A forcing starts a change in temperature, but the climate answers back. Some responses make the change bigger, some make it smaller. These are feedbacks, and they decide how much Earth finally warms.
Key idea
A positive feedback strengthens the change that caused it; a negative feedback weakens it. The strongest negative feedback is Earth's own glow: a warmer planet gives off more infrared ($\sigma T^4$). Positive feedbacks include water vapor (warmer air holds more of this greenhouse gas) and ice and snow albedo (melting uncovers darker surfaces that absorb more sunlight). Cloud feedbacks are probably positive overall, but less certain.
Why it works
Each feedback is measured in W/m² per degree of warming. The IPCC's best estimates: Earth's extra glow, about −3.2; water vapor (with the change in how temperature falls with height), about +1.3; ice and snow albedo, about +0.35; clouds, about +0.4. The net is about −1.15 W/m² per °C: still negative, so the climate settles, but much less negative than the glow alone. Warming at balance = forcing ÷ (minus the net feedback). With only Earth's glow, doubling carbon dioxide (+3.7 W/m²) would warm about $3.7/3.2 \approx 1.2$ °C; with all the feedbacks, about $3.7/1.15 \approx 3.2$ °C.
Worked example: Ice and albedo
Warming melts sea ice and uncovers dark ocean. Is this a positive or negative feedback?
- The ocean absorbs much more sunlight than ice (lower albedo).
- More absorbed sunlight means more warming, which melts more ice.
- The change strengthens itself: a positive feedback.
Answer: Positive
Worked example: Warming with feedbacks
A forcing of +3.7 W/m² acts on a climate whose net feedback is −1.5 W/m² per °C. How much does Earth warm at balance?
- Warming = forcing ÷ (minus the net feedback) = $3.7 \div 1.5$.
- $\approx 2.5$ °C.
Answer: About 2.5 °C
Common mistake
Thinking a positive feedback is a good thing. In science, "positive" means only that the change strengthens itself, whatever its effects; "negative" means it weakens itself. Melting ice speeding up warming is a positive feedback even though few would call it good. Check: ask whether the response pushes in the same direction as the original change.