Ad blockers: How online publishing is fighting back
When Apple said it was going to allow ad blocking on the iPhone version of its web browser last September, it escalated a conflict that had been building in the digital economy for years.
Programs such as Adblock Plus, AdFender and Popup Blocker Pro have long claimed to protect consumers from intrusive web ads that slow down our browsers and hoover up personal data.
Around 200 million people globally are estimated to use such apps.
But for businesses that depend on web advertising, ad blockers could cost an estimated $21.8bn (£15bn) in lost revenue a year.
While some dispute this figure, most in the industry agree that ad blockers threaten publishers' ability to provide content free at the point of use.
Now publishers and advertisers are fighting back; the battle of the blockers is heating up.
Free London-based business newspaper CityA.M. is one of several publishers to have tried blocking visitors to its site if they have an ad blocker switched on.
The firm, which has been working with the anti-ad block start-up Rezonence, says about a quarter of its readers use ad blockers, but that there has been "no perceivable drop in traffic" since it launched the strategy in October 2015.
"We have had a trickle of complaints, but not many," digital editor Emma Haslett tells the BBC, adding that the firm had extended the strategy - which was initially aimed at Firefox users - to all internet browser types.
Others, like Conde Nast's men's magazine GQ, have tried charging ad blocker-users for access, while US business magazine Forbes is asking users to turn their blockers off in exchange for an "ad-light" experience.
If readers comply, Forbes says, they will be shown no welcome ad, no video ads inserted between paragraphs, and no ads between posts.
If they don't, they will be denied access to the site's content.
"Since we started in December, four million desktop visitors, or over 40% of those asked, have either disabled their blockers or whitelisted Forbes.com, gaining access to content and the ad-light experience," says chief product officer Lewis Dvorkin.
Whitelisting means telling an ad blocker or security program to treat a certain website as safe.
'Coding tug of war'
Not everyone is convinced by this approach, however.
Johnny Ryan of anti-blocking start-up PageFair believes ad blocking walls are relatively easy to bypass.
"Often publishers simply update their code, forcing the ad blocking community to get working again. It is a coding tug of war," he says.
Instead, PageFair's technology enables publishers to display "respectful and unobtrusive ads" that do not get filtered out by ad blockers.
In a variation on the theme, former Mozilla co-founder Brendan Eich has launched a browser called Brave, which replaces the targeted ads on websites with "clean" ones that do not record user data or slow down browser speeds.
It claims it will be able to pay publishers a higher proportion of ad revenues generated by cutting out middlemen. And it plans to roll out a system that lets web users make micropayments to publishers they like, in exchange for an ad-free experience.
Privacy app Ghostery is also adopting a more nuanced approach. It shows consumers which ad-serving technologies a website has switched on, and then offers the choice to block or whitelist those services.
According to the Association of Online Publishers, companies also need to display ads that don't annoy consumers in the first place.
This is why mobile video is proving so popular as an ad format - it's entertaining if done well and therefore more acceptable to consumers. Mobile video will account for 87% of global advertising spend by 2018, according to ZenithOptimedia.
Sponsored articles and videos - so-called native advertising - could be another solution, say some, being less obtrusive than traditional pop-up and banner ads and harder for ad blockers to detect.
How much of a threat this poses to independent journalism is another debate, however.
While offering more appetising advertising is vital, ensuring it does not affect the functioning of a website is equally important.
The problem, says Tej Rekhi of ad management company Sizmek, is that too many sites are "over-tagged" because of their ads and that this slows down page-loading speeds.
"Publishers need to make use of the technology that exists to unify tags in one place, allowing for faster load times and better results," he says.
Innovations such as Facebook's Instant Articles - which lets users load articles on mobiles 10 times faster than the traditional mobile web - point to a way forward.
'House in order'
But is all this concern about ad-blockers overhyped?
App market analytics firm firm App Annie tells the BBC that - despite an initial surge of interest after Apple's announcement - the most popular ad blocking programs have all dropped out of the App Store's top 100 most downloaded apps and now rank much lower.
And Stuart Miles, editor of tech news site Pocket-lint, says: "The media likes to stir things up when it comes to ad blocking. But in reality, the majority of people aren't bothered because they have to install something, and they never get round to it."
That said, Andrew Frank, digital marketing expert at technology research firm Gartner, believes ad blocking is growing quickly and its effects could be severe "for an industry that already faces challenges".
Facebook warned investors in January that ad blockers could "adversely affect" future profits.
Ciaran O'Kane, founder of ad tech research company ExchangeWire, doubts we'll see affected companies "die on the vine", but says they will have to adapt.
"Media companies have certainly sacrificed some of the user experience online for the sake of extra revenue.
"So I think this is a great opportunity for the ad industry to finally get its house in order."
Follow Technology of Business editor @matthew_wall on Twitter.