Behind the scenes of water vapour 

    Global warming continues to rise, with greenhouse gases (GHG) polluting the atmosphere. Concerningly, a commonly overlooked GHG is plain old water vapour. Astonishingly, water vapour accounts for roughly 97% of the total GHG.   

    What is water vapour? 

    Water vapour is the gaseous state of water and it is invisible. It is also the primary form of atmospheric moisture.  

    Water evaporates from the Earth’s surface and rises on warm updrafts into the atmosphere, condensed into clouds. These clouds are responsible for rainy and snowy days on Earth. The cycle in which water evaporates and finally falls back to the surface is essential for our planet’s survival. In the process, heat and energy are transferred from the Earth’s surface to the atmosphere, transported to other regions, and consequently serve as a second source of heat for our planet (besides sunlight).  

    Nevertheless, water vapour is classified as a greenhouse gas. It is the most critical greenhouse gas there is as, without it, Earth would not be a habitable place. On the flipside, in connection with other greenhouse gases, water vapour is dangerous too.  

    How is it measured?

    Under typical atmospheric conditions, water evaporates and then condenses again, creating a cycle. As a result, the amount of water vapour in the atmosphere is relatively constant. However, with other greenhouse gases polluting the air, the amounts of water vapour in the atmosphere increases.  

    The amount of water vapour is measured in humidity. A humidity of 50% means that the air contains 50 per cent of the water that it could hold. With rising temperatures, though, the air can hold more and more water, which means that the relative amount of water vapour would decrease.  

    Contrary to the rising temperatures due to global warming, scientists have predicted that relative humidity will stay about the same, meaning that the atmosphere will hold more water. 

    How is more water in the air a problem?

    According to NASA, it is assumed that the relative percentage of moisture in the air will stay about the same. This is in spite of warm air being able to hold more water. Nevertheless, proportionally more water will also evaporate, meaning more water will be in the atmosphere, which leads to a constant relative humidity. 

    Climate models that assume that future relative humidity will remain constant predict more significant increases in the Earth’s temperature in response to increased carbon dioxide than models that allow relative humidity to change. The constant-relative-humidity assumption places extra water in the equation, which increases the heating.  

    The graph below shows the relationship between the percentage of humidity and temperature in degrees Celsius.

    Source: NASA 

    The effect of too much water vapour 

    The addition of the non-condensable gases (i.e.CO2, CH4, N20, O3) causes the temperature to increase, leading to an increase in water vapour that further increases the temperature. This is the so-called positive feedback effect.  

    On the other hand, there is also the possibility of a negative feedback effect. In that scenario, the rising vapour in the air decreases the temperature as more clouds reduce the number of sun rays reaching the Earth’s surface. This is due to clouds reflecting sunlight. 


    Whether the possible positive or negative feedback loops are more probable remains an active area of climate science research and debate. However, currently there is more evidence to suggest that the positive feedback loop is more prevalent. Consequently, a rising amount of humidity in the atmosphere will further contribute to global warming.  

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