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Warmer air holds more moisture; as climate warms, atmospheric water vapor increases (Clausius-Clapeyron relation). Since water vapor is a potent greenhouse gas, increased humidity amplifies warming (positive feedback). The Clausius-Clapeyron constraint provides robust estimates of humidity feedback strength, which paleoclimate data can verify by comparing model-derived humidity to proxy indicators.
From your work on climate sensitivity and radiative feedbacks, you know that the climate system responds to a forcing (like increased CO₂) not just directly but through a cascade of amplifying and dampening feedbacks. Water vapor feedback is the single strongest positive feedback in the climate system, roughly doubling the warming that would occur from CO₂ alone. Understanding why it works — and why paleoclimate evidence confirms its strength — requires connecting two ideas you already know: the greenhouse effect and the physics of phase transitions.
The Clausius-Clapeyron relation states that the saturation vapor pressure of water increases approximately exponentially with temperature — roughly 7% per degree Celsius of warming. This is not a climate model assumption; it is thermodynamics. Warmer air can hold more water vapor before condensation occurs, and observations consistently show that relative humidity stays roughly constant as climate changes. This means that absolute humidity tracks temperature: warm the atmosphere by 1°C, and it holds about 7% more water vapor. Since water vapor absorbs and re-emits infrared radiation across broad spectral bands, this additional moisture traps more outgoing longwave radiation, warming the surface further, which increases humidity further, and so on.
This feedback loop does not run away to infinity because each successive round of amplification is smaller than the last — the system converges on a new, warmer equilibrium. But the total amplification is substantial. Climate models estimate that water vapor feedback contributes roughly 1.5–2 W/m² of additional radiative forcing per degree of surface warming. Paleoclimate records provide a critical test of this estimate. During the Last Glacial Maximum (~21,000 years ago), global temperatures were about 5–6°C cooler than today, CO₂ was ~180 ppm, and the atmosphere was significantly drier. When climate models are run with LGM boundary conditions and produce the observed cooling, the water vapor feedback they calculate matches what proxy data (ice-core gas compositions, tropical sea-surface temperature reconstructions) independently suggest. If models had the feedback strength wrong, they would systematically over- or under-predict LGM cooling.
The paleoclimate context also clarifies why water vapor is a feedback and not a forcing. Water vapor's atmospheric residence time is only about 10 days — it is constantly cycling through evaporation and precipitation. It cannot accumulate independently the way CO₂ does. Instead, its concentration is *enslaved* to temperature through Clausius-Clapeyron. If you magically doubled atmospheric water vapor without changing temperature, the excess would rain out within days. This is why paleoclimatologists treat water vapor as an amplifier of other forcings (orbital changes, volcanic CO₂, ice-albedo shifts) rather than an independent driver. Its reliability as a feedback — grounded in basic thermodynamics rather than complex ecosystem or ice-sheet dynamics — is precisely what makes it one of the best-constrained components of climate sensitivity.
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