It's easier than you think being green -- at least when it comes to investing through exchange-traded funds.The latest play on this increasingly crowded sector comes from Van Eck, which launched its Market Vectors-Global Alternative Energy ETF ( GEX) on the NYSE Wednesday. The new entry tracks the performance and yield of the Ardour Global Alternative Energy Index (Extra Liquid), which includes stocks of companies all over the world involved in alternative and renewable energy technologies. The listing comes at a time when you no longer have to own An Inconvenient Truth on DVD to be thinking about investing in alt-energy. The Supreme Court ruled last month that the Environmental Protection Agency has the power to curb greenhouse gas emissions. Many think this decision will eventually lead to regulations that make "cleaner" energy sources such as wind, solar and geo-thermal more attractive in the U.S. And the European Union vowed in March that 20% of its energy needs will come from renewable sources by 2020. The Market Vectors-Global Alternative Energy ETF is meant to give investors exposure to the most-widely held and traded "pure plays" in the sector. The Ardour Global Alternative Energy Index (Extra Liquid) screens 250 companies from the U.S., Europe and Asia that derive at least 50% of their revenue from alternative energy. Only the 30 stocks with the highest average daily trading volume and largest float-adjusted market capitalization actually make the index, which is rebalanced quarterly.