What this lesson teaches
I can weigh human influence through the carbon cycle and greenhouse effect against natural causes.
Syllabus 8834, Topic 1.2, 1. The Science of Climate Change. How significant is the influence of human activities on Earth's climate compared to natural factors?
Make a guess
Plants grow faster when there is more carbon dioxide in the air, taking some of it back out. What kind of feedback is that?
- A forcing, because it changes the energy balance.
- A positive feedback, because it is good for plants.
- Not a feedback, because plants are not part of climate.
- A negative feedback, because it opposes the rise.
Show the answer
A negative feedback, because it opposes the rise.
A negative feedback reduces the first change. Faster plant growth removes some carbon dioxide, though water, nutrients and heat limit it.
A feedback loops back to the first change, and human emissions can trigger both amplifying and limited stabilising responses.
A forcing starts a change in Earth's energy balance. A feedback is a response that then changes the first change. Human carbon dioxide emissions are a forcing. Warming puts more water vapour in the air, which traps more heat. This amplifies the warming, so it is a positive feedback.
Warming can thaw permafrost, frozen ground full of old plant matter. As it rots, it releases carbon dioxide or methane. Heat and drought can kill trees and fuel fires, releasing more carbon. These added gases boost warming further. Clearing forests or draining peat makes ecosystems more exposed to these responses.
There is also a stabilising response. More carbon dioxide can help plants grow faster, taking some carbon out of the air. This is a negative feedback, because it opposes the rise. But water, nutrients and heat stress limit it. How much carbon is stored, and for how long, also depends on the species and later decay.
Sort these
Is each one a positive feedback, a negative feedback, or a forcing that starts a change?
Groups: Positive feedback (amplifies), Negative feedback (opposes), Forcing (starts a change).
- A big volcanic eruption fills the stratosphere with aerosols
- A warmer Earth gives off more heat to space
- More carbon dioxide helps plants grow and take in carbon
- Burning coal releases carbon dioxide into the air
- Warming adds water vapour to the air, which traps more heat
- Melting sea ice exposes darker water that absorbs more heat
- Thawing permafrost rots and releases methane
Show the answer
Positive feedback (amplifies)
- Warming adds water vapour to the air, which traps more heat The response strengthens the first warming, so it is a positive feedback.
- Melting sea ice exposes darker water that absorbs more heat Less reflection means more heating, which boosts the first warming.
- Thawing permafrost rots and releases methane Warming causes the release, and the methane adds more warming, which amplifies it.
Negative feedback (opposes)
- A warmer Earth gives off more heat to space More heat escaping as Earth warms pushes back against the warming.
- More carbon dioxide helps plants grow and take in carbon Caused by the rise, it removes some carbon, so it works against the change.
Forcing (starts a change)
- A big volcanic eruption fills the stratosphere with aerosols The eruption is not caused by warming, so it is a forcing, not a feedback.
- Burning coal releases carbon dioxide into the air Human emissions start the change in Earth's energy balance; they are not a response to it.
People create loops too. Hotter weather raises air-conditioning use. If that power comes from fossil fuels, emissions rise and warming grows. Call this a societal response loop. Harm people experience can also push them to cut emissions and protect sinks. State each link, and do not call a policy a physical feedback.
A cooling action is not automatically a negative feedback. It must be caused by the first change to count. For example, adding reflective aerosols is a forcing, unless the change is itself caused by the climate response. And a negative feedback may only reduce warming, not stop it.
- Forcing
- Starts a change, like human emissions.
- Positive feedback
- Amplifies the first change.
- Negative feedback
- Opposes the first change, often only partly.
Step by step
Name the first change
Warming or a carbon dioxide rise.
Follow the response
What changes because of it?
Loop back
Does it amplify or oppose the first change?
Worked example: Plant growth has limits
NASA research shows plants absorb more carbon when carbon dioxide rises, but water and nutrients limit this. So it is a real negative feedback. But forests will not automatically cancel out continued fossil fuel emissions.
Watch out for this
Any action that cools the climate is a negative feedback.
A feedback must be caused by the first change. A separate cooling action is a forcing.
Check your understanding
Warming raises air-conditioning use, fossil power output rises, and emissions add more warming. How is this loop classified?
- Positive, because it reinforces the first warming.
- Negative, because air conditioners cool rooms.
- Neither, because human choices cannot affect climate.
Show the answer
Positive, because it reinforces the first warming.
Yes. Each step boosts the original change.
Check your understanding
Which of these is a positive feedback?
- Warming thaws permafrost, releasing methane that causes more warming.
- More carbon dioxide boosting plant growth, which absorbs some carbon.
- Adding reflective aerosols to the atmosphere, which cools it.
Show the answer
Warming thaws permafrost, releasing methane that causes more warming.
It amplifies the first change.