the fact that that rate of change is faster than organisms and systems can adequately adapt to.
And even this assumes that there is some sort of linear progression towards a semi-stable state instead of churning, unpredictable "seasons" for tens of thousands of years before the climate settles into something.
And even this assumes that there is some sort of linear progression towards a semi-stable state instead of churning, unpredictable "seasons" for tens of thousands of years before the climate settles into something.