Chaos and Climate
One final comment on internal influences relates to the concept of chaos. We will see in a later unit that the climate system can be described by a set of time-dependent, coupled, nonlinear, first-order, partial differential equations. Professor Ed Lorenz, meteorologist at MIT, discovered some thirty years ago that systems described by such a set of equations have the property of being "almost intransitive".
"There are extremely simple and also very complicated systems of equations possessing solutions which behave in one manner for an extended period of time, and then change more or less abruptly to another mode of behavior for an equally long time. Such systems have been described as almost intransitive." (Lorenz, 1970)
Solutions to these equations give a set of weather conditions that vary over a confined range of values for an extended period of time and then more or less abruptly change to some other confined range for another (but likely different) extended period. A simple example of this is the dripping water faucet which drips at a steady rate for a period of time, then suddenly, for no apparent reason, drips very rapidly for a short period and then reverts to a slower (but different from the original) rate. The theory describing these systems governed by time-dependent, nonlinear differential equations is called "chaos theory".
The climate system is thought to possibly have such multiple states; that is, the climate (or some components of the climate system) could be stable for a period of time and then abruptly, and for no overtly evident reason, change to another stable regime. There is evidence that certain components of the climate system have done this. The circulation in the north Atlantic Ocean is believed to have gone through an abrupt change in which the Gulf Stream, instead of tracking northeastward off the East Coast of the United States and heading toward Scandinavia, at one time switched very abruptly over about fifty years (that's abrupt on geological time scales) to an easterly direction toward the Mediterranean Sea. This caused an abrupt cooling of the climate in Scandinavia.
In summary, almost-intransitivity is an inherent characteristic of the dynamics of the climate system that may, for seemingly small or unknown reasons, launch some component of the climate system into a pattern not previously seen.