Forcings and Feedback Loops

Forcings

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.

Feedback Loops

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 disasterous 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.

negative feedback loop

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.  

positive feedback

 

 

Last modified September 25, 2008 by Dennis Ward.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Content for Climate Change Education Courses

Looking for online content that can be used for a climate change education course or module? Pages linked below can be used to support an introductory climate change education for either a unit or a full...more

World Leaders Developing a New Plan to Help Earth’s Changing Climate

Leaders from 192 nations of the world are trying to make an agreement about how to limit emissions of heat-trapping greenhouse gases, mitigate climate change, and adapt to changing environmental conditions....more

What is Climate?

Climate in your place on the globe is called regional climate. It is the average weather pattern in a place over more than thirty years, including the variations in seasons. To describe the regional climate...more

Earth's Greenhouse Gases

Less than 1% of the gases in Earth's atmosphere are called greenhouse gases. Even though they are not very abundant, these greenhouse gases have a major effect. Carbon dioxide (CO2), water vapor (H2O),...more

Space Missions to study Earth's Atmosphere & Climate

Television weather forecasts in the space age routinely feature satellite views of cloud cover. Cameras and other instruments on spacecraft provide many types of valuable data about Earth's atmosphere...more

Modeling the Future of Climate Change

Predicting how our climate will change in the next century or beyond requires tools for assessing how planet responds to change. Global climate models, which are run on some of the world's fastest supercomputers,...more

Effects of Climate Change Today

The world's surface air temperature increased an average of 0.6° Celsius (1.1°F) during the last century according to the Intergovernmental Panel on Climate Change (IPCC). This may not sound like very...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA