With the growing threats due to climate change – rising sea levels, devastating storms and tidal flooding – it's no mystery why some of the world's most iconic cities and natural wonders are at risk. And while there's no clear-cut answer on the rate at which many cherished places the world over will be underwater, with the impending long-term effects of climate change – including the melting polar ice cap – we have a very narrow window before there will be dramatic repercussions, says Costas Christ, Chairman of the National Geographic World Legacy Awards and sustainable travel expert. "We have a window of 10 or 20 years at most before we set in motion the temperatures that we can't turn back," he says. Happily, the outlook isn't all bleak. Our travel choices and actions make a difference, Christ says. "What can we do as travelers? We can choose those companies that are practicing and embracing sustainability," he explains. By rewarding companies that are substituting plastics, generating less waste, offsetting their carbon footprint, using renewable energy and supporting national parks and fragile ecosystems, among other sustainable practices, we can advance conservation efforts, travel responsibly and have a positive long-term impact. With that in mind, here are six cities starting to submerge, and expert-endorsed tips for limiting your carbon footprint and aiding conservation efforts on your next trip. [See: 5 Inspiring Places to See Before They Disappear.]Miami Beach, FloridaWith frequent coastal flooding and high tides in vulnerable, low-laying areas, it's no surprise Miami Beach, Fort Lauderdale and other areas across southern Florida are at risk of sinking into the ocean at a fast rate. At only about 3 feet above sea level, Miami is particularly susceptible to flooding. The Intergovernmental Panel on Climate Change projects that sea levels could surpass 3 feet at the turn of the century and some studies suggest daily flooding could lead to a rise as high as 6 feet. And according to a 2015 report from the Risky Business Project, an estimated $15 billion in property across Florida could be threatened by accelerated sea levels. Undoubtedly, increasing sea levels and flooding will impact Florida, but on the upside, hundreds of millions of dollars have been funneled into the city of Miami to raise sea walls and elevate streets, among other forward-thinking preventative measures, to protect Miami Beach.Venice, ItalyDubbed the city of water, Venice has long been revered for its enchanting canals. But with increasingly high tides, turbulent winds and flooding in the Venetian lagoon, the Italian mecca is disappearing quickly. While the Mose flood barrier initiative was introduced in the late 1980s to block up to 3 meters of water during high tides, it is not yet completed. Aside from the city being precariously built over water, an ever-growing increase in floating cruise ships is causing significant damage to the city as well, Christ says. So, in short, if you want to experience the city's legendary gondolier rides, awe-inspiring cathedrals and charming palazzos, now is the time to plan a trip – before it's too late. Mexico CityMexico City has sunk nearly 30 feet since the end of the 19th century. Why is one of the world's largest metropolises dropping so rapidly? A growing population of more than 21 million people has led to limited water resources as the city pumps groundwater from aquifers, underground geological formations that absorb and contain water, leading to land subsidence, or gradual sinking. To add fuel to the fire, world-renowned buildings like the Palace of Fine Arts are sinking quickly and clay layered below the city has cracked, leading to many leaning structures. While engineers have implemented measures to help fix subsidence and right buildings on an incline, time may be limited to see cherished monuments.[See: Going Green: 5 Top Eco-Friendly Trip Ideas.] New York CityWith legendary attractions – from the Empire State Building to the Statue of Liberty and the Metropolitan Museum of Art – the Big Apple is a must-see destination. Yet, Manhattan frequently encounters severe storms (such as Hurricane Sandy in 2012) and climatologists forecast such natural disasters to take place more often due to warmer temperatures and higher sea levels that make these storms increasingly hazardous. And if the sea increases by 2.5 feet, which some experts predict could take place as early as 2050, New York City's infrastructure will be impacted drastically. The silver lining: innovative urban planning initiatives like the Big U, a $335 million proposed project that would create a network of barriers to block the city from sea-level increases associated with severe storms. New OrleansIf you want to experience Mardi Gras and admire architectural gems of the French Quarter, now is the time to plan a trip to the Crescent City. A recent study conducted by NASA and the California Institute of Technology analyzing subsidence in and around New Orleans through NASA airborne radar shows parts of the city are dropping by up to 2 inches annually. While the Big Easy has faced a host of environmental catastrophes, including the Hurricane Katrina levee-system disaster, it's also sinking at a steady rate. More than 50 percent of the city is below sea level today and estimates show three quarters of the city could drop below sea level as early as 2050. "The issue of sinking cities is certainly well-related to climate change," says Jamie Sweeting, vice president for sustainability at G Adventures and president of the Planeterra Foundation, a nonprofit organization dedicated to contributing financial resources to emergency response, conservation and other charitable causes. "Rising sea levels are going to be more acutely linked to big-weather events," he explains, noting that scientists agree that natural disasters will take place more frequently and at a more profound level. [See: 15 Must-Visit National Park Attractions.]Bangkok, Thailand Aside from its low-lying landscape and susceptibility to severe storms, Thailand's capital city is in danger also because of overdevelopment and subsidence caused by too much water getting pumped from the city's aquifers. Catastrophic flooding and severe rain also put a strain on the city, as do its host of soaring skyscrapers and its heavy railway. In fact, some experts say the city could be underwater by 2030 if severe storms persist. That said, Thailand's National Reform Council committee has proposed creating a floodwall to protect the area from rapidly increasing sea levels – a project estimated to cost more than $14 billion to protect Bangkok and its surrounding areas.It's no longer a question of whether places around the globe are at risk. The real question is how far we will go to promote sustainable tourism, Christ says. "When will we see a day when hotels are not generating tons and tons of plastic water bottles or carbon dioxide in the area?" Christ asks. In addition to traveling responsibly, tourists can let hotels and tour operators know they care to incentivize them to make a difference and leave a positive impact on the environment, he says.