Global investments in renewable energy aren't bearing fruit and the world should focus on securing reliable supplies of fossil fuels, U.S. energy and interior secretaries said this week as they worked on convincing Europe to buy more U.S. oil and gas. The U.S. has become Europe's top oil and gas supplier on the back of the U.S. shale boom and its companies are seeking to boost the share as the European Union moves to fully cut off remaining Russian energy imports.