Offshore energy plan revealed

US President Barack Obama has marked Earth Day by announcing a plan to permit offshore electricity generation from wind, waves and ocean currents.   The move will help to reduce carbon emissions and fight global warming, while creating jobs, said Obama.  He also said that wind could create as much as 20 percent of US electricity demand by 2030, and insisted that his plan for offshore energy would not damage the economy.  "As with so many clean energy investments, it's win-win," he said, predicting that offshore energy would create as many as 250,000 jobs.   Coastal US states such as New Jersey and Delaware have already developed plans to build offshore renewable energy projects.   Obama has indicated that combating climate change is one of his administration's top priorities.   He has pledged to pass legislation establishing a "cap and trade" system to reduce greenhouse gas emissions, although members of Congress from both parties may attempt to block the changes, or water them down.   "The choice we face is not between saving our environment and saving our economy, it's a choice between prosperity and decline," said Mr Obama, as he announced the move, in a speech at a wind tower factory in Iowa.   "Good for environment and great for our economy."