Doctor Dolittle

In early Victorian England, Matthew Mugg (Anthony Newley) takes his young friend Tommy Stubbins (William Dix) to visit eccentric Doctor John Dolittle (Rex Harrison) for a sick duck that Tommy had found in the streets, Dolittle, a former physician, lives with an extended menagerie, including a chimpanzee named Chee-Chee (Cheeta), a dog named Jip, and a talking parrot named Polynesia (the uncredited voice of Ginny Tyler). Dolittle claims that he can talk to animals. In a flashback, he explains that he kept so many animals in his home that they created havoc with his human patients, who took their medical needs elsewhere. His sister, who served as his housekeeper, demanded that he dispose of the animals or she would leave; he chose the animals. Polynesia taught him that different animal species can talk to each other, prompting Dolittle to study animal languages so that he could become an animal doctor.
While treating a horse, Dolittle's lack of human empathy offends the horse's owner, General Bellowes (Peter Bull). Bellowes' niece, Emma Fairfax (Samantha Eggar), chides Dolittle for his irresponsibility and rudeness to her uncle. Matthew falls in love with her at first sight. After she has gone, Dolittle admits he also finds her attractive.
A friend of Dolittle's sends him a rare Pushmi-pullyu, a creature that looks like a llama with a head on each end of its body. Dolittle takes the creature to a nearby circus, where the Pushmi-Pullyu becomes the star attraction. The doctor befriends a circus seal named Sophie who longs to return to her husband at the North Pole. Dolittle disguises her in women's clothing to convey her to the coast, and then throws her into the ocean. Fishermen mistake the seal for a woman, and have Doctor Dolittle arrested on a charge of murder. General Bellowes is the magistrate in his case, but Dolittle proves he can converse with animals by talking with Bellowes' dog. Though Dolittle is acquitted, the vindictive judge sentences him to a lunatic asylum. Dolittle's animal friends rescue him, and he, Matthew, Tommy, Polynesia, Chee-Chee and Jip set sail in search of the legendary Great Pink Sea Snail. Emma stows away, seeking adventure. They randomly choose their destination: Sea-Star Island, a floating island currently in the Atlantic Ocean.
The ship is torn apart during a storm, but everyone washes ashore on Sea-Star Island. Emma and Dolittle admit they have grown to like each other. The party is met by the island's natives, whom they mistake for hostile savages. The populace are in fact highly educated and cultured from reading books that have washed ashore from innumerable shipwrecks. Their leader is William Shakespeare the Tenth (Geoffrey Holder); his name reflects the tribe's tradition of naming children after favorite authors. William explains that they are wary of strangers coming to the island, and that the tropical island is currently endangered because it is drifting north into colder waters. Mistrust leads the islanders to blame the doctor and his party. Dolittle persuades a whale to push the island south, but this causes a balancing rock to drop into a volcano, fulfilling a prophecy that dooms Dolittle and party to be burned at the stake. The whale also causes the island to rejoin the mainland, fulfilling another prophecy that dictates that the doctor and his friends be heralded as heroes, and they are freed. While treating the animals on the island, Dolittle receives a surprise patient - the Great Pink Sea Snail, which has caught a severe cold. Dolittle discovers that the snail's shell is watertight and can carry passengers. Dolittle sends Matthew, Tommy, Emma, Polynesia, Chee-Chee, and Jip back to England with the snail. Emma wishes to stay on the island with him, but the Doctor is adamant that a relationship would never work. She finally admits her love for the Doctor, and kisses him goodbye.
Dolittle cannot go back because he is still a wanted man. Furthermore, he wishes to investigate the natives' stories of another creature, the Giant Lunar Moth. After his friends leave, Dolittle realizes painfully that he has feelings for Emma. Sophie the seal arrives, accompanied by her husband. They bring a message: the animals of England have gone on strike to protest his sentence, and Bellowes has agreed to pardon him. Dolittle and the islanders construct a saddle for the Giant Lunar Moth, and Dolittle flies back to England.

Question: WHy is the tropical island currently endangered?
Answer: It is drifting north into colder waters.