Google has been unusually specific and has revealed on its blog that there will be a significant algorithm update on the 21st April 2015, that will expand their use of mobile-friendliness as a ranking signal. Although this sounds alarming, it is to be expected to be the biggest change since Panda and Penguin (targeted a limited set of sites using spammy tactics), while this will affect all searches on mobile - that's why Google says it's significant.
Will the update affect all pages on a site?
Google has announced that this update will take place in ‘real-time’, meaning that if changes are made to make websites more mobile-friendly, as soon as the changes have been indexed by Google the benefits will be realised. Google has also said that this algorithm update will impact sites on a page-by-page basis, meaning that only those pages that are not mobile-friendly will be impacted, rather than the wider domain. This will mean that websites not deemed to be ‘mobile-friendly’ will suffer from reduced visibility within organic search results. It’s also possible that this reduction in visibility may extend to desktop as an additional incentive to webmasters to improve mobile experiences. We'll explain how you can test your pages are mobile friendly below.
Why is Google implementing these changes?
Despite the promotion of YouTube, Android and Google+, Google is still heavily reliant on AdWords to generate the majority of their revenue. If a search does not offer value to users, then they may end up going elsewhere, representing a risk to Google’s revenue stream.
At the moment, mobile search results largely match desktop and are typically ranked based on the merits of the desktop site based on the combination of on-page and off-page ranking factors. However, it seems Google believe this isn't acceptable with the tipping point of mobile traffic outstripping desktop traffic by Q2 2015 already passed.
Google does not want to be serving half of its users with sub-optimal search results and are therefore making efforts to mitigate this issue.