Earths average temperature used to be 33°C warmer, +15°C, than it would be without GHGs. It is now more than a degree warmer than that, rapidly closing on 2°C warmer.We sometimes mistakenly think greenhouse gases and carbon dioxide (CO2) are synonymous. After all, many climate change articles talk about CO2 induced global warming without mentioning any of the other contributors. But greenhouse gases encompass a wide range of components. At a high level, greenhouse gases divide into two broad categories: condensable and non-condensable components. Condensable gases are those whose concentrations are dependent on temperature.
The primary greenhouse gases are water vapor, carbon dioxide, methane, ozone, nitrous oxide, and chlorofluorocarbons. Water vapor is the only condensable gas in this group, and chlorofluorocarbons constitute the only purely human-made component. The distinction between condensable and non-condensable gases is important because non-condensable gases are the atmospheric components keeping our planet livable.
Water vapor differs from non-condensable greenhouse gases like carbon dioxide (CO2) or methane (CH4), whose concentrations hold relatively steady as temperatures change. 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 them, the earth’s average temperature would be about -18 degrees Celsius. A sheet of ice would cover the planet.
So CO2 is the driver, water vapor, increasing as the world warms. Water vapor is the positive feedback increasing the warming from CO2 and the other GHGs.If human activity had not increased the non-condensable gas concentrations by burning fossil fuels, today’s water vapor concentrations would be the same as before the industrial revolution. We don’t directly add more water vapor to the atmosphere. Instead, human activity adds more CO2, methane, nitrous oxide, and chlorofluorocarbons, causing temperatures to rise. Once the atmosphere is warmer, then it absorbs more water vapor.
Because water vapor is a condensable greenhouse gas, the maximum amount of moisture in a fixed volume of air relates directly to air temperature. Warmer air holds more moisture, hence more water vapor. But when air fully saturated with water vapor cools, the vapor condenses into a liquid. This condensation effect forms clouds and creates precipitation like rain and snow.
Clouds are thought to generally increase warming over all. The types and heights of clouds are changing and this increases the warming of clouds over all. This is not well understood. Clouds forming at night trap IR below them, reflecting it back to the surface hence warming the surface. Driving through the midnorth of SA as I did last week in showery weather the car thermometre would drop by up to 2°C when clouds covered the local sky, rising again when the clouds moved away.Cycles within cycles
The saying that every action creates an equal, but opposite reaction holds true for water vapor amplification. Additional water vapor leads to more clouds. Because clouds are fluffy and white on top, they reflect sunlight into space. Less sunlight then reaches the earth’s surface, so less solar heat is available for atmospheric warming. This process encourages cooling.
The dual heating and cooling roles of water vapor play out daily. Extra water vapor leads to increased heat retention, but it also creates more clouds to block sunlight and encourage cooling. However, the steady rise in average global surface temperatures over the past 100 years attests to a fact; warming is winning out over cooling in this battle.
The real impact
Weather is one of the earth’s engines for heat dissipation, moving heat from hot areas to cooler areas, smoothing out large temperature variations. This process forms the global water cycle with heat evaporating water in some areas and precipitating this water over other regions. Since oceans cover 70 percent of the earth’s surface, evaporation from the oceans is a major part of the water cycle. A warmer atmosphere means more water evaporates from the oceans, and precipitation increases. These changes impact weather patterns worldwide.
Studies of ocean salinity, led by Lijing Cheng from the Institute of Atmospheric at the Chinese Academy of Sciences, tracked surface salinity changes over six decades. Their findings show how global warming amplifies the water cycle, creating a more dynamic range of environmental conditions—high salinity zones get saltier, and low salinity areas become even fresher.
The results are important because they indicate a future where dry regions become dryer and wet zones wetter. Extreme weather events will become more frequent, and large populations will grapple with regional shifts in water availability.
Dry seasons become longer and dryer, so the West Coast burns. Decreasing water flow in the Colorado River threatens 40 million people and billions of dollars in agriculture. More frequent hurricanes pound the USA’s Gulf and East Coast, submerging towns below ocean waters. Water vapor is the atmospheric link to these amplified weather patterns. It is also a greenhouse gas able to create its own positive feedback loop, further accelerating an already rapid rise in global temperatures.