If you believe the world’s tourism hot spots are already saturated, think again — specifically about so-far mostly untapped markets of international travelers from China and India. When they add hundreds of millions more visitors over the next decade or two to jam-packed peak-season crowds, places that think they face a breaking point now will remember this summer as the good old days.

Tourism worldwide has been swelling for decades, and its impact falls disproportionately in Europe, especially southern Europe’s sun belt. Barcelona, an achingly charming place whose popularity has rocketed since it hosted the Olympics 32 years ago, is a case in point.

The city, a densely packed dreamscape of 1.6 million inhabitants, had fewer than 2 million tourists in 1990. This year, city officials expect nearly 13 million, accounting for roughly 38 million overnight stays. That doesn’t include millions more cruise ship passengers or the day-trippers who join the throngs on the Rambla pedestrian boulevard, Antoni Gaudi’s Sagrada Familia cathedral and other must-see sights.

Rents in the city have soared, up almost 70 percent in a decade, driven by Airbnb and similar platforms, along with digital nomads and holiday home buyers. Roughly 150,000 people have tourism jobs in Barcelona, but many can’t afford to live in the city.

Similar concerns beset Lisbon, Athens, Las Palmas, Ibiza and other prime destinations. They are victims of their own success, awash in tourism dollars and increasingly fearful of losing working-class residents, neighborhood grocery stores — and their souls.

No European city has been as thoroughly overrun by tourists as Venice, where 70 percent of its population has been driven out since the 1950s and more than 20 million annual visitors cram the city’s fairy-book three square miles. This year Venice imposed a $5.30 entry fee on day-trippers, a gesture that seems pointless.

Pushback and protests by locals on the tourist onslaughts are intensifying — not only in southern Europe but also in Amsterdam, Tokyo, Seoul and elsewhere. In Barcelona, where tourism was almost universally seen as positive 15 years ago, nearly one-quarter of residents now say it has brought negative effects.

“The likely scenario is that the tourism industry keeps on doing what it’s been doing, and tensions between residents and tourists will rise,” Sebastian Amrhein, a tourism scholar at Germany’s Rhine-Waal University, told me.

In Barcelona, I spoke with Jordi Valls, deputy mayor for the economy, tax, economic promotion and tourism, who took the job last year. Almost immediately he determined that tourism, which accounts for almost one-fifth of the city’s nonpublic economy, was at its limit — and also unstoppable.

“I can’t control the demand,” he told me. “I can only affect the supply.”

In pursuit of that goal, the city has banned room rentals in private apartments and, by 2028, plans to abolish another 10,000 short-term rentals through Airbnb and the like. The city has also raised the municipal tax on overnight stays, capped new hotels in the city center and limited the size of tour groups.

Part of the idea is to ensure public spaces aren’t colonized by visitors wielding selfie sticks. That’s proving hard enough in some parts of town. Then there’s the long-term stress that tourists exert on water, electricity, transport and other resources.

An equally stiff challenge is protecting the character of residential neighborhoods where apartments have been snapped up by outsiders. The few thousand new units Barcelona plans to build annually aren’t likely to slow the city’s steady march toward anglophone gentrification.

Squeezing supply tends to raise prices, shifting available capacity to richer tourists at little or no benefit to the workforce. In the trade, that’s known as “quality tourism,” a euphemism for deep-pocketed visitors whose lavish travel habits take a major toll on the climate.

The end of the pandemic brought a wave of “revenge” tourists bingeing on new sights after long sojourns in their living rooms. Projections last summer that travel would ease this year have proved wrong: Revenge tourism is becoming rampage tourism, now surpassing pre-pandemic records.

Tourists are a golden goose for many places, especially the sun-and-sand meccas of southern Europe, whose economies depend disproportionately on them. Few places will muster the political will anytime soon to slash numbers meaningfully.

But if governments don’t proactively diversify their economies to become less tourist-reliant, and redistribute tourism revenue more aggressively to demonstrate its benefits, then social protest or resource overload might force their hand — unless tourists themselves, sick of the crowds, abandon destinations altogether.

The writing is on the wall — literally. In the Canary Islands, anti-tourist activists have lately taken to posting phony signs near the beach: “Closed due to overcrowding.”