Although this study doesn't discuss C02 emissions, it does discuss energy use and the potential impact of internet advertising.

Unfortunately, it only discusses the impact on client-side devices, the energy consumption of hardware used to serve advertisements (networking, servers) is left to a future study.

>Strikingly, uBlock Origin has the potential to save the average global Internet user more than 100 h annually.

>So, for example, the 1.35 × 1010 kWh saved globally for using uBlock Origin is equivalent to more than 1.0% of the electricity generated per year from coal in the United States, which is responsible for the premature deaths of about 52,000 American every year from air pollution [43,44].

>Globally, the results with the most efficient open source ad blocker tested, uBlock Origin, would be even more substantial: ad blocking would save consumers more than $1.8 billion/year.