Drew blogged about Google Author Rank back in April, and the rate of authorship markup adoption has been steadily increasing since then. It’s ridiculously easy to add the markup in WordPress and Google is dangling a pretty big carrot for us all: Better click through in the SERPs due to the nice avatar you get next to your results.

Google continues to encourage the use of the tag, and very shortly, its usage will be the norm. The question is, “Why?”

Everyone knows that Google uses links to determine the topic, authority, and trustworthiness of websites (though we don’t know exactly how they do so.) In the past (and now,) the root domain of a link has been a very strong signal of the quality of that link. So, if my website is linked to from a New York Times article, that link is very valuable to me.

The problem with this is that root domain signals can be noisy. The NYT, for example, delivers great, authoritative news, but they also have a classifieds section, which are simply ads (and not that authoritative.) Google attempts to differentiate these links through page segmentation, but authorship allows them to go a step further. By verifying and separating the authors from the sites they write on, Google can now quantify how authoritative a specific author is. SEOMoz allows pretty much anyone to blog on their website. After Author Rank is widely implemented, Google can easily differentiate between a link from an authoritative source like Rand Fishkin, and a link from a non-authoritative source (like a no-name blogger.)

This has big implications for link diversity. We’ve always looked to the “root domain count” as a signal for site authority. Conventional wisdom said, “the more linking root domains, the better.” Basically, the more websites (not pages) linking to your website, the more important your site is.

So, many webmasters (me included) offered up their content as guest bloggers to as many other webmasters that would take them (with some exceptions.) The reason was simple: If you were to guest blog on 3 different sites, the link profile would look like this:

So, guest blogging was a good thing, simply for the fact that you could increase your root domain link count. Now, Google presumably looks past the root domain to the actual author. So, if the same author guest blogs on 3 different sites and links to their own site, it looks like this:

So, guest blogging is still a good thing for other reasons, but we presumably don’t get the benefit of the increased root domain link count. We now want to increase the number of authors linking to our website (and really the number of authoritative authors linking to us.) The analogous diagram to root domain count in the world of Author Rank looks like this:

It’s important to note that the authorship markup is not currently the norm, and Author Rank is most definitely not rolled out to its fullest extent. Today, root domain count is still a solid signal. However, be prepared for it to become less significant, as its replaced by Author Rank.