Let’s do a thought experiment.  Imagine there is a salad bowl sitting (upright) on your kitchen table.  Imagine, also, there is a marble resting in the bottom of the salad bowl.  If you slightly disturb the marble with your finger, the marble will roll around the bottom of the bowl.  If you disturb the marble a bit more, the marble will roll up the side of the bowl and then roll back down to the bottom.  In this situation, the marble is said to be in a “stable equilibrium,” because the marble remains inside the bowl (equilibrium) despite reasonable-sized disturbances.

Now, imagine removing the marble from the bowl, turning the bowl upside-down, and resting the marble on the flat base of the bowl.  Although the marble will remain within the boundary of the flat base (equilibrium), even a relatively small disturbance will roll the marble off the base, down the side of the bowl, across the kitchen table and onto the floor.  In this situation, the marble is said to be in “unstable equilibrium,” because of the tendency of the marble to roll (far) out of position with even a small disturbance.  Once on the floor, the marble is again in equilibrium:  it will stay on the floor unless some force lifts it back to the tabletop.

Now, let us consider global warming.  For tens of thousands of years (and perhaps much longer), our planet has maintained roughly the same average temperature.  Yes, there were a few ice ages, but …