I still believe you are missing the point, Dave - the RATE of change is the all-important factor. There is evidence for occasional geologically rapid atmosphere/sea chemistry changes in the fossil record - the Siberian Traps being a classic - where the whole system appears to have gone out of balance worldwide.The anthro GHG change is going at breakneck speed.
If I may offer an analogy. Climbers attempting peaks like Everest make sure that they spend a week or more on a slow approach march to Base Camp at about 17,500 feet ASL. This allows their systems to become acclimatised. However, there has been more than one occasion where e.g. journalists have helicoptered straight in and the sudden change to rarefied air has led to them becoming seriously ill with oedema - cerebral and/or pulmonary. A rapid change in environmental properties is, I suggest, liable to risk similar deteriorations.
Cheers - John