Jeju’s coastline comes into view long before the plane lands. From the window of a descending Airbus A320 appears rows of white wind turbines dotting the coastline. A closer look reveals more turbines farther inland, joined by clusters of solar panels spread across former citrus fields.

Long known as a rustic holiday destination 290 miles (about 470 kilometers) south of Seoul, Jeju has spent more than a decade reinventing itself as South Korea’s clean energy testbed. The island aims to become carbon neutral by 2035, pioneering technologies and policies that could eventually be adopted across the country.

“Jeju was way