The Great Barrier Reef in Australia is dying.
Extensive parts of the 2,300kmlong reef have become irreparably damaged and permanently bleached, say scientists, and its future is in serious jeopardy.
A study published in science journal Nature in March found that warmer than normal water temperatures for a second year in a row caused vast coral bleaching along a 800km section of the reef north of Cairns last year.
In pristine stretches north of Port Douglas, Queensland, there are areas where 67 to 83 per cent of the reef has died.
This is bad news for tourists who have yet to visit the world's largest coral reef system, which was listed as a Unesco World Heritage site in 1981.
Other travel attractions may soon disappear as well.
Given the higher temperatures, glaciers around the world are melting more rapidly, which is in turn a danger to low-lying tourist destinations such as the Maldives and Venice, which are under threat from rising sea levels.
Climate change is not the only culprit though.
People are travelling more than before, putting a strain on historic monuments, including Machu Picchu in Peru, and hot spots such as Venice, where angry citizens have held street protests against the record numbers of tourists.
Residents of European cities Mallorca, Rome, Barcelona and Dubrovnik have also complained that tourists are clogging infrastructure, increasing pollution and raising the cost of living, and are demanding something be done to tackle the problem.
Urban development has also led to the demise of places such as Tokyo's iconic Tsukiji fish market, which is closing its doors and moving to make way for the Japanese capital's 2020 Olympic buildings.
It is not all bad news, however.
Mr Alex Malcolm, founder and managing director of Jacada Travel, a luxury travel agency which specialises in bespoke experiences worldwide, says cities and sites can be protected if tourism is wellmanaged. "The danger is when huge numbers of tourists flood a destination without any planning or without sensitivity to the local environment," he says.
Governments can implement a high-value, low-impact form of tourism, like what Bhutan has done by limiting the number of tourists allowed into the country at any one time.
"It is one of the best tourism models as it keeps the pressure of numbers low, while making sure the people and the environment can still benefit hugely," says Mr Malcolm.
Travellers should also be educated about best practices when they travel, says Mr Brett Tollman, global chief executive of The Travel Corp, a travel group which includes brands such as Trafalgar Tours and Insight Vacations.
Travellers should search online to make sure they are travelling with an environmentally and socially responsible company, he says, and make sure not to buy products made using endangered plants or animals.
"In protected areas, stick to the places, trails and spaces open to visitors and leave rocks, plants and other natural objects as you find them," he adds.
The United Nations World Tourism Organisation said there were 1.235 billion international tourist arrivals worldwide last year, an increase of about 46 million compared with 2015.
These numbers are unlikely to slow down anytime soon, say experts, who point out that the historic, cultural and natural treasures that travellers long to see risk destruction if the industry is not careful.
Tourists may have less control over factors such as urban development and climate change, but Mr Tollman says that those visiting destinations under threat should do so in a "responsible and sustainable manner".
"The hope is that the appreciation for these destinations... increases through the act of experiencing them," he says.
Here are five destinations around the world under threat.
If you have yet to tick Antarctica off your bucket list, you should do so soon, says Mr Nico Heath, co-founder and director of Lightfoot Travel, a luxury travel agency.
On average, a staggering 160 billion tonnes of ice is breaking off every year, he says, and Antarctica already has 40 per cent less sea ice than it did a century ago.
In July, one of the largest icebergs recorded, A68, a trillion-tonne iceberg 5,800 sq km in size, broke off the West Antarctic Peninsula, one of the fastest-warming areas on Earth, said Antarctic and Southern Ocean Coalition, an international protection and advocacy group.
Antarctica is also at the top of the threatened destinations list for Mr Alex Malcolm, founder and managing director of luxury travel agency Jacada Travel, which specialises in responsible and sustainable travel.
As more ice melts, the survival of the native wildlife - such as whales, penguins and seals - of this unique continent will be in danger, he says, adding: "It's a very special place, truly magnificent and pristine and unlike anywhere you have travelled, with stunning scenery and wildlife. Few destinations can compare in the spirit of adventure and sense of accomplishment you feel, having visited the White Continent."
Ice fields around the world are under threat as well, from the Swiss Alps to the Himalayas to the famed snow cap of Mount Kilimanjaro, which has shrunk by more than 85 per cent since 1912.
Scientists have predicted that the last of the African glaciers will be gone in 20 years, at the current melting rate. Similarly, the United States Geological Survey has reported that Glacier National Park in Montana has lost more than 120 glaciers in the last century.
In Iceland, Solheimajokull glacier on its southern coast is shrinking at an average of 100m a year and, in Alaska, rising temperatures mean that precipitation that used to fall as snow is now falling as rain.
For travellers, the message is clear. If glaciers are on your hit list, go soon.
The best time to visit Antarctica is in summer from late November, when penguin chicks start hatching, to the end of March, when whale and seal sightings are more common.
On the vast Mongolian steppe, the unique life of the local people is as much a draw for tourists as the breathtaking expanse of the landscape.
Mongolian pastoral herders are one of the world's last nomadic cultures. For thousands of years, they have raised sheep, goats and cattle on the grasslands and are wholly dependent on their herds for sustenance and livelihood.
The animals, which provide milk, meat and even clothes and homeware, are moved from one grazing ground to another, while the herders live in gers, a portable, traditional tented structure.
The chance to visit a nomadic Mongolian family, learn about its traditional way of life and spend a night in a ger has been a highlight for many travellers.
Unfortunately, these days, the herders are more likely to move their gers to the nearest city than to the nearest steppe, as life on the plains becomes increasingly difficult.
Rising temperatures have exacerbated the dzud, a periodic weather phenomenon which creates unusually dry summers and harsh winters.
Dry summers mean the grass cannot grow as thick or as high, so when the bitter cold sets in, many of the herders' animals starve and die.
