Aviation and the environment are on a collision course. The number of airline flights worldwide is growing and expected to skyrocket over the coming decades. Aircraft emissions pollute the air and threaten by 2050 to become one of the largest contributors to global warming, British scientists have concluded.

Much remains unknown about climate change and the role aviation plays, though climate scientists express particular concern about jet emissions in the upper atmosphere, where the warming effect from some pollutants is amplified.

Now, aviation is believed to be less a factor in the Earth's warming than power plants or vehicular traffic. But its emissions are considerable. On a New York-to-Denver flight, a commercial jet would generate 840 to 1,660 pounds of carbon dioxide per passenger. That's about what a typical driver generates with an SUV in a month.

With the projected explosion in worldwide travel, air pollution from aviation is a growing concern among scientists, and it's drawing increased scrutiny from governments, particularly in Europe.

"It's an issue that has to be addressed," says Brenda Ekwurzel, a climate scientist for the Union of Concerned Scientists, an environmental advocacy group.

David Travis, a climate science professor at the University of Wisconsin-Whitewater, says aircraft emissions "are currently one of the fastest-growing contributors to global warming."