> So if we are still in an ice age, then the gradual increase in global temperatures is all part of that

For the last 10,000ish years the Earth has been in an interglacial, ie. the least cold stage of an ice-age. Glacial-interglacial cycles have been occurring for the last 2 million years or so, which is the Quaternary Ice Age.

The increasing global mean temperature in very recent times is on top of being in the warmest part of an ice age, it cannot be attributed to part of the natural cycle. The natural cycle would be due to start cooling sometime in the next few thousand years and transition back to a glacial episode. Anthropogenic warming has eliminated that possibility and the fear is that the Earth could exit Ice Age mode entirely and switch into hot-house mode with virtually no ice at the poles at all, correspondingly higher global sea levels and a lot more energy in the climate system for extreme weather events to become a regularity.

> So where do scientists draw the line between the global warming caused by humans, and the global warming that’s part of the natural cycle of things?

Somewhere around the year 1900. Take your pick, determining an exact date is a bit of a moot point by now.