A story on the BBC website: Are hydrogen trains the future of UK travel?—Trains powered by hydrogen could be a reality in the UK by the ''early 2020s', according to UK Transport Secretary Chris Grayling, pushes the idea of hydrogen fuels as "clean". 

But according to the American Chemical Society, “Forget the CO2. Water vapor [H2O] is the most important greenhouse gas. It controls the Earth’s temperature.”

It is to be noted that while "...water vapor is the largest contributor to the Earth’s greenhouse effect. On average, it probably accounts for about 60% of the warming effect, ... water vapor does not control the Earth’s temperature, but is instead controlled by the temperature."

In other words, when Earth's climate warms, the atmosphere can hold more water vapour which adds to the warming.

When Earth's climate cools, the atmosphere is capable of holding less water vapour, adding to the cooling.

And according to the American Chemical Society, "There is also a possibility that adding more water vapor to the atmosphere could produce a negative feedback effect.

This could happen if more water vapor leads to more cloud formation. Clouds reflect sunlight and reduce the amount of energy that reaches the Earth’s surface to warm it. If the amount of solar warming decreases, then the temperature of the Earth would decrease. In that case, the effect of adding more water vapor would be cooling rather than warming.

But cloud cover does mean more condensed water in the atmosphere, making for a stronger greenhouse effect than non-condensed water vapor alone – it is warmer on a cloudy winter day than on a clear one. Thus the possible positive and negative feedbacks associated with increased water vapor and cloud formation can cancel one another out and complicate matters.

The actual balance between them is an active area of climate science research." 

Used as fuel, burning hydrogen produces pure water vapour which adds to atmospheric greenhouse gases and their effect.

And anyone who says climate science is settled is a charlatan, a fraud, irresponsible, and/or ignorant! 

Mike Priaro, P. Eng.

Calgary

403-281-2156