Climate change is happening because the earth is warming at an accelerated rate, a significant portion of that acceleration is due to human activity, and not taking measures to mitigate it will have disastrous consequences for life as we know it.
In other words: Earth is heating up, it's kinda our fault, and if we don't fix it, we're screwed.
This is the consensus of the vast majority of the world's scientists who study such things for a living. Case closed. End of story.
How do we know this to be true? Because pretty much every reputable scientific organization on the planet has examined and endorsed these conclusions. Thousands of climate studies have been done, and multiple peer-reviewed studies have been done on those studies, showing that somewhere between 84 and 97 percent of active climate science experts support these conclusions. In fact, the majority of those studies put the consensus well above 90%.