The external forces acting on the car are gravity and friction. Willis didn't mention those forces. The cruise control can kick in to oppose changes in these forces by introducing an opposing force of the engine. It can add fuel to increase or decrease the engine power, apply brakes to oppose the engine, and shift gears to increase or decrease the power of the engine. It will do this when it detects a change in speed. The change in speed trigger will only come about when there is a change in forcing.
Lets assume that his car is moving forward in a straight line (same horizontal direction). When one or other force changes it will affect the speed. Gravity won't affect the car on the flat, but it will kick in when the car goes up or down a hill. (Gravity will no longer be perpendicular to the velocity of the car.) Adding gravity to the mix affects the speed of the vehicle. The cruise control detects this result of a change in forcing. It detects a change in the speed. In response to the detected change in speed, the cruise control increases or decreases the supply of fuel, and/or applies the brakes and/or changes gears. It can apply an opposing force. It can change the force of the engine.
Under normal circumstances, the Earth doesn't have a fuel tank at its control, despite what Willis might claim. Its supply of fuel is the sun, but that's not controlled by Earth.
However, we humans are very inventive. We've discovered a fuel tank right here on our planet. It's not controlled by natural forces, it's controlled by us. It's called fossil fuel, or coal, or oil, or natural gas. In one form it is the very same fuel as is used in Willis' motor car.
Using Willis Eschenbach's example of a car - think of how our earth has been going along a reasonably flat straight road at a fairly steady pace. Since civilisation began, there've been no big hills - maybe a few gentle undulations now and again. The sun provides pretty well the same amount of energy to keep us on an even keel. Our orbit hasn't caused big bumps. There've been some volcanoes, but nothing drastic. No comet collisions or asteroid strikes.
We're like adolescents who are bored and looking for thrills. So we decide to open up the throttle and inject more fuel. We've added a force in one direction, with virtually nothing to oppose it. We've done that by burning fossil fuels and increasing the amount of greenhouse gases in the atmosphere. This is like stepping on the accelerator while on this flat straight road. The car accelerates. The planet gets hotter. We keep our foot pressed to the floor and the car keeps accelerating, going faster and faster. We keep burning fossil fuel and adding more greenhouse gases, and the earth gets hotter and hotter.
There is no governor to kick in. There is no opposing force. All there is is feedback. We've taken over from the cruise control and put our foot to the floor.
Related reading from HotWhopper
- On forcing and feedback with Willis Eschenbach - July 2015
- Wondering Willis Eschenbach's Thunderstorm at WUWT - October 2013
- Denier Weirdness: Wondering Willis Eschenbach wonders does "an ice age cometh"... - October 2013
- Wondering Willis' Original Weather Hypotheses - December 2013