A stock is a quantity held somewhere; flows are the rates in and out.
Stock A quantity held in a system at a given time.
Step 1: Something to Notice
Watch first. The explanation comes later.
A forest holds a constant amount of carbon for decades while individual trees grow, die and decay continuously.
What is actually constant?
Step 2: Find Out
Isolate the loop type and run the same disturbance through each.
Step 1 — Predict
A lake holds a constant level. What does that tell you?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Steady state means the flows balance
Not that they have stopped — a lake at constant level pours through continuously.
Damping feedback opposes a change
So the system returns toward where it started.
Amplifying feedback adds to it
So a small departure grows. This is what "positive feedback" means.
And lags make management hard
The response arrives after the cause, so correction tends to overshoot.
Positive feedback is the destabilising one
Negative feedback returns a system towards equilibrium; positive feedback amplifies a departure. Melting ice exposing darker water, which absorbs more heat, is the standard example.
Systems respond with a delay
Effects can appear long after the cause and continue long after it stops. Lags make problems easy to underestimate while they are building and hard to reverse once they are visible.
Step 4: A Common Mistake
Lots of people think
“A system at steady state is one where nothing is happening.”
Step 5: Why a Bathtub Is the Most Useful Model in the Subject
The same idea somewhere new.
Water level rises when the tap runs faster than the drain, whatever the absolute rates are. Applied to carbon, that means emissions falling is not the same as concentration falling — the level keeps rising as long as the tap exceeds the drain, and stabilising the level requires matching them. A great deal of confused public discussion about emissions targets dissolves once the stock and the flow are kept apart.
Step 6: Think It Through
Practice makes it stick.
The Constant Lake
Problem 1 of 2
A lake holds a constant level. What does that mean?
Tap and Drain
Problem 2 of 2
Emissions fall but stay above absorption. What happens to the concentration?
Stocks and Flows
1 of 5
What is a stock?
2 of 5
Steady state means what?
3 of 5
Which feedback destabilises?
4 of 5
Why do lags cause overshoot?
5 of 5
Does falling emissions mean falling concentration?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Does a system at steady state have nothing happening in it?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain stocks, flows and feedback using a real environmental example.
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- A stock is a quantity held; flows are the rates in and out.
- Steady state means the flows balance, not that they stopped.
- Damping feedback stabilises and amplifying feedback destabilises.
- Lags hide consequences and cause correction to overshoot.