United Airlines said on Thursday it has set a goal to cut its greenhouse gas emissions by 50 percent over the next few decades to help reduce its carbon footprint and its dependence on fossil fuels. The third-largest U.S. air carrier will invest more than $2 billion a year in more fuel-efficient aircraft, expanding its use of low-carbon biofuels in daily flights and implementing ways to better conserve fuel. "This is not only good for the environment but guards against oil price instability," Aaron Stash, a United manager of environmental strategy and sustainability, told reporters. Fuel costs account for a major portion of airlines' expenses, and rising oil prices over the past year have eaten in to industry profits, sending carriers scrambling to mitigate the impact.