Air

Why Focus on CO2 when Water Vapor Is a "Stronger" Greenhouse Gas? 0526.rtf - CO2 and water vapor play very different roles within global warming.  The former is a forcing, while the latter is a feedback.  As CO2 rises, it creates the baseline forcing to raise temperatures and, critically, keep them elevated.  This allows an increase water vapor to exist in the air, through something called the Clausius-Clapeyron relationship.  This increase in specific humidity acts as a feedback, magnifying the warming, but not in a runaway fashion, since it only stays in the atmosphere for a very short amount of time.

     When we pump enough CO2 into the atmosphere to raise temperatures by 1°C, the air can hold 7% more water vapor, fed by increased evaporation from the surface.

     Water vapor also moves heat around the whole climate system.  On the surface, either over water or land, evaporation acts as a coolant.  You feel this when you sweat or when you step out of the water after a swim.  Energy is absorbed when water changes from a liquid state to a gas state.  This is why forests feel cool, each tree is evaporating tonnes of water a day from their leaves, drawing energy from its surroundings.

     Since warmer air can hold more water vapor, land surfaces lose moisture more quickly as temperatures rise.  Once dry and as vegetation cooling capacity drops (either through plant stress or land use change), this can lead to faster land area warming and more intense heat waves.  This effect also dries out soil moisture, leading to agricultural drought and higher fire risk.

     When water vapor condenses at altitude, it changes from the gas state to a liquid or solid state (depending on the temperature). Clouds have varying effects on warming, depending on their properties. One of the main effects is that they reflect more sunlight back into space, since they are brighter - with a higher albedo than the land or ocean surfaces beneath them. 

     This is a significant aspect of global warming, since as temperatures have risen, cloud area and brightness have dropped. Since 2000, there has been a drop of over 1.5 million sq km of global average cloud cover. To put that area into context, it’s the land area size of Mongolia.

     Cloud cover can also add to the greenhouse effect - by trapping outgoing infrared radiation below it.  This is especially true with high-level cirrus clouds.  The effect is so strong that the warming from aircraft vapor trails accounts for half of the aviation industries warming effect, equalling their carbon emissions impact.

     As water vapor rises higher in the air column, it cools and eventually condenses into clouds.  In doing so, it releases that stored latent heat into the upper troposphere.  These convection dynamics are a primary engine for atmospheric circulation and weather patterns.

     Note below that water vapor mass (specific humidity) rose from 7.74 to 8.08 g/kg of air over 40 years, or 4.3%.  Meanwhile, relative humidity did not change.

Section Map: Water