[wyattpeak]

Honestly, I started working this out because I thought it would be negligible. But I think you're right to doubt. Leaving aside the question of how much a human produces, since two people have suggested human output is neutral and I don't know enough to question it:

It takes an average of 0.10kcal to walk up/down a step, averaged.[1]

2.2kg of CO2 are emitted per 2000kcal of consumption (I just averaged table 3 for want of a better idea)[2]

37 steps in a staircase (TFA, 46 total - 9 flat)

3.7kcal burned, 3.7kcal * 1.1g/kcal ~= 4g CO2 per person per trip

Obviously very rough, but unless I've made an order-of-magnitude error it's in the same ballpark.

[1] https://pubmed.ncbi.nlm.nih.gov/9309638/

[2] https://nutritionj.biomedcentral.com/articles/10.1186/s12937...