SEATAC, Wash. – Sea-Tac Airport is the fastest growing in the nation, and it’s growth shows little sign of slowing down.

Simultaneously, efforts continue to reign in the airport's carbon footprint. Now, the Airports Carbon Accreditation Program for North America recognizes Sea-Tac as a level three airport along with San Francisco International.

In its rankings, the ACA said the airport has not only mapped its carbon output, those exhaust streams coming directly from the airport, but now includes other sources of carbon from a range of business partner emissions. The ranking even includes how much comes from passengers and staff going to and from the airport.

Only one airport in the U.S. scores higher, Dallas-Ft. Worth, which is considered "carbon neutral" by the ACA.

Sea-Tac's been spending years reaching its goal.

In 2013, it began hooking planes up directly to the airport's massive central air conditioner, which pumps warm and cool air, depending on the season, directly into the planes. Many planes can now plug directly into the electrical grid so an aircraft's auxiliary power unit, a small jet engine usually located in the tail, can be turned off. The auxiliary power unit is usually noticed from its high pitched whine as the airplane sat at the gate.

One of the latest moves includes requiring ride services, such as Uber and Lyft, to meet carbon reduction goals by heavy use of hybrid and electric vehicles.

"We're the first airport in the world to successfully put in environmental requirements on ride share apps like Uber and Lyft," said Stephanie Meyn, an atmospheric scientist, who serves as the airport's Climate Protection Program Manager.

Taxis have also been required to use similar vehicles for several years.

"We're looking at every opportunity and not just looking to offset or purchasing our way out of reductions," said Meyn.

Meyn cited DFW's partial use of carbon credit offsets to meet its carbon neutral ranking. The credits can be purchased from other industries, such as utilities, who build up credits by installing things like wind farms.

Another work in progress is the use of electric ground vehicles, such as baggage tugs, push back tractors, and powered baggage loading ramps. Once all powered by gas or diesel fuel, Alaska Airlines has already converted more than 200 vehicles into electric ones, with the rest of the airlines expected to implement over the next several years.

"Five years from now, the vision is, when you park an aircraft at the gate, the bags, everything from that point is emission free as it moves into the airport. All of the bag tugs, belt loaders and push back tractors will be all electric," said Meyn.

Carbon is considered a danger to the environment, because many scientists have blamed it for raising global temperatures.