Every summer, the most popular European destinations get stuffed to the gills with tourists, who outnumber locals by many multiples, turning hot spots into sweaty, selfie-stick-clogged, “Disneyfied” towns. They offer a taste of a growing global threat: Across the world, thanks in part to rising affluence, travel is becoming a more widely shared pastime. International trips were up 6 percent in the first half of the year, surpassing experts’ forecasts, according to the United Nations’ World Tourism Organization.

This growth might once have been considered unambiguously good news. But the world’s most popular destinations cannot expand to accommodate an infinite flood of visitors. Advocates of curbing tourism say too many visitors are altering the character of historic cities, and making travel terrible, too.

“It’s a level of tourism which is degrading the enjoyment that residents have, but it’s also degrading the tourist experience, because the tourist who is endlessly queuing behind backpacks of hundreds of other tourists is not discovering the real or the authentic place,” said Justin Francis, the chief executive of Responsible Travel, a company that arranges “sustainable” travel for customers.

What’s to blame? In addition to broad prosperity, there’s technology, defined very broadly.

Over the last few decades, innovations in aviation — wider, more efficient jets and the rise of low-cost airlines — significantly reduced the cost of flying. Bigger cruise ships capable of holding many thousands of passengers now take entire floating cities to coastal ports (which is why Venice recently banned these). Then there are the many splendors enabled by the internet, among them online booking, local reviews, smartphone mapping, and ride-hailing and home-sharing, which have collectively democratized pretty much every step involved in travel.

Finally, as in almost every other issue these days, there is the influence of social media.

“You can’t talk about overtourism without mentioning Instagram and Facebook — I think they’re big drivers of this trend,” Mr. Francis said. “Seventy-five years ago, tourism was about experience seeking. Now it’s about using photography and social media to build a personal brand. In a sense, for a lot of people, the photos you take on a trip become more important than the experience.”