There are plenty of reasons of CTR to decrease.

- Increase of mobile traffic, which "in general" has a lower CTR than desktops,
- Check your coverage too. Since mid December, coverage decrease for lot of publishers, certainly due to some kind of brand protection system deployed by Adsense, which has to verify (bot) the content of a page before displaying ads. As a result, the first visitor of page, in a while, will be served thrid parts ads, instead of Adsense ones,
- Also, in the past, there were a significant amount of advertisers, having misleading ads, which could cause "unwanted" clicks,
- Adsense might also be better at detecting fake clicks, from bots,
- Part of your increase of traffic might not be humans,
- Users ad blindness,
- ...

Ad blocking in all its shapes and forms are not helping advertisers, nor publishers.
[webmasterworld.com...]
[webmasterworld.com...]

One thing I didn't mention is the drop appears to start quite markedly at the beginning of the year. Ie. on the graph there is a quite marked divergence at the start of the year for RPM - which hasn't happened before.

@Travis thanks for the reply but as I said the drop seems too sharp to be any of the ongoing headwinds such as mobile / ad blindness etc. Also, I have done well out of mobile traffic for quite a few years and coverage is actually up during the timeframe I'm comparing.
As for the increase in traffic, this is not coincidental as it corresponds to improvements made to these sites.

I suppose the most important question is if anyone else has (reliably / not anecdotally) experienced this kind of drop recently. Or is there something I should be looking to fix.

Digging into this a little more I'm now wondering if last year was the outlier.
For some reason the CTR was somewhat higher than the previous year. CPC is still on a fairly constant decline though....

@surfgatinho

I disagree with most of what has been said thus far. Let begin with what I do agree with.

- Check your coverage...

This is specially important given that you say:

the drop appears to start quite markedly at the beginning of the year.

In December AdSense/Adwords rolled out the "Brand Safety" update that prevents AdWords from bidding on auctions for page impressions when the page has not been previously crawled. (previously is really in the previous 2 weeks or so). Low coverage can signal that you are impacted by this. Typically if you have a site with where you have many pages and each gets few visits then you will see a big impact, whereas if your sites has few pages but get visits from many users there will be no impact.

Ad blockers... There will definitely impact your earnings but the impact is not measurable through AdSense stats as the adblockers block the page views from appearing, thus there will be no measurable impact in terms CPC, CTR, or RPM. So yes it's a factor, but it is on top of what you are seeing.

- Increase of mobile traffic,
I disagree her, not because I don't think there is mobile traffic or that the RPM associated with mobile is less than desktop. My issue is that this is often framed as a problem without a solution, a new reality that one must accept. That is simply not true. First having a responsive website does not mean that your site and AdSense placements are optimized for mobile. A long desktop page that simply collapses the content into one tall column is responsive and "mobile friendly" but the experience may still suck for mobile users. From an ad placement perspective, showing ads on long slow to load page means that many users will never see the ads. The top ads will not have loaded and the user will have scroll down the page. Then the lower ad placements will not be seen because the user will bounce before reaching it. Users need to see the ads.

Mobile now accounts for the biggest share of traffic for nearly every website, designing a site for desktop and adapting it to fit mobile is the wrong approach. In my view the issued should be framed as "poorly optimized ad placement/site layout for mobile" as opposed to "increase in mobile traffic".

As for the other points raised by Travis, I agree that these are valid issues but I think there impact on earnings is marginal at best in most instances. In instances where this is not the case, then the problem is generally obvious and a problem to be addressed in and of itself.

AVV - Active view viewable, is everything!
AVV is a new metric which measure the viewability of ads. AdWords does not offer CPM bids it only offers Active View CPM, any CPM bids you see in your account are coming from third party networks. What this means is that an impression is paid for based on its viewability. If there are many ads requests occurring that are never being seen by users then you get a low AVV. This is n't simply about AV-CPM bids, it should be obvious that if user can't see an ad then they can't click on the ad. So increasing AVV has positive benefit for CTR and for all bid types.

Case Study:
Over the period described in the OP, on my site I was able to increase my AVV from around 40% to well over 60%, in that same period my RPM has doubled and the bulk of my earnings now comes from Mobile users. This was achieved by redesign a responsive website to a site layout to has been optimized for mobile device.

@NickMNS Thanks for the informed response. I think you've covered most of the usual suspects and would second pretty much all you say - which is a shame as I think all the low hanging fruit have gone now. AVV is over 60% on all my sites and I've been building mobile first(ish) sites for a few years now. I even have a couple of sites with AdRecover.

Anyway, having not done any productive work today I have spent quite a few hours trawling through the past few years of AdSense earnings and I am now thinking the first quarter of last year was exceptional. This would explain why no one else is seeing the same drops I am.
Hard to explain why but CTR was up across several sites and with a large sample size (i.e. millions of visitors). CPC seems to have been in permanent decline for years, but this is fairly gradual.

So, my current hope is that, given last year was my best to date, as long as things don't get worse the next few quarters should be back in line with the rest of last year.