Deregulation of the nomadic herds in the early 1990s, after the collapse of the Soviet Union - under which numbers of herds were capped and nomads were given designated land to graze - meant any number of animals could be herded and left to graze in any location, which has led to overgrazing.
And without government subsidies of cattle feed during the dzud, whole herds have collapsed, decimating the livelihood of thousands of herders.
Some nomads are also leaving the fields for the cities or mining towns by choice, eager for a more modern way of life and better educational opportunities for their children.
As a result, 600,000 former herders have migrated to Ulaanbaatar in the past three decades, doubling the Mongolian capital's population, about 60 per cent of whom now live in ger districts surrounding the city centre.
3 MACHU PICCHU
Located high in the steep Peruvian mountains, Machu Picchu was undisturbed for centuries, its narrow cobbled alleys and network of roads known only to the local people.
That all changed in 1983, when the 15th-century Incan citadel was declared a Unesco World Heritage Site and it was placed high on every intrepid traveller's bucket list.
But the structure - which is thought to have been built as an estate for Incan emperor Pachacuti - was never designed to cope with hordes of tourists, who littered and destroyed the delicate pathways and crumbling stone structures, says Mr Nico Heath, co-founder and director of Lightfoot Travel agency.
The Peruvian government has been struggling to mitigate tourism's damaging effects on Machu Picchu and the famous Incan Trail, which leads to it, for over a decade. A few years ago, Unesco and the Peruvian government agreed that no more than 2,500 people should visit the site a day, but more than 1.2 million people toured the site last year, well over the daily limit.
Now, visitors are required to enter with an official tour guide in small groups of up to 16 people in either the morning or afternoon.
Visiting time is limited to four hours and selfie sticks, tripods and strollers have been banned.
"Ancient sites have experienced a boom in tourism, increasing pollution and risk further damage by travellers who are not educated about the monuments' preservation," says Ms Blanche Drysdale, a travel consultant at Scott Dunn luxury travel agency in Singapore.
She says that to protect the structures, travellers should be careful to not leave any trash behind, use a reputable guide and refrain from touching the monuments, especially the intricate carvings.
"As tempting as it might be to do so, our skins' natural oils act like acid on the stone and speed up corrosion," she adds.
4 DEAD SEA
For thousands of years, travellers have ventured to the banks of the Dead Sea to float in its saline waters and slather themselves with its mineral-rich clay.
One of the world's first health resorts, the Dead Sea is also one of the saltiest bodies of water in the world, almost 10 times more salty than the ocean, and its minerals have been used in cosmetics and health remedies for generations.
Unfortunately, it is drying out.
Bordered by Jordan, Israel and Palestine, it has lost about a third of its surface area since 1960, when booming populations in the surrounding countries began diverting large amounts of water from the Jordan River and other tributaries, preventing them from reaching and replenishing the sea.
Chemical industries in Jordan and Israel have also drawn large amounts of seawater to extract salts and minerals for products such as fertilisers.
All this has meant that the fresh water reaching the sea cannot make up for the region's high evaporation rate and water levels continue to fall by about 1.3m every year.
The receding water levels have caused thousands of sinkholes to form along the shore, forcing some of the region's best known beaches, hotels, roads and agricultural communities to close.
Many hotels still in business have had to construct lifts or walkways so guests can reach the banks of the sea, which used to be at their doorstep.
Plans are under way to build a pipeline, which will pump desalinated water from the Red Sea into the Dead Sea to halt its decline.
But the ambitious billion-dollar "Red-Dead" project, scheduled for completion in 2021, is not without controversy. Environmentalists say this might damage the Dead Sea's unique ecosystem and does not address the root problem.
Tourists to the area should be conscious of their water consumption and choose a hotel such as the Movenpick Resort & Spa Dead Sea, which uses treated waste water for irrigation and outdoor cleaning, as well as water-saving taps and showerheads.
The city was the victim of a terror attack on Thursday, which left at least 13 people dead and 100 people wounded after a van was driven into pedestrians in Las Ramblas, a tree-lined street in the centre of Barcelona which is popular with tourists.
Unrelated to the attack, tensions between the locals and tourists have been simmering in the city for the past couple of years.
The locals say too many tourists have clogged their streets, polluted their city and raised the cost of living. They are protesting in record numbers, leaving anti-tourist graffiti at popular sightseeing spots and slashing the tyres of rental bicycles and tour buses.
In 1990, 1.7 million tourists visited the Catalan capital. In 2012, the number had surged to 7.4 million.
According to a report in The Guardian, including day trippers from cruise ships, 30 million people visited Barcelona last year, an astronomical increase for a city with a population of just 1.6 million.
The city has started cracking down as a result.
A law putting a hold on new hotel developments and holiday apartment licences went into effect earlier this year, and Segway tours and electric scooters were banned from the Old City and the seafront this month.
Some feel that a cap should be placed on the number of daily cruise visitors as well.
Such sentiments are an increasingly common phenomenon in popular destinations around Europe, including Dubrovnik in Croatia and Mallorca, Spain.
This influx, coupled with the narrow, winding nature of some of Europe's old town centres, has caused cities such as Barcelona to feel choked. In fact, so many tourists have crowded Venice's delicate infrastructure that Unesco warned that it would place the city on its list of endangered heritage sites, if the Italian government does not do something to drastically stem the tide of tourism.
To avoid the masses, consider visiting these cities in their off season, from February to June and September to November, when there are fewer crowds and prices drop.
Consider leaving the typical tourist sites to explore spots off the beaten track. Visit the hipster-artsy Gracia neighbourhood instead of the Gaudi buildings or head to the Sant Andreu farmers' market in place of the famous, but very touristy La Boqueria, for your fix of jamon and olives.