A feedback returns to the original change. Human emissions can trigger amplifying responses as well as limited stabilising responses.
A forcing starts a change in the energy balance; a feedback responds to that change and then alters it further. Human carbon dioxide emissions are a forcing. Warming that increases atmospheric water vapour strengthens the greenhouse effect, amplifying that initial warming: a positive feedback.
Warming can thaw permafrost and expose organic matter to decomposition, releasing carbon dioxide or methane. Heat and drought can also increase tree mortality and fire under suitable fuel and ignition conditions, reducing uptake and releasing carbon. Added greenhouse gases reinforce warming. Forest clearance or drainage can make ecosystems more vulnerable to these responses.
A stabilising carbon response also occurs. Higher carbon dioxide can increase photosynthesis and plant growth, drawing some additional carbon from the atmosphere. This negative feedback opposes the initial carbon dioxide rise. Water, nutrients, heat stress, species and later decomposition limit how much carbon is stored and for how long.
People can create reinforcing or opposing responses too. More heat may increase cooling demand; if extra electricity comes from fossil fuels, emissions add further warming. Conversely, experienced impacts can prompt stronger emissions cuts and sink protection. Call this a societal response loop and state each link, rather than labelling any environmental policy a physical feedback.
A cooling influence is not automatically a negative feedback. For example, adding reflective aerosols is a forcing unless their change is itself caused by the initial climate response. Negative feedback also does not guarantee that warming stops: it may only reduce its size.
Step by step
Name the initial change
Specify warming, carbon dioxide increase or another perturbation.
Follow the response
Identify what changes because of that perturbation.
Return to the start
Decide whether the final effect amplifies or opposes the initial change.
Worked example: Carbon dioxide fertilisation has limits
NASA research describes enhanced plant uptake under higher carbon dioxide, but also limits from water and nutrients. It is therefore reasonable to identify a negative carbon feedback and unreasonable to infer that forests will automatically cancel continued fossil emissions.
Watch out for this
Any action that cools climate is a negative feedback.
A feedback must close a causal loop. An independently imposed cooling action can be a forcing instead.
Check your understanding
Warming increases electricity use for cooling, fossil generation rises, and additional emissions increase warming. How is the loop classified?
- Negative because air conditioners cool rooms.
- Positive because the final effect reinforces the initial warming.
- Neither, because human decisions cannot interact with climate.