The reading is the kun'yomi reading, but it's pretty easy. How do you read all the other "to climb" words? 登る is one example. 上る is another. They're all のぼる, and so is this. Unlike the other climbing words, though, this suggests there's no effort involved. That's why you're "ascending" and not climbing, because climbing is a bit of work.
I felt like I was ascending to heaven as I read my college acceptance letter over and over.