The theme of this year's day is "Eco-tourism - Key to Sustainable Development".

"How can we deny that today humanity is, unfortunately, living an environmental emergency?" the Pope asked in his message.

"A certain uncontrolled tourism has contributed, and continues to contribute to such destruction, caused also by tourism facilities built without plans to respect the impact on the environment," he said.

The Pope said that tourism was too often the subject of egotistical desire for pleasure rather than contemplation of nature's wonders.

Tourism had to respect the environment and protect it for future generations, he said, adding that people should look for the spiritual dimension of the appreciation of nature.