In physics, force can be defined as any external agent that causes a change in the motion of a free body, or that causes stress in a fixed body (Newton's First Law of Motion). In addition to the classic example of a "body," force can also be applied to any simple or complex system. Take for example, the way a heating and air conditioning system regulates the temperature in a building—as long as the temperature remains constant, neither heating or cooling is needed. If any force is applied to this system (e.g. a change in sunlight, wind, or air temperature), the system heats or cools the building until equilibrium is re-established (more on this in a moment). The outside force is sometimes called a forcing.
In the context of climate science, one of the most often-referred to forcings is radiative forcing, is a measure of how internal or external factors affect climate. Internal forcing is part of the natural chaos of the climate system, for example ENSO (El Niño). External forcing may be natural (e.g. volcanic eruptions or solar fluctuations) or anthropogenic (e.g. increasing greenhouse gases or aerosols). External forcing can change the Earth's energy balance, and hence its climate patterns.
When the average condition of a system is relatively constant over time it is said to be stable or in equilibrium. This reflects the system's ability to react to both internal and external forces. The ability to self-regulate is a characteristic of stable systems that are usually controlled by internal adjustments known as feedback mechanisms. There are two general types of feedback, positive and negative. As shown below, positive and negative in this context have nothing to do with the desirability of the outcome—a positive feedback loop can have disastrous results!
Negative feedback tends to minimize the effect of a disturbance and return to the normal state. For example, you might hypothesize that global warming leads to increased evaporation and therefore more cloud cover. The clouds would reflect more incoming solar radiation, which would in turn result in cooling and reduce the temperature rise.
Positive feedback, on the other hand, increases the effect of the disturbance and can destabilize a system in such a way that returning to the initial state is almost impossible. Returning to our example, you might hypothesize that global warming would result in increased evaporation and therefore an increase in the amount of water vapor in the atmosphere. Since water vapor is a greenhouse gas, this might trap outgoing terrestrial radiation, which would of course cause further warming.
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