Water vapor is a potent greenhouse gas, and it accounts for about 60% of the warming in the earth’s lower atmosphere. However, this fact is often confused with the causes of global warming. Skeptics of human-induced global warming sometimes incorrectly blame water vapor for the rise in average global surface temperature over the past 100 years. Water vapor amplification enhances atmospheric heat retention, but water vapor is not the primary cause of global warming.
Greenhouse gases contain condensable and non-condensable components. Water vapor is a condensable greenhouse gas whose concentration in the atmosphere depends on temperature. So, the maximum amount of water vapor in a fixed volume of air relates directly to air temperature. Warmer air holds more moisture. When air, fully saturated with water vapor, experiences a temperature drop, the water vapor then condenses into a liquid. In the atmosphere, this condensation effect forms clouds and creates precipitation like rain and snow.
Water vapor differs from non-condensable greenhouse gases like carbon dioxide (CO2) or methane (CH4), whose concentrations hold relatively steady with temperature fluctuations. These non-condensable greenhouse gases play a critical role in maintaining the earth’s surface temperature in a range that supports life as we know it. Without these non-condensable gases, the earth’s average temperature would be about -18 degrees Celsius. The planet would be covered by a sheet of ice. Without the non-condensable greenhouse gases, the frigid air could then only hold a tenth of the moisture it contains today.
The water vapor feedback loop
Water vapor is an enhancer or amplifier of atmospheric warming but it doesn’t start the cycle. The warming feedback process begins with an injection of non-condensable greenhouse gases into the atmosphere. Fossil fuel emissions provided the greenhouse gases that kickstarted the Anthropocene warming we experience today. Increased concentrations of CO2 and methane from emissions trap additional heat in the lower atmosphere. Temperatures rise, and then the warmer atmosphere holds more water vapor. Like a shot of adrenaline, the additional water vapor provides another burst of heat absorption.
Thus, the feedback cycle begins. Now, with the additional heat captured by extra water vapor, the atmospheric temperature rises again, and even more water vapor incorporates into the air. Non-condensable greenhouse gases like carbon dioxide and methane are the root cause of global warming, but water vapor is the magic enhancer.
Cycles within cycles
The saying that every action creates an equal, but opposite reaction holds true for water vapor amplification. More water vapor leads to more clouds. Now, clouds are fluffy and white on top and help reflect sunlight back into space. Because less sunlight reaches the earth’s surface, less solar heat is available for atmospheric warming. This process encourages cooling.
These two sides of this environmental coin play out daily. Increased water vapor provides for increased heat retention but creates more clouds that block sunlight and encourage cooling. However, the steady rise in average global surface temperature over the past 100 years attests to the fact that warming is winning out over cooling in this battle.
ArcheanWeb:
Regional temperatures: Location, location, location (Source: ArcheanWeb) – https://archeanweb.com/2019/12/13/regional-temperatures-global-temperature/ Also:
Sources:
It’s Water Vapor, Not the CO2 (ACS) – https://www.acs.org/content/acs/en/climatescience/climatesciencenarratives/its-water-vapor-not-the-co2.html Also:
How hot could the earth get? (By Vivien Cumming; BBC) – http://www.bbc.com/earth/story/20151130-how-hot-could-the-earth-get Also:
Air Moisture Holding Capacity (The Engineering ToolBox) – https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html Also:
Feature Image: Pirogue on the Mekong under grey clouds before a storm (By Basile Morin) (Modified) – https://commons.wikimedia.org/wiki/File:Pirogue_on_the_Mekong_under_grey_clouds_before_a_storm.jpg – This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license. – https://creativecommons.org/licenses/by-sa/4.0/deed.en
