I have some wetlands only a couple thousand yards from me. They are continually in a state of change (as is the whole planet). Some years they are dry as a bone, others they are a source of flooding and all states in between. It is a great place to catch glimpses of herons now and then.
So these folks, like Mann and Jones, try to isolate a small period of time and extrapolate that into some 'modeled' prediction.
Haw, Mother Nature passes by in her SUV, eases the throttle a bit as she passes then laughes her ass off.