THERE IS ONLY ONE country on earth that is truly, deeply, accidentally green. In 2006, the World Wildlife Fund did the global math, cross-indexing social factors like education and life expectancy with each population's ecological footprint and global biocapacity. The poor countries were huddled on the left, underdeveloped. The rich soared up to the right, overconsuming. Out of the 150 nations studied, only here, in the rigidly ruled kingdom of Dr. Castro, were human beings developing at a statistically sustainable rate.

WWF isn't alone in concluding that Cuba is doing something right. The United Nations lists it as one of the only countries in the Caribbean that have stopped and even reversed deforestation, with 22 percent of the island covered in everything from palms and pines to mangroves and ceiba trees. Fidel Castro has declared himself the island's chief ecologist, intervening in everything from the design of lobster boats (he added live wells, to ensure that juveniles return to the sea still kicking) to the retrofitting of sugar mills (to run on bagasse, the biomass left behind by the sugar harvest). He ordered the planting of 348 million trees in the 1960s, and as recently as 2007 Cuba claimed it would put 135 million more in the ground. At the Rio Earth Summit, in 1992, Fidel received rapturous applause as a prophet of low-consumption ethics, even as George H.W. Bush was criticized for ignoring both him and the resulting treaties.

Since then, Cuba's national assembly has enshrined sustainable development in the constitution, designated 20 percent of the country for conservation, and organized an islandwide drive to install fluorescent lightbulbs and rip out old electricity-sucking refrigerators. When Castro learned that spearfishing was damaging the Jardines de la Reina national park, an 830-square-mile archipelago in Cuba's south, he simply banned it—even though spearfishing there was his favorite relaxation.

There can be no question that something genuinely important is now at stake in Cuba. If you think you know the Caribbean, think again. Cuba is the Caribbean: It contains nearly half the landmass (ten times that of Jamaica) and a third of the population (11 million people) in that round sea. And in a region stressed by development, habitat loss, overfishing, and low environmental standards, Cuba retains the richest biodiversity in the Caribbean, with perhaps half of her 20,808 known terrestrial species found nowhere else. Its seas are like no other: The northern coast stretches in a bow almost 1,000 miles long, with crystalline waters, blue holes, and some 1,000 barrier islands barely fathomed since Hemingway sent his U-boat hunters stalking their channels in Islands in the Stream. In the south, nutrients well up from the 25,000-foot-deep Cayman Trench, enriching a 70-mile-wide continental shelf. As much as 50 percent of Cuba's southern coast is healthy red and black mangrove forest, the largest such forest—and fish nursery—in the Caribbean. Its 2,200-square-mile Zapata Swamp is the Caribbean's largest wetland, home to 900 plant species, and its reefs are intact to a standard unknown elsewhere in the region.

Hold on a minute: Cuba? For as long as I've been reporting on the island—15 years now—I've listened skeptically to declarations about Cuba's ecological achievements that seemed to defy the brown reality I saw on the ground. Eurogreen Web sites trumpet wind farms; I found the beaches of the north coast stained with tar from a low-tech, mismanaged oil industry. The head of the Center for Cuban Studies, in New York, told me how organic gardening was greening Havana; in the countryside, I found crude state agriculture despoiling the earth. The United Nations gave biosphere status to a wildlife reserve in the Pinar del Río province surrounding an "eco-village" called Las Terrazas; this turned out to be mostly some phony thatched huts and a sandwich shop used by tour buses.

If the present is dubious, the future may be worse. After nearly five decades of Castro and a collective 12 years of Bush, regime change is the new reality. Now 81, Fidel is on life support; his 76-year-old brother Raúl is in the wings; and presidential candidates from Barack Obama to John McCain have spoken cautiously of a new beginning with Cuba. Lifting the U.S. embargo would send millions of American tourists and billions of investment dollars flooding into Cuba, turning pristine coastline into—well, think Ayn Rand with a cement mixer. Cubans today are hungry and cash-starved, but tightly controlled; a sudden collapse of the country's political system would unleash them—and foreign investors—on the forests and seas.

In Cuba, facile lessons are abundant (Michael Moore, call your doctor!), but even a jaded visitor like me has to acknowledge that a particular mix of Cuban factors—utter economic incompetence, visionary green policies, and a dash of red brutality—has conspired to deliver the big island into the 21st century with an almost 19th-century set of natural assets. This success comes with a question: Is it possible that Cuba, a deliberate refutation of the consumer-crazed, gas-guzzling, climate-warming American way of life, might someday offer us genuine alternatives? If Supertanker America strikes a reef, wouldn't we need some quick lessons from a neighbor about urban gardening, low-impact living, and fitting two guys onto one bicycle?