In celebration of Earth Month, United Airlines is enhancing its carbon-offset program to allow the airline's MileagePlus frequent traveler program members to redeem their miles for offsets to cover the carbon emissions associated with their air travel and/or cargo shipments, the airline said today. The Chicago-based carrier's existing program allows United customers to calculate and purchase offsets for travel and cargo shipments. But customers now can use either miles or money. The new mileage component is believed to be the first of its kind to be offered by a United States-based carrier. When MileagePlus customers opt to offset their emissions after booking on united.com, they may select from a list of United Eco-Skies partners to redeem their miles for the offset, including forest conservation near San Francisco, wind power in Texas and biodiversity preservation in Belize. The airline's carbon-offset projects were designed in collaboration with Sustainable Travel International, a global, non-governmental organization that specializes in innovative, sustainable tourism and conservation solutions. In 2013, United (NYSE: UAL) was recognized as the Eco-Aviation Gold Airline of the Year by Air Transport World for its leadership in environmental efforts. Since 2006, United has recycled more than 23.5 million pounds of aluminum cans, paper and plastic. The company launched its Eco-Skies Community Grants program in 2012 as a way to encourage United employees who volunteer their time and efforts to environmental organizations in their communities. More Info here.