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Positive feedbacks amplify climate changes: ice-albedo feedback (melting sea ice reduces surface reflectivity, warming further); water vapor feedback (warmer air holds more moisture, a potent greenhouse gas). These are the two largest feedbacks in climate models, approximately doubling the warming from CO₂ alone. Negative feedbacks (cloud, lapse-rate) partially offset these, determining overall climate sensitivity.
Compare global mean temperature and sea ice extent from satellite data. Use radiative transfer models to quantify water vapor contribution to outgoing radiation.
A climate feedback is a process where an initial temperature change triggers a secondary effect that either amplifies or dampens the original change. You already understand the general concept from climate feedbacks and sensitivity — now we examine the two most powerful positive feedbacks in Earth's climate system and why they roughly double the warming you would get from a CO₂ increase alone.
The ice-albedo feedback works through reflectivity. Ice and snow are bright — they reflect 60–90% of incoming solar radiation back to space. Open ocean or bare land, by contrast, absorbs most of that energy. When warming melts some ice, the newly exposed dark surface absorbs more sunlight, which causes more warming, which melts more ice. This is a textbook positive feedback loop. You may recognize this mechanism from paleoclimate contexts: during glacial-interglacial transitions, ice-albedo feedback helped amplify the small orbital forcing changes (Milankovitch cycles) into full ice age swings. Today, Arctic sea ice decline is a real-time demonstration — the Arctic is warming roughly two to three times faster than the global average, partly because this feedback is actively operating.
The water vapor feedback is the single largest positive feedback in climate models. The Clausius-Clapeyron relation — which you studied as saturation vapor pressure — tells you that warmer air can hold exponentially more moisture, roughly 7% more per degree Celsius. Water vapor is itself a potent greenhouse gas, absorbing and re-emitting infrared radiation across broad wavelength bands. So when CO₂ warms the atmosphere, the air holds more water vapor, which traps more outgoing radiation, which warms the atmosphere further. Crucially, water vapor is a feedback, not a forcing — it responds to temperature rather than independently driving it. If you removed all CO₂ forcing, water vapor concentrations would drop as temperatures fell, because the atmosphere simply could not hold as much moisture.
These two feedbacks do not operate in isolation. As ice melts and exposes ocean, evaporation increases, adding more water vapor to the atmosphere. Meanwhile, other feedbacks push back. The lapse-rate feedback is negative: in a warmer world, the upper troposphere warms faster than the surface (especially in the tropics), which increases outgoing radiation and partially offsets surface warming. Cloud feedbacks remain the largest source of uncertainty — low clouds that reflect sunlight are a negative feedback, but high thin clouds that trap outgoing radiation are positive, and predicting how cloud cover will change is notoriously difficult. The net effect of all feedbacks together determines equilibrium climate sensitivity — the total warming per doubling of CO₂. Current best estimates place this at roughly 2.5–4°C, with ice-albedo and water vapor feedbacks responsible for most of the amplification beyond the ~1.1°C direct radiative effect of doubled CO₂.
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