Here’s a paper from a few years back with a low end estimate of 11MTCO2e for a year of digital advertising https://www.sciencedirect.com/science/article/pii/S019592551...

For comparison that one year is about the same as the entire history of ethereum, which gets a lot of criticism. Or about 4 world cups.

And a crazy 10x that on the high end of the estimates. Be interesting to see some new numbers on this.