While online ads consume substantial resources (energy, etc.), traditional ads did as well. Consider the cost of printed pages, or even making your TV run for slightly longer than it otherwise would because your TV show lasts 30 minutes with ads even though it would last 22 minutes with no ads.

An interesting environmentalist argument would be, to ask everyone to pay more to producers of all kinds (online or otherwise), to prevent the "need" for extra resource consumption on any medium.

It "should" be an easy argument because people are clearly already paying for ads, they just don't see how. For instance, your electric bill would go down if you knew your TV would run for a half hour less every day.