Scientists can’t say for sure that humans are the cause of global climate change. They’re still a little uncertain, and that’s a good thing. It means the science is working the way it should.
A well-educated friend of mine, a climate-change skeptic, once told me that he didn’t believe anything coming out of the big computer models that scientists use to reason about the complex nonlinear feedbacks driving the Earth’s climate system. He has a point: Researchers are doing the best they can in the midst of great complication and uncertainty.