Correlation does not always imply causation. We live in a world with imperfect information and each of us has to decide the best course of action based on the evidence available. The phrase “global warming” is just a shorthand way of saying that we are adding massive amounts of the energy to the system controlling weather. When we see weather changing whilst knowing that we’ve added a massive amount of energy to the system then a reasonable person concludes causation. It’s the most likely explanation.