The world’s richest nations have long been fueled by oil, coal and natural gas, but President Obama warned Friday that less affluent countries trying to take the same path will put the planet “under water.” In an interview with Facebook founder Mark Zuckerberg, Mr. Obama said he hoped social-media “connectivity” will help convince developing nations to eschew fossil fuels, which contribute to rising carbon-dioxide levels in the atmosphere but are also less expensive and more reliable than green energy alternatives. “In terms of the problems we have to solve, energy is a classic example, the issue of climate change,” Mr....