The narrative around climate change, i.e., that it's happening and that we need to do something about it, is not demoralizing. What's demoralizing is that the people with the power, money, and influence to take the biggest steps refuse to do so. Someone has figured that the only way to get them to care is to badger them with the facts until they do, and a lot of us are okay with that. It's preferable to the alternative, which is ecoterrorism. Trying to sweep the situation under the rug while things become more troubled and people become more desperate just makes the latter more likely. In this case, the opposite of mopey and depressed isn't optimism, it's rage.