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 the planet’s temperature eventually returned to the more moderate level that we have today. Thus, despite disturbances toward cooler temperatures, our planet’s temperature has been in a “stable equilibrium.”
Global warming, however, represents a new challenge to our planet’s temperature-regulating system. As our planet warms, glaciers (both on mountains and on the polar ice caps) melt. As they melt, they shrink, exposing more of the earth’s dark surface. As more of the earth’s dark surface is exposed, the planet absorbs more of the sun’s energy (because white glaciers reflected much of the sun’s energy, whereas the dark ground absorbs more of the sun’s energy). As the planet absorbs more of the sun’s energy, it warms up…which, in turn, melts more glaciers…which, in turn, raises the temperature…which in turn melts more glaciers….
Here, of course, is the problem: although our planet’s temperature-regulating system has shown stable equilibrium when the temperature turns colder, scientists don’t have any information about whether our planet’s temperature-regulating system will display stable or unstable equilibrium when the temperature turns warmer. For all we know, our planet’s temperature might behave like the marble on the upside-down salad bowl: forever running away from its current level. The result: temperatures that go higher and higher until all the glacers are melted, at which point the planet might establish a new equilibrium at a temperature much hotter than we have today. And what might this cause? We’ve already seen some of the end results: increased coastal flooding, increased hurricane intensity, decreased inland rainfall (drought), decreased winter snowfall in the Rocky Mountains (a precursor to drought)…the list goes on. Significantly, such a hotter global temperature might be a very stable equilibrium, such that even a total halt in the burning of fossil fuel might be insufficient to return the planet to its current temperature.
This is why scientists (and many others) are so concerned about global warming. It isn’t just because of the small temperature rise they see today. It’s because they simply don’t know if the planet’s temperature will adjust itself back to where it was before we began burning fossil fuel (stable equilibrium), or if the temperature will soar to a point of no return (unstable equilibrium).