We learnt this at university in the 90ies, but big money does not listen. Steam from water, vapor, mist are much more greenhouse gas than others. Especially on local levels, like urban micro climate (think of a million cars steaming water in the air) but also on higher layers. Read this. ATM there’s no valid path around public transport and electric vehicles. Good-bye, privately owned combustion engine. That includes fuel cells.
“we have also considered, for the first time, previously ignored changes in stratospheric [that is, in the second-lowest layer of the atmosphere] water vapour and stratospheric ozone in our calculations of hydrogen’s GWP,” “