Feedbacks and tipping points
Why the climate warms more than CO₂ alone would cause, what actually holds it back, and how to read a headline about tipping points without panic or dismissal.
Climate feedback
A change caused by warming that in turn changes how much the planet warms. An amplifying (positive) feedback adds to the warming. A damping (negative) feedback reduces it. 'Positive' here means 'reinforcing', not 'good'.
Warming melts snow, the darker ground underneath absorbs more sunlight, and that adds more warming. That loop is an amplifying feedback.
A microphone near a speaker
Hold a microphone near its own speaker and a small sound gets picked up, amplified, played, picked up again: a loop. Climate feedbacks loop the same way, turning an initial push from CO₂ into a bigger warming. Where the analogy breaks: the speaker squeals out of control, but Earth's feedbacks don't run away. A stronger brake, which you'll meet shortly, grows faster than they do, so the warming settles at a higher level.
Check yourself
Kaveh reads that as the Arctic warms, sea ice shrinks and more dark ocean is exposed each summer. What kind of feedback is this, and why?
- Damping, because melting ice cools the water around it
- Amplifying: dark water absorbs more sunlight than ice
- Not a feedback, because sea ice melting doesn't raise sea level
- Damping, because open water lets heat escape into space
Show the answer
Amplifying: dark water absorbs more sunlight than ice
Yes. Ice reflects most sunlight; open ocean absorbs most of it. Less ice means more absorbed heat, which melts more ice.
THE BIGGEST AMPLIFIER
Warmer air holds more water vapour
For every 1 °C of warming, air can hold about 7% more water vapour, and water vapour is itself a greenhouse gas. So CO₂ warms the air, the air takes up more vapour, and the vapour traps more heat. This one loop roughly doubles the warming CO₂ would cause on its own.
Water vapour doesn't drive the change by itself: any extra rains out within days. It follows the temperature, which is why it acts as an amplifier and not a starting push.
Other amplifiers
- Ice and snow: less white surface means more sunlight absorbed, as Kaveh saw.
- Permafrost: frozen Arctic soils hold more carbon than the atmosphere does today. As they thaw, microbes slowly release some of it as CO₂ and methane.
- Clouds: the most uncertain feedback, because some clouds cool and others warm. The IPCC judges their overall effect is likely to amplify warming.
Check yourself
Does each one amplify warming or hold it back?
- Warmer air holding more water vapour
- A warmer planet giving off more heat to space
- Thawing permafrost releasing methane
- Oceans and plants absorbing part of our CO₂
- Snow melting earlier and exposing dark ground
Show the answer
Amplifies: Warmer air holding more water vapour, Thawing permafrost releasing methane, Snow melting earlier and exposing dark ground
Holds back: A warmer planet giving off more heat to space, Oceans and plants absorbing part of our CO₂
THE BRAKES
What stops warming running away
The strongest brake is simple physics: a warmer object radiates more heat, and this grows faster than all the amplifiers combined, so Earth settles at a new, higher temperature. A second brake is the carbon sinks: oceans and land have absorbed roughly half of the CO₂ we've emitted. As warming grows they keep absorbing, but they take up a smaller share.
It's why the water vapour loop on Earth levels off instead of boiling the oceans, and why the other half of our CO₂ is the half that stays and warms.
Climate sensitivity
How much Earth eventually warms if CO₂ in the air doubles from its pre-industrial level, once all the fast feedbacks have played out. It's the net result of amplifiers and brakes together. The IPCC's best estimate is about 3 °C, likely between 2.5 and 4 °C.
Doubling pre-industrial CO₂ means about 560 ppm. On its own, with no feedbacks, that would warm Earth only a little over 1 °C; the feedbacks take it to around 3.
Check yourself
Farhad says: 'Feedbacks amplify warming, so even if the world reached net-zero CO₂ emissions, temperatures would keep climbing on their own for centuries.'
Show the answer
False
False, according to the IPCC. Once net CO₂ emissions reach zero, warming from CO₂ is expected to roughly level off. Feedbacks make each tonne warm more, but the brakes keep them from running on by themselves. Some slow changes, like sea-level rise, do continue for centuries.
TIPPING POINTS
When a system shifts to a new state
A tipping point is a threshold past which part of the climate system changes in a self-sustaining way that's hard to reverse. Candidates include the Greenland and West Antarctic ice sheets, the Amazon rainforest and the Atlantic overturning circulation. The IPCC calls these low-likelihood but high-impact, and says the risk rises with every increment of warming.
Losing an ice sheet would play out over centuries to millennia, not in a year. The IPCC expects the Atlantic circulation to weaken this century and judged an abrupt collapse before 2100 unlikely, though not ruled out.
Two ways to misread a tipping-point story
'It's game over'
Treats one threshold as the end of everything. But there's no single cliff edge for the whole planet, and every tenth of a degree avoided still lowers the risk and the damage.
'It's just hype'
Treats low likelihood as zero. But these changes would be huge and hard to undo, so a small chance still matters, the way you'd insure a house against fire.
Check yourself
Shirin sees a headline: 'Ice sheet may be nearing a tipping point, scientists warn.' Her friend replies, 'So we're past saving, why bother?' What's the most accurate response?
- He's right: once one tipping point is near, nothing else can be changed
- The headline must be exaggerated, since tipping points are only a theory
- Risk grows with warming, so cutting emissions still lowers it
- Tipping points only matter for polar bears, not for people
Show the answer
Risk grows with warming, so cutting emissions still lowers it
Right. Tipping-point risk climbs with warming, and ice-sheet changes unfold over centuries. Less warming means lower odds and a slower pace.
Lesson recap
- Feedbacks are loops: amplifying ones (water vapour, ice, permafrost, likely clouds) add to warming.
- Brakes are stronger: a warmer Earth radiates more heat, and sinks still absorb about half our CO₂, so warming doesn't run away.
- Climate sensitivity sums it up: about 3 °C for doubled CO₂; at net-zero CO₂, warming roughly levels off.
- Tipping-point risk rises with each increment of warming, so every tenth of a degree avoided matters.