as it says in the right hand pane next to the graphic, past...

as it says in the right hand pane next to the graphic, past volcanic activity is difficult to quantify as events earlier than 1950s have not been measured as accurately or as finely as they are today, so researchers are reliant on media reporting. smaller volcanic activity can present in local news and not in national new or global media.

so the numbers of smaller volcanic activity identified are minimum. ice core sampling can help to a limited extent as small eruptions usually don't show up clearly in core samples.

on water vapour: theres buckets of site discussing this. some are quite dubious. I can back this link https://skepticalscience.com/water-vapor-greenhouse-gas-intermediate.htm and https://theconversation.com/how-rising-water-vapour-in-the-atmosphere-is-amplifying-warming-and-making-extreme-weather-worse-213347

its easy to search though Q..... dyor.