group-think. Because it is a topic where most people promote some group-think, so when this topic is introduced, there is a high prior probability that is was introduced for bad reasons.