Not so long ago, families coming to the shore in Montgat, Spain, just outside Barcelona, built sand castles, played paddle games and lounged around on the broad band of sand that was the picture of the European summer vacation. But now in some places, there is hardly enough room to put down a towel.

“From here to there,” said Sofia Mella, 19, as she pointed far out to sea, “it was all sand.”

Climate change is changing Europe into a summer paradise lost. Across Spain, Italy, Greece, France and beyond, sand-devouring storms, rising seas, asphyxiating temperatures, deadly floods and horrific wildfires have year after year turned some of the continent’s most desired getaways into miserable locales to get away from.

As southern Europeans dream of fjords, the traditional hot spots and fixtures of travel agency package deals no longer seem so desirable. This past week, even though temperatures broke across much of the continent, experts and officials warned that the next heat wave would come soon, endangering vulnerable locals and making tourists question what they had gotten themselves into. Even activists seeking to free their cities from the scourge of overtourism saw no silver lining to the brutal conditions.

“It’s hell,” said Daniel Pardo Rivacoba, who lives in Barcelona, and who spoke for a group fighting overtourism — and its climate-change fueling flights. He saw the scorching sun not as an ally to keep the tourists away, but as a common enemy that will melt them both.

Last year, Mr. Pardo Rivacoba’s group went viral by organizing demonstrations against overtourism that including spraying tourists with water guns. This year, he said, it was so hot that “we used the water guns on ourselves.”

Across the continent, June was the cruelest month. So far. In Rome, tourists rotated around the city’s sites as if spinning in an outdoor microwave. Opera singers in Verona passed out in their costumes.

But Spain has become the least fun-in-the-sun destination. Temperatures in the southwestern city of El Granado reached nearly 115 degrees (46 degrees Celsius), a national record for June. And there were other grim metrics. Last year, floods in Valencia killed more than 200 people; this year, experts say excess deaths, especially among the ailing and elderly, have risen sharply with the temperatures.

Climate change is also transforming the Spanish landscape, including the beach in Montgat, where increasingly frequent storms have washed much of the sand away.

“Every time we come, there is less and less sand,” said Susanna Martínez, 40, who had been going to the beach in Montgat with her family for a decade.

Barcelona, only a few miles away, has reported losing 30,000 square meters of sand over the past five years. Marina d’Or, outside Valencia farther south, was envisioned by developers and families across Spain as a seaside resort, an emblem of the country’s beach holidays. Now, storms have washed some of its beaches away, too. Experts have estimated the loss of hundreds of thousands of square meters of beach across the country, and warned about desertification.

Spain knows it has a problem. In a time of right-wing opposition to Europe’s environment-protecting Green Deal regulations — “greenlash,” it has been called — the progressive government of Spain has embraced an ecological transition.

“The main risk humankind faces today, undoubtedly, is climate change,” said Sira Rego, a minister in the government led by Pedro Sánchez. She called responding to those changes the country’s “priority in terms of security.”

The government, proud of its well-performing economy, is working to attract hundreds of billions of euros in investments in sustainable energy to create hundreds of thousands of new green jobs. It seeks to invest in temperature prediction systems to foresee heat waves, and to train health care workers who will be forced to treat more heat-related illnesses. It is trying to increase energy efficiency and reduce greenhouse gas emissions.

At more local levels, cities like Barcelona are also trying to mitigate the damage wrought by a changing climate.

Laia Bonet, the city’s first deputy mayor responsible for ecology and urban planning, said that Barcelona was “especially exposed to the effects of climate change,” and that it was working to address the reality of a hotter city with higher sea levels and eroded beaches.

The city’s priority, she said, is protecting vulnerable residents with hundreds of climate shelters. But it is also investing 1.8 billion euros, more than $2 billion, to make buildings greener, expand green spaces, install 200 shade structures, and replace some pavements with dirt to better absorb and repurpose rain water.

The city is also using sand recovered from construction projects to help preserve its beaches, which are a beloved public space for locals to cool off, as well as a guard against storm surges, and a critical element in the city’s identity as a tourist destination. About 15 percent of Barcelona’s economy comes from tourism.

She said climate change had forced the city to think of its overtourism problem differently, to come up with measures that addressed both. “These are progressive policies that cities can implement as an antidote,” she said.

In Seville, the southern Spanish city sometimes called “the frying pan” of Europe, nuns have long hung white drapes around their cloisters to keep the sun out. But the city has more recently started covering its narrow streets with white sheets, pulled across the rooftops like canopies. Parts of the city are using an ancient system of underground ducts to bring cooler air to the surface, while another project pumps water runoff into shaded public spaces, often cooled by mists. The city has started naming heat waves to make them more palpable and memorable.

That is important, experts say, because once the heat waves pass, people tend to forget about them. So scientists in recent days have rushed out estimates that the heat wave may have tripled the death toll in afflicted regions across the continent. The goal, one of the scientists has said, is to focus attention on the dangers of extreme heat when people were extremely hot.

But instead of changing their behavior, many Southern Europeans suffering during the sweltering heat waves have resorted to hatching escape plans. In Barcelona, three older Spanish women sitting in the shade across from the city’s cathedral fantasized about cloudy Galicia in the north. And Romans with healthy travel budgets have started looking longingly at damp, chilly, often overlooked nations.

“It could happen that if it gets too hot people could come to Belgium,” said Ann Verdonck, 45, from near Antwerp, who was on vacation with her family in Barcelona, where she said last month’s temperatures were untenable. “And then we will have too many people.”

But Mr. Pardo Rivacoba, the activist, said extreme heat would not get rid of overtourism because the tourism industry is so cynical. If the summers became too hot, he feared, the industry would work to move school vacations to months when families could travel.

“The tourism industry,” he said, is “ready to literally burn every piece of the calendar.”

Still, some locals considered the heat a reprieve from the suffocating masses of tourists.

“If it’s going to be this hot, at least I’ll be able to walk by the Sagrada Familia,” said Mercedes López, 67, a Barcelona native who lived by the famous, and famously swamped, landmark designed by Antoni Gaudí.

But she said a decimated tourism industry would only cause economic misery. And with extreme heat, locals would try to get out, too. “If this heat keeps coming,” she said, “we’re going to have to move to Norway or Finland.”

Her friend Consol Serra, 74, did not find that a sustainable solution.

“If we’re dealing with the heat now,” she said, “it’ll reach them eventually too.”

Ana Rojas Martín contributed reporting.