Tesla is focused on a niche: Luxury / high-performance cars, which is a tiny fraction of the market in units, and even if they were wildly successful they would not even make a blip in the carbon emission statistics.
It is for now, but see http://news.ycombinator.com/item?id=5310911 for discussion from the weekend about Elon's post from a few years ago about his plans to enter the mainstream with Tesla.
Isn't that a category error? From what I know Tesla is interested in building good electric cars, where luxury/high-end gives them the profit margins to enable further improvements. Though I'm sure it plays some role in their marketing, I don't know that they are currently measuring their success in terms of overall carbon emission statistics.