I would venture to guess that most people are pessimistic because of the extreme hyperbole that finds its way in the headlines or even into Politicians mouths. It is extremely common for the media or Politicians to throw out crazy things like "If we don't enact this, our planet isn't going to be here in 20 years" or "If we don't make this change now, in 15 years, all of the East Coast of the United States will be under water. Every time those things don't come true, it makes rational people more and more skeptical of what is behind that kind of talk, given that it is very rarely rooted in hard core scientific studies. When you peel back the layers of the onion, it's usually one of two things. An agenda driven by money (getting some financial benefit from the legislation that they are proposing) or a passion for change that is a little misguided.
If people were a little more honest about things up front, it would go a long way. For instance, do carbon emissions have an impact on our environment? Yes. Do they have the impact that most of these people would like us to believe they are having? No. Now, that doesn't mean that we shouldn't do the responsible thing and slowly pivot towards things that have less of an impact on our environment, but it doesn't mean that we need to do it immediately or half of the world is going to die tomorrow.
