How can advocates of AGW rule out natural causes if they don't know with certainty what caused past warmings

To my knowledge no one knows for sure why the climate has warmed or cooled in the past, yet they are able to say matter factly that all natural variables have been ruled out. How can they be ruled out if no one knows exactly what those variables are. Causes of Past warmings are at best speculative, just as I believe this one is as well.