You "believe" stuff when there is no verifiable proof. I believe in God. I cannot prove he exists, if you are a sceptic you won't believe.
People claim they believe in man-made global warming/climate change. They cannot prove it exists, but they believe fervently.
I suppose I am a climate agnostic. I used to own a book that used weather, or climate if you prefer, as a means of predicting future events. The author used past weather to map human events and offered a view of the future. Not surprisingly, I picked up the tome for either free or next to nothing at a library book sale. The whole theory is crackpot. You have to love the 1970s.
Weather models fail to forecast with much accuracy 24, heck 12 hours, in advance of a storm. Yet we are supposed to change our way of life, our economy, our behavior based on weather models for ten or twenty years from now? Those same models, ten years ago, predicted a much different reality. In fact they were dead wrong. Too bad I lost my free book, it may have been more accurate.