By 2030, many areas in the United States are likely to see storm surges combining with rising sea levels to bring waters at least four feet above the local high-tide line, according to a report released by Climate Central, a nonprofit research group. Nearly 2.6 million homes are on land that would be inundated. The Obama administration is trying to plan for a country altered by shifts in precipitation, higher oceans and more intense periods of heat. It is rethinking infrastructure projects and creating a new plan for how to manage plants and wildlife in the face of global warming. Every agency is required to come up with a plan by June for how to adapt to climate change.