BERLIN, March 29 -- President Obama's chief climate-change negotiator said Sunday that the United States would be "powerfully, fervently engaged" in global talks to reduce carbon emissions but warned that a difficult path lay ahead.

Todd Stern, a Washington lawyer and former Clinton White House official, said at a U.N. conference in Bonn, Germany, that despite high expectations, Obama did not have a magic solution for fashioning a global climate-change treaty by the end of the year.

The Obama administration reversed years of U.S. policy Monday by calling for a treaty to cut mercury pollution, which it described as the world's gravest chemical problem.

Some 6,000 tons of mercury enter the environment each year, about a third generated by power stations and coal fires. Much settles into the oceans, where it enters the food chain and is concentrated in predatory fish like tuna.

Almost everyone has seen the ENERGY STAR label on products in the market place - it’s on computers, light bulbs, appliances and many other common household products. What does Energy Star mean?