It's been a long time since I was (briefly) involved with Globe At Night (https://globeatnight.org/). Back then - before cell phone sky measurements - they accepted SQM readings or naked-eye star counts. NASA even funded a free SQM for astronomy clubs that committed to submitting GAN data. There was indeed information about the need for 'dark adaption' - I believe they recommended 20 minutes outside before making your 'count' - and noted that children involved with GAN needed to be accompanied by a responsible adult to keep the kids safe out in the dark for 20 to 30 minutes.
I'm not sure what "The skies are getting 10% brighter every year" even means - The sky over big cities like LA? The sky in general over North America or India, or.... ?? - but I think that their data at least shows that over large swaths of land with significant human habitation (i.e., not remote deserts, etc.) - there is a noticeable brightening of the sky on an annual basis. There may be exceptions - some believe that interior New England and Upstate New York is getting darker - as population and economic activity declines. In one of the areas I live - the NY/PA border area (and South) - any trend in light pollution over the past 10 years would be caused by gas frack'ing. At least one of those older dark sky maps shows a big light patch - a good deal brighter than Binghamton NY or Scranton PA - in a forested hilly area with almost no human population. What? Gas fracking! It was the major source of light pollution over a multi-state area for several years - gone for now - but who knows for how long?
Plus - we are limited to the data we have. DarkSky International was considering installing a system of auto SQMs thru the New England / New York area to get good data on light pollution as population declines. Alas - no $$.
My personal experience with the astro club NASA-provided 'free' SQMs -- we had lots of interest at first - but it gradually declined - then a member took the SQM to a star party - did not return it - claimed he didn't know what we were talking about - etc - a sad end of the SQM data flow from our club.
"It's been a long time since I was (briefly) involved with Globe At Night (https://globeatnight.org/). Back then - before cell phone sky measurements - they accepted SQM readings or naked-eye star counts. NASA even funded a free SQM for astronomy clubs that committed to submitting GAN data. There was indeed information about the need for 'dark adaption' - I believe they recommended 20 minutes outside before making your 'count' - and noted that children involved with GAN needed to be accompanied by a responsible adult to keep the kids safe out in the dark for 20 to 30 minutes."
A couple of points:
First, the Globe At Night still accepts user input from either the phone app or an SQM and there doesn't appear to be a means of discerning how much is derived from either source. However, I think it's a safe bet the lion's share of the data originates from user impressions via the phone app for obvious reasons, i.e. general availability of phones versus an additional expenditure for an SQM.
Second, information about dark adaptation is one thing but staring at an illuminated phone is quite another and I'm not convinced that such information that may have been provided would be enough for a novice observer. Moreover, I don't believe the methodology in doing so lends itself to a research grade or level of rigor yet the study evidently presents itself as such and apparently relies on the statistical power of the number of observers for its data and results.
"I'm not sure what "The skies are getting 10% brighter every year" even means - The sky over big cities like LA?"
I'm in agreement with your statement although that's just one of other similar examples that could be provided. Yet, that 10% figure, as noted in the link to a science site in the OP, is the one that's has been echoed in various journals as well as in the popular media. And, of course, it's highly unlikely the user input data is coming from anything other than fairly populated areas and much less so from more remote and darker areas so the results are likely skewed towards outcomes derived from brighter skies.
Finally, the fact that seemingly no one has previously reported the app being insensitive under a relatively dark sky, i.e, SQM readings of 21.6 or greater (and possibly less), indicates a significant issue with the methodology since it appears almost all of the data originates under bright sky conditions that can be derived from the app...
