Melody Cheung, 16, St Paul's Secondary School
While tourism generates millions of dollars in revenue, many seem to forget the harm it causes. For example, a record 130,000 tourists visit the Forbidden City in Beijing every day, much higher than the ideal capacity of 30,000. The packed crowds, and their litter, may cause serious damage to the ancient structure.
Despite the tremendous income the central government earns from allowing so many guests to visit the Forbidden City, no amount of money would be enough to make up for the loss of cultural and historical value.
It's the same with human disturbance of coral reefs. The US Coral Reef Task Force identified 'recreational overuse' as one of the six top threats facing America's reefs.
In developing countries, the rapid expansion of tourism isn't always accompanied by effective conservation efforts. Once ecosystems or structures are damaged, it will take a lot of time and money to restore them - if they can be repaired at all.
Also, travellers can spread infectious diseases around the world. A pandemic could kill millions of people and destroy world economic growth. Then there's global warming. Most tourists travel by air, and planes release huge amounts of carbon dioxide into the atmosphere. This causes the Earth's temperatures and sea levels to rise, posing a serious threat to low-lying areas.