Costa Rica, population 5 million, wants to wean itself from fossil fuels by 2050, and the chief evangelist of the idea is a 38-year-old urban planner named Claudia Dobles who also happens to be the first lady. Every country will have to aspire to something similar, scientists say, if the world is to avert the most dire consequences of global warming. And while Costa Rica's carbon footprint is tiny compared to other countries, Ms. Dobles has a higher goal in mind: Getting rid of fossil fuels would show the world that a small country can be a leader on an awesome problem and improve the health and well-being of its citizens in the bargain. Costa Rica's green bid, though fraught with challenges, has a head start. Electricity comes largely from renewable sources already - chiefly hydropower, but also wind, solar and geothermal energy. The country has doubled its forest cover in the last 30 years, after decades of deforestation. Now, if its decarbonization strategy succeeds, it could provide a road map to others, especially developing countries. For Ms. Dobles, the top priority is fixing transportation. It is the largest single source of Costa Rica's greenhouse gas emissions. The National Decarbonization Plan, as it's called, envisions electric passenger and freight trains in service by 2022. Under the plan, nearly a third of all buses would be electric by 2035, dozens of charging stations would be built, and nearly all cars and buses on the roads would be electric by 2050.