1.3 What are positive and negative feedbacks?

In a system, things interact. Sometimes they interact to reinforce a given effect – a positive feedback; sometimes they interact to dampen an effect, holding the system in check – a negative feedback. A couple of examples are given below.

One example of a positive feedback in the current climate system is that increasing CO₂ from human activities warms the atmosphere. After some time the surface ocean temperatures begin to rise (this takes a long time because it takes a lot more energy to heat a given amount of water than it does for the same amount of land or air). The combined effect of warming atmosphere and warming surface ocean waters leads to melting of polar ice. When the ice melts it exposes either land or sea. Ice is much more reflective than either land or sea, so reducing the extent of ice means that more of the Sun's radiation is absorbed by the Earth. This leads to more warming, so more melting of ice, less reflectivity and more absorption of solar radiation, more warming and so on.

A warmer atmosphere warms the surface ocean waters. Oceans are an important ‘sink’ for CO₂ (they remove CO₂ from the atmosphere), but this effect weakens as ocean temperature increases. So, a warming ocean removes less CO₂ from the atmosphere, increasing the amount of CO₂ in the atmosphere, further warming the surface ocean waters, and so on.

One important point about positive feedback loops is that it really doesn't matter where you enter the cycle (which comes first), it's still a positive feedback. For example, it doesn't matter whether an increase in temperature warms the atmosphere, which weakens the ocean sink effect, which leads to an accumulation of CO₂ in the atmosphere leading to further warming, etc.; or whether an accumulation of CO₂ in the atmosphere leads to a warmer atmosphere, which leads to a weakening of the ocean sink effect, etc. For more on this, see Section 9 ‘Why are some scientists sceptical about climate change?’

Increasing cloud cover is an example of a negative feedback. A warmer atmosphere holds more water vapour. If this condenses to form clouds, then (depending on the type of cloud, its altitude and other factors) more cloud means higher reflectivity, so less absorption of incoming solar radiation, so a cooling effect.

These web-based resources provide additional information

• An explanation of feedbacks can be found at the Encyclopedia of the Atmospheric Environment.

• RealClimate hosts an in-depth discussion of internal feedback effects, especially water and clouds.