Listen to this lesson (podcast-style overview)
Climate Feedbacks and Sensitivity
Why climate is more sensitive than you might expect
If you doubled CO₂ and nothing else changed, Earth would warm by about 1.2°C. But the real climate warms by roughly 2.5–4°C because of feedbacks: processes triggered by warming that produce additional warming or, in some cases, partial cooling. Feedbacks are the multipliers that determine how much warming any given emission causes.
What Is a Climate Feedback?
A climate feedback is a process that responds to an initial change in temperature and either amplifies it (positive feedback) or dampens it (negative feedback). Feedbacks do not cause warming by themselves. They only operate in response to an initial forcing, such as rising CO₂. But they can dramatically alter how much warming a given forcing produces.
The concept of climate sensitivity captures the combined effect of all feedbacks. The most commonly used measure is Equilibrium Climate Sensitivity (ECS): the long-term global warming expected from a doubling of atmospheric CO₂ once the climate system reaches a new equilibrium. The IPCC AR6 assessed ECS to be likely in the range of 2.5°C to 4°C, with a best estimate of 3°C. This is the tightest constraint ever placed on this value, narrowed significantly compared to previous assessments.
Analogy: A Microphone in Front of a Speaker
Positive feedbacks are like placing a microphone in front of its own speaker. A small sound is amplified by the speaker, picked up again by the microphone, amplified again, and so on until you get a loud screech. In Earth's climate, a small warming from CO₂ triggers feedbacks that produce additional warming, which trigger more feedbacks. The key difference is that Earth's feedbacks are not unlimited. Negative feedbacks (mainly the Planck response, where a warmer Earth radiates more heat to space) ultimately bring the system to a new, higher equilibrium rather than a runaway state.
The Major Positive Feedbacks
Water vapour feedback (strongest positive feedback): A warmer atmosphere holds more water vapour (following the Clausius-Clapeyron relationship, roughly 7% more water vapour per 1°C of warming). Since water vapour is a potent greenhouse gas, this significantly amplifies the initial warming. The water vapour feedback roughly doubles the warming that would occur from CO₂ alone.
Ice-albedo feedback: As temperatures rise, snow and ice melt. Snow and ice have high albedo (reflecting 80–90% of sunlight). The dark land and ocean surfaces that are exposed absorb far more solar energy (only 6% reflectivity for ocean). This additional absorption drives further warming. This feedback is particularly powerful in the Arctic, where it helps explain why Arctic temperatures are rising more than twice as fast as the global average.
Cloud feedbacks (uncertain, but likely positive on net): Clouds can both warm (trapping outgoing infrared radiation) and cool (reflecting incoming sunlight). How clouds change with warming is one of the most complex questions in climate science. Low-level marine clouds (which are primarily cooling clouds) appear to decrease in coverage as the climate warms, producing a net positive feedback. The IPCC AR6 concluded with high confidence that cloud feedbacks are positive on net.
Permafrost and carbon cycle feedbacks: Arctic permafrost contains enormous stores of organic carbon (approximately twice as much carbon as currently in the atmosphere). As warming thaws permafrost, microbial decomposition releases CO₂ and methane, adding to atmospheric greenhouse gas concentrations and driving further warming. This feedback is slow but potentially very large over centuries.
The Key Negative Feedback: The Planck Response
The most important stabilizing feedback is the Planck response (sometimes called the blackbody radiation feedback). As Earth's surface warms, it radiates more energy to space. The amount of radiation increases with the fourth power of temperature. This means that no matter how many positive feedbacks amplify warming, a new equilibrium is ultimately reached at which the warmer surface radiates enough additional energy to restore balance.
This is why Earth's climate is not in runaway feedback but settles at a new, higher equilibrium temperature. The Planck response prevents positive feedbacks from becoming infinitely self-amplifying.
Calculating Climate Sensitivity Step by Step
Step 1: CO₂ forcing alone: Doubling CO₂ from 280 to 560 ppm adds a radiative forcing of approximately 3.7 W/m².
Step 2: Without feedbacks: The Planck response alone would produce about 1.2°C of warming per 3.7 W/m² of forcing.
Step 3: With feedbacks: Water vapour feedback roughly doubles the response; ice-albedo and cloud feedbacks add more. The combined effect gives the IPCC best estimate of approximately 3°C for ECS.
Note: Reaching the full equilibrium takes centuries, since the ocean must warm throughout its depth. Transient warming (what we experience over decades) is lower than ECS because the ocean has not yet fully warmed.
Tipping Points: When Feedbacks Become Self-Sustaining
Some feedbacks can become self-sustaining beyond a critical threshold, even if the original forcing is removed. These are called tipping points. The IPCC defines them as "critical thresholds beyond which a system reorganizes, often abruptly and/or irreversibly."
| Tipping Element | Estimated Threshold | Potential Consequence |
|---|---|---|
| Greenland Ice Sheet | 1.5–2°C above pre-industrial | Up to 7 metres of eventual sea level rise |
| West Antarctic Ice Sheet | 1.5–2°C regional warming | Up to 3.3 metres of sea level rise |
| Amazon Rainforest dieback | 3°C global + 40% deforestation | Savannification, large carbon release |
| Atlantic Overturning Circulation (AMOC) | 3–4°C (uncertain) | Disrupted European climate and Atlantic ecosystems |
| Permafrost carbon release | Gradual, accelerating above 2°C | Centuries-long self-reinforcing CO₂/CH₄ release |
Transient vs Equilibrium Warming
Because the ocean absorbs so much heat and warms slowly, the surface warming we observe today is less than the eventual equilibrium warming for current CO₂ concentrations. The difference between what we have already experienced and the full committed warming is called unrealized warming or committed warming.
Even if all greenhouse gas emissions stopped today, the climate would continue warming by an estimated 0.3–0.5°C due to the thermal inertia of the ocean. This committed warming underscores the urgency of early emissions reductions: every tonne emitted today adds not just to near-term warming but to long-term committed temperature.
Scientists use feedback analysis techniques in climate models to isolate each feedback. By running a model with one feedback turned off at a time and comparing the result to the full model run, researchers can calculate each feedback's contribution in W/m² per degree of warming (the feedback parameter). The total warming is determined by how negative the sum of all feedback parameters is: more negative means more stable, less sensitive climate; closer to zero means higher sensitivity.
Observations also provide constraints. The rate at which ocean heat content has risen compared to surface temperature change gives an observational estimate of effective climate sensitivity that broadly confirms model-based ECS estimates.
Key Takeaways
- 1Climate feedbacks are processes that amplify or dampen an initial temperature change - they are not independent drivers but multipliers
- 2Equilibrium Climate Sensitivity (ECS) is estimated at 2.5-4°C for a doubling of CO₂, with a best estimate of 3°C (IPCC AR6)
- 3The water vapour feedback is the strongest positive feedback, roughly doubling the warming from CO₂ alone
- 4The Planck response (increased radiation from a warmer surface) is the key negative feedback that prevents runaway warming
- 5Tipping points represent thresholds beyond which feedbacks become self-sustaining and potentially irreversible, making early action critical