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The Role of Water Vapour in Climate Change

"Water vapour is the most powerful greenhouse gas, so why are we so concerned about carbon dioxide and methane?" This is a question that climate skeptics challenge us with, with the implication that what is happening is not because of human-generated climate change emissions and so we can continue as we have been.

Even without climate skeptics, I myself have been asking the same question. From my investigations, I have discovered the following:

All greenhouse gases work by reducing net radiation outflow of energy from the earth, so that the earth warms. (i.e. they 'entrap' heat.) Water vapour contributes about 60% of global warming. Carbon dioxide contributes around 25%. But the water vapour effect is symptom more than a cause.
Most of the 60% is a multiplier effect, multiplying what is caused by carbon dioxide and other greenhouse gases. The process by which it does so is that as the atmosphere warms, the air absorbs more water vapour from oceans etc. and the additional water vapour increases global temperatures more.
Moreover, water vapour keeps condensing out at rain, snow, clouds, etc. It cannot increase above 100% humidity. Water vapour that enters the atmosphere does not last long before it precipitates back out again.
By contrast, carbon dioxide and other greenhouse gases are non-condesable, and keep on increasing their amount in the atmosphere.
So, increase in water vapour in the atmosphere is a symptom more than a cause of greenhouse warming.

Here are a few pages that explain all these:

Steamy Relationships: How Atmospheric Water Vapor Amplifies Earth's Greenhouse Effect [NASA 2022]

Water Vapor and Climate Change [American Chemical Society 2023]

Is Global Warming Caused by Water Vapor? How to think about the No. 1 greenhouse gas. [Brendan Koerner on Slate, 2008]

Explaining how the water vapor greenhouse effect works [skepticalscience 2015]

This page, "abxn.org/ccge/water.vapour.html", is an expression of part of a project to understand the links between climate change, global economy and other matters including society's beliefs and aspirations. It is designed to stimulate thinking and discourse. Comments, queries welcome.

This page is written on behalf of the CCGE Group by Andrew Basden, but the views expressed herein are his and not necessarily those of the other members of the Group. Written on the Amiga with Protext in the style of classic HTML. Copyright (c) Andrew Basden 2008 - present, but you may use this material subject to certain conditions.

Created: 1 June 2023 Last updated: