Well it's easy enough to recombine hydrogen and oxygen. Step well back and add a spark will do it! Safely controlling the process is the key to using hydrogen as an energy source. Liquid (compressed) hydrogen was a vital fuel in Saturn V rockets. As for affecting humidity, generally speaking the atmosphere would take care of that via the water cycle. There would be some effect but it would be far more benign than adding carbon dioxide, which is far more slowly dealt with, which tends to accumulate and which acidifies the oceans, not forgetting the greenhouse effect of course. And the hydrogen we'd be using would have come from earthly water in any case. We'd just be putting it back where we got it from, which was not from buried resources that have been undisturbed for hundreds of millions of years.