According to the documentary Cowspiracy (which was very good for those who haven't seen it), animal agriculture is the leading emitter of greenhouse gases. It seems that after that documentary came out, this reality has been more acknowledged, whereas before it was relatively unknown to the general public. At least that's my sense. I believe this is also addressed in the TV series Years of Living Dangerously, another title I'd recommend.

Should people change their diets? Good question. If it could be determined with certainty that the human race will become extinct if every person did not change, then I think they should, because they would be directly responsible for our extinction. But as that is not the case and there are too many uncertainties, it does not seem necessary (in the absolute sense), but still advisable, in my opinion. We should all do our part to reduce our carbon footprint and negative environmental impact as we are "running over budget" so to speak. Changing diet should be part of that plan. To all the meat lovers that can't give it up, a reduction is still better than nothing